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Fun experiment to do at home using neutralisation

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How To Make a Volcano for the Science Fair

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Shows the damage that strong acids can do to a person and how students shouldn't play around with them.

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Gardaí believe drain-cleaning fluid may have used in an attack on a number of teenagers in Co Waterford last week.

The incident occurred in the Earlscourt housing estate on the Dunmore Road at around 11pm last Thursday.

Gardaí said a number of youths were involved in a "serious altercation", and it is understood "a corrosive substance" was used.

"Three youths required medical attention for burns and were taken to University Hospital Waterford Hospital for treatment," gardaí said.

Four male teenagers arrested on Saturday have been released without charge.

Gardaí said a file is being prepared for the Director for Public Prosecutions (DPP) and that the investigation is ongoing.

Tega Agberhiere (16), an underage Irish soccer international, and two of his friends were allegedly splashed with a corrosive liquid during the altercation.

There was speculation the liquid might have been from a car battery but gardaí believe now that it could have been drain cleaning fluid. "It's some sort of acid," a garda familiar with the investigation said on Tuesday.

Tega Agberhiere (17) was brought to Cork University Hospital. Photograph: Damian Tiernan via Twitter

Republic of Ireland senior manager Mick McCarthy, Under 17 national team manager Colin O'Brien and Under 16 coach Paul Osam were among those to send messages of support to Tega and his two friends.

"I know Tega is a very promising young footballer and I hope and pray that he is back on the pitch in the very near future. My best wishes to Tega, to his friends and to their families," said McCarthy.

Taoiseach Leo Varadkar, speaking in Waterford on Tuesday, said he was "really horrified" by the incident.

"It's the kind of thing you hear about happening in other countries. I don't recall it happening any time in Ireland any time recently.

"You really have to wonder what kind of poison exists in the mind of somebody who would throw acid on another person, somebody who would try to disfigure somebody. It's terrifying, quite frankly.

"The people who were responsible will face the full rigours of the law and I'd really encourage anyone at all who has information to come forward because nobody wants to see that kind of thing happening in our country."

Gardaí are trying to establish a motive for what happened and are hoping to get more information when the victims of the assault are in a position to be fully interviewed. The incident is not believed to have been racially motivated.

Tega plays for Waterford FC's under-17 side and has represented Ireland at underage level. He was born in Waterford and is studying for the Leaving Certificate at Waterpark College.

His parents are originally from Nigeria and moved to Ireland in 2002. His mother, Christie, said her son suffered first-degree burns on his face and was continuing to receive treatment in hospital.

He was transferred to Cork University Hospital on Tuesday morning.

"I'm angry, I'm not happy about the whole thing because it shouldn't have happened to anybody," she told RTÉ's Morning Ireland.

Ms Agberhiere said her son was preparing for his Leaving Certificate and has had football trials with a number of clubs in the United Kingdom.

"It has affected the whole community. . . . It's really shocking, I'm in shock. I'm still in shock. Even an adult, emotionally, you would be damaged," she added. "As a mother, I have to lift his spirit and make him positive."

Padraig Sullivan, who plays underage hurling for Waterford, also sustained injuries to both legs and will need skin grafts . A third boy in the group sustained back injuries.

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Teenager suffers serious facial burns after acid thrown during assault

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Gardaí believe drain-cleaning fluid may have used in an attack on a number of teenagers in Co Waterford last week. The incident occurred in the Earlscourt housing estate on the Dunmore Road at around 11pm last Thursday. Gardaí said a number of youths were involved in a "serious altercation", and it is understood "a corrosive substance" was used.

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Patryk Lis Sciecne Collaborative Planning Document

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Paper on the history of acid rain which helped me with explaining acid rain to students.

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Abstract

Because of its serious large-scale effects on ecosystems and its transboundary nature, acid rain received for a few decades at the end of the last century wide scientific and public interest, leading to coordinated policy actions in Europe and North America. Through these actions, in particular those under the UNECE Convention on Long-range Transboundary Air Pollution, air emissions were substantially reduced, and ecosystem impacts decreased. Widespread scientific research, long-term monitoring, and integrated assessment modelling formed the basis for the policy agreements. In this paper, which is based on an international symposium organised to commemorate 50 years of successful integration of air pollution research and policy, we briefly describe the scientific findings that provided the foundation for the policy development. We also discuss important characteristics of the science-policy interactions, such as the critical loads concept and the large-scale ecosystem field studies. Finally, acid rain and air pollution are set in the context of future societal developments and needs, e.g. the UN's Sustainable Development Goals. We also highlight the need to maintain and develop supporting scientific infrastructures.

Introduction

Acid rain was one of the most important environmental issues during the last decades of the twentieth century. It became a game changer both scientifically and policy-wise. For some time, particularly during the 1980s, acid rain was by many considered to be one of the largest environmental threats of the time. Observations of fish extinction in Scandinavian surface waters and forest dieback on the European Continent were top stories in the news media. Even in North America acid rain received large public and policy attention.

During the cold war, with almost no contacts between East and West, acid rain broke the ice and formed an opening for scientific and political collaboration, resulting in a treaty under the United Nations' Economic Commission for Europe (UNECE), the Convention on Long-range Transboundary Air Pollution (often mentioned as CLRTAP but in this paper we call it the Air Convention) signed in 1979. Eight protocols have been signed under the Air Convention committing parties to take far-reaching actions, not only with respect to acid rain but also with respect to several other air pollution problems (Table 1). Emissions of all key air pollutants have been reduced significantly and for the most important acidifying compound, sulphur dioxide, emissions in Europe have decreased by 80% or more since the peaks around 1980-1990 (Fig. 1).

Table 1 The Convention on Long-Range Transboundary Air Pollution and Its Protocols

European emissions of sulphur dioxide (SO ₂-black), nitrogen oxides (NO x, calculated as NO ₂-green) and ammonia (NH ₃-blue) 1880-2020 (updated from Fig. 2 in Schöpp et al. 2003)

In this paper, we present and discuss how the acid rain problem became a key environmental issue among industrial countries from the late 1960s and the following decades (Fig. 2). We view the problem from a science-to-policy interaction perspective, based on a Symposium in Stockholm in the autumn 2017 organised to manifest 50 years of international air pollution science and policy development. The Symposium involved both a testimony from a number of those involved in science and policy during the first decades of the history but also a discussion of what we have learned and how the experience can be used in the future. Further information about the symposium and its outcome can be found at http://acidrain50years.ivl.se.

Our historical review will be limited to some of the issues brought up at the Symposium. For more information on the early history see Cowling ( 1982). A comprehensive description of the acid rain history has recently been published by Rothschild ( 2018). The history of the first 30 years of the science-policy interactions under the Air Convention is also described in Sliggers and Kakebeeke ( 2004).

Short historical review

The discovery and the early acid rain history

In a deliberatively provocative article in the Swedish newspaper Dagens Nyheter in October 1967, entitled "An Insidious Chemical Warfare Among the Nations of Europe", the Swedish scientist Svante Odén (Fig. 3) described a new and threatening environmental problem-Acid Rain. He pointed to the significant decrease in pH of rainwater and surface waters that had occurred over the previous decade and linked it to the large and increasing emissions of sulphur dioxide in Europe.

The discovery received immediate attention by the Swedish government and, a few weeks after Odén's article, the minister of industry presented the issue at the Organisation for Economic Cooperation and Development (OECD), but it did not receive any political attention at that time. The issue was also brought up in OECD's Air Pollution Management Committee by the Swedish delegate Göran Persson. Also, here the message was met by scepticism and the common opinion among the members in the committee was that sulphur dioxide was a local problem, which easily could be solved by tall stacks. It was not until Persson felt he was going to "loose the case" he "played his last card" and pointed to the observations of intercontinental transport of radioactivity from the Chinese nuclear bomb experiments. The opinion then changed and the meeting agreed that acid rain might be an issue to look into. From now on, OECD and the western world realised that air pollution might be a problem of international political dimensions.

Odén's discoveries were to a large extent based on the regional precipitation networks that were running in Sweden and Europe. In 1947, the Swedish scientist Hans Egnér set up a Swedish network to investigate the importance of atmospheric deposition for the fertilisation of crops. In 1954, the network was expanded forming the European Air Chemistry Network (EACN) through initiatives by Egnér, Carl Gustav Rossby, and Erik Eriksson (Egnér and Eriksson 1955; see also Engardt et al. 2017). Data from these networks together with a Scandinavian surface water network set up by Odén in 1961 formed the basis for Odén's observations on the ongoing acidification (Odén 1968).

Acid rain and many of its ecological effects were, however, recognised long before 1967-1968. In fact, many features of the acid rain phenomenon were first discovered by an English chemist, Robert Angus Smith, in the middle of the nineteenth century! In 1852, Smith published a detailed report on the chemistry of rain in and around the city of Manchester, England. Twenty years later, in a very detailed book titled "Air and Rain: The Beginnings of a Chemical Climatology", Smith first used the term "acid rain" and enunciated many of the principal ideas that are part of our present understanding of this phenomenon (Smith 1872). Unfortunately, however, Smith's pioneering book was substantially ignored by nearly every subsequent investigator.

In Norway salmon catches decreased substantially in the early 1900s and in 1927, Professor Knut Dahl hypothesised that acidification of surface waters could be a factor of importance for the extinction of fish. Later Alf Dannevig assumed that "The acidity of a lake is dependent on the acidity of the rainwater and the contributions from the soil" (Dannevig 1959).

Based on detailed field observations and experimental studies both in England and in Canada, beginning in 1955 and continuing through 1963, Eville Gorham and his colleagues built a significant foundation for contemporary understanding of the causes of acid precipitation and its impacts on aquatic ecosystems, agricultural crops, soils, and even human health (Gorham 1981; Cowling 1982). Thus, Gorham and his colleagues as well as Dahl and Dannevig had discovered major aspects of the causes of contemporary changes in the chemistry of atmospheric emissions and deposition and their effects on aquatic ecosystems.

But these pioneering contributions, like those of Smith a century earlier, were not generally recognised-neither by scientists nor by society in general. Gorham's researches, like those of Smith a century before, were met by what Gorham himself acknowledged as a "thundering silence", not only by the scientific community, but also by the public at large.

It was not until 1967 and 1968 when Svante Oden published both his deliberatively provocative article in Dagens Nyheter and his carefully documented Ecological Committee Report (Odén 1968) that the acid rain problem was brought to both public and scientific considerations. The report included a huge body of scientific and policy-relevant evidence that long-distance transport and deposition of acidifying pollutants were causing significant environmental and ecological impacts, even in countries far away from pollutant-emitting source areas in other countries.

The Swedish case study and the OECD project

Two years after Odén's article, the Swedish government decided to prepare a "case study" as a contribution to the UN Conference on the Human-Environment in Stockholm 1972 (Royal Ministry of Foreign Affairs and Royal Ministry of Agriculture 1972). Bert Bolin at the Stockholm University was appointed chair of the study, which included Svante Odén, Henning Rodhe, and Lennart Granat as authors. The report included a broad environmental assessment of the sulphur emission problem including sources, atmospheric and surface water chemistry, and effects on ecosystems and materials. Finally, it also included scenarios and estimated costs for environmental damage and control; in fact it was probably the first full systems analysis of an environmental problem.

In the report, a first estimate was made of the relative contributions of domestic and foreign emissions to the sulphur deposition in Sweden (Rodhe 1972). Estimates were also made of the effects of sulphur emissions on excess mortality and showed that 50% of the Swedish lakes and rivers would reach a critical pH level within 50 years (assuming continuation of present emission trends). Even if some aspects of the report received criticism, the overall case study was well received by the UN conference and in its final report (see http://www.un-documents.net/aconf48-14r1.pdf) regional air pollution was explicitly mentioned (§85) with a citation of the Swedish study.

The Swedish initiative in the OECD resulted in a collaborative project to investigate the nature and magnitude of the transboundary transport of emitted sulphur dioxide over Western Europe, in which 11 countries participated. To initiate the project, a Nordic organisation on scientific research, Nordforsk, was asked to plan and develop methodologies for the investigation. Scientists and institutions from Norway, Sweden, Denmark, and Finland established an expert group in April 1970, which became central for the development and implementation of the OECD project. The Norwegian Institute for Air Research (NILU) offered through its director Brynulf Ottar to coordinate the project. The project included emission inventories, measurements of atmospheric concentrations, and deposition, together with model development and application for the assessment of the transport. A key part of the model calculations was to prepare the so-called "blame matrices", through which the transport of pollutants between countries could be quantified.

The main conclusion from the OECD project, published in 1977, was that "Sulphur compounds do travel long distances in the atmosphere and the air quality in any European country is measurably affected by emissions from other European countries" (OECD 1977). Even if there still were hesitations about the magnitude of the transport, the common opinion was that transboundary transport of air pollution is an issue that needs collaboration across national borders. These conclusions paved the road for a pan-European scientific collaboration on air pollution, the European Monitoring and Evaluation Programme (EMEP) starting in 1977. The findings from the project also formed the basis for the Air Convention (Table 1). EMEP was already from the beginning included in the Convention as a key element, strongly contributing to the scientific credibility of the policy work.

Threats to forests boosted the interest

In 1980, the German scientist Bernhard Ulrich warned that European forests were seriously threatened from atmospheric deposition of sulphur. From his long-term experiments in the Solling area, he concluded that the high deposition of atmospheric pollutants had seriously changed the soil chemistry (Ulrich et al. 1980). Ulrich pointed to the links between sulphur deposition and the release of inorganic aluminium. His findings became a policy issue not only in Germany but in Europe as a whole, and even in North America. The alarms-often exaggerated-went like a wildfire through media and changed many attitudes throughout Europe. Newspapers were filled with photos of dying forests, in particular from "The Black Triangle", the border areas between Poland, East Germany, and Czechoslovakia, characterised by large combustion of brown coal with high sulphur content. Forest inventories showed crown thinning and other effects on forests, but it became difficult to finally determine that acid deposition was the (only) cause for the observed effects.

The increasing interest in regional air pollution also paved the way for the first international agreement on emission control under the Air Convention. As a start, countries with a large interest in taking actions formed a "club" under the Convention, aiming for a 30% reduction in emissions. This ambition then became the basis for the first emission reduction protocol, the Sulphur Protocol signed in 1985. While Germany and some other West European countries acted almost immediately on the alarms, the progress in emission control in Eastern Europe was very slow during the 1980s, even though several of these countries signed the protocol. In fact, substantial decrease in emissions did not take place in the East until after the break-down of the communist regimes and the industrial collapse around 1990.

Critical loads and advanced policies

One of the most well-known characteristics for the control of the acid rain problem is the concept of Critical Loads (Nilsson 1986; Nilsson and Grennfelt 1988). The Executive Body, the highest decision-making body of the Air Convention, decided in 1988 that new negotiations on the control of sulphur and nitrogen emissions should be based on critical loads, and all parties to the Convention were requested to prepare their own critical load maps. The Netherlands offered to take a lead and prepared mapping manuals and initiated an international network, which became crucial for the scientific and policy acceptance of the concept (Hettelingh et al. 1991; De Vries et al. 2015; Fig. 4). (The critical loads concept is further discussed later in the paper)

The outcome of emission control of SO2, NOx, and NH3 between 1990 and 2010 presented as maps on exceedance of critical loads of acidity. Such maps have played an important role for illustrating outcomes of future policies as well as of actions taken (from Maas and Grennfelt 2016)

When critical loads became a basis for further protocols, Integrated Assessment Models (IAMs) offered a method to calculate how to achieve a prescribed ecosystem effect reduction in the most cost-effective way. A couple of different approaches were developed, but the model at the International Institute for Applied Systems Analysis (IIASA) became the official model on which the Second Sulphur Protocol signed in 1994 was agreed (Hordijk 1995).

When revising or developing a new protocol for nitrogen oxides the concept could, however, not be used in the same way as for sulphur and acid deposition, since the NO x emissions contributed to several effects and, in addition, a strategy would need to take additional compounds into account. Instead, a more advanced approach was suggested by which both several effects and several compounds could be considered simultaneously (Grennfelt et al. 1994, Fig. 5). IIASA and other bodies under the Air Convention were asked to develop an integrated assessment model that fitted into a broader approach and a more comprehensive model was developed, which made it possible to simultaneously take into account the effects of acidic deposition, nitrogen deposition, and ozone-the so-called multi-pollutant, multi-effect approach. The calculations became the basis for the Gothenburg Protocol (GP) that was signed in 1999 (Amann et al. 1999). The GP and the parallel EU National Emissions Ceilings (NEC) Directive from 2001 outlined control measures for 2010 and beyond.

Links between sources and effects used as an illustration in the preparation of the Gothenburg Protocol. From Grennfelt et al. 1994

After 2000-Health effects and integration with other policies became main drivers

The basis for the GP was almost entirely ecosystem effects. Around 2000, however, public health effects from air pollution became increasingly important. Large epidemiological studies indicated that air pollution was a significant source of premature deaths and that particles were a main cause of the health effects (WHO 2018). When the European Commission started its work to revise the NEC directive, health effects became central and the Air Convention followed. Further studies have supported the role of air pollution for health effects and when the GP was finally revised in 2012, health effects dominated as a policy driver for the establishment of national emission ceilings, and for the first time particulate matter was included in an international protocol (Reis et al. 2012).

When considering further actions after signing the GP in 1999, it was realised that for some pollutants under the Air Convention, emission control needed to be considered over larger geographic scales than Europe and North America alone. Ozone was of particular importance, since long-term objectives in the form of critical levels and public health standards could not be reached without taking into account sources outside the areas considered so far. Future policies therefore needed to include the ozone precursors methane and to some extent carbon monoxide. A task force on Hemispheric Transport of Air Pollution (HTAP) was set up under the Convention in 2004, with a primary objective to quantify the intercontinental transport of pollutants. The outcome of its work clearly showed the importance of considering air pollution in a wider geographic perspective than had been done so far (Dentener et al. 2010).

Climate change has for more than a decade become an issue of increasing interest for air pollution science and policy. In many cases, the emission sources are the same and there are obvious co-benefits (and some trade-offs) in handling them together. One aspect that has received large interest is the option to decrease short-term temperature increase through control measures directed towards atmospheric pollutants that also contribute to the warming of the atmosphere, in particular black carbon and methane (for methane both by itself but also as a tropospheric ozone precursor) (Ramanathan et al. 2001). Compounds contributing to both air pollution effects and to the radiation balance in the atmosphere have been named Short Lived Climate Pollutants (SLCPs). SLCPs thus also include compounds that are cooling the atmosphere, i.e. small secondary aerosols, e.g. sulphate particles. Recent research has focused on a better understanding of these compounds' contribution to both air pollution and climate as well as on opportunities for selective control of these compounds (e.g. Sand et al. 2016).

Reactive nitrogen species are another group of compounds that has received increased attention after the turn of the century. Around 2006 several initiatives were taken in Europe, including a special task force on Reactive Nitrogen under the Air Convention, a large-scale EU project on nitrogen, and the preparation of a European Nitrogen Assessment (Sutton et al. 2011). Here nitrogen was considered both as a traditional atmospheric pollutant and within a societal and industrial context. A cascade perspective, where one fixed nitrogen molecule could contribute to a series of effects before it returns to molecular nitrogen again, was introduced (Galloway et al. 2003). The studies have pointed to the importance of the agricultural sector for the intensification of reactive nitrogen cycling, determined by food production mechanisms and dietary choices.

North America

In North America, the acid rain problem developed to a large extent in parallel with the situation in Europe. Lake acidification became already from the beginning a main driver, and monitoring programmes were set up both in the United States and Canada (Driscoll et al. 2010). The US National Atmospheric Deposition programme (NADP) started in 1976 and is still running. Both countries have taken part in the Air Convention activities and have signed most of the protocols and achieved decreases in SO ₂ emissions of the order of 80% between 1980 and 2015. The US has however taken a different approach with respect to policy in comparison to Europe. Instead of developing a strategy based on integrated assessment modelling, it was decided to establish an emissions trading programme for the large electric generation sources under the Clean Air Act (See also UNECE 2016).

Characteristics of the science-policy interactions

In this section we will, from a science-policy perspective, briefly discuss some characteristics of the history of acid rain and transboundary air pollution that have become central for the international collaboration, not only on air pollution but also for international environmental collaboration in general. We will bring up monitoring, modelling, and data collection (including field experiments and long-term studies carried out in order to understand and quantify effects to ecosystems), development of bridging concepts that have served the implementation of strategies, and finally the dynamics in the science-policy interactions.

Monitoring, modelling, and data collection

Monitoring of atmospheric concentrations, deposition, and ecosystem effects has been a key for understanding the causes, impact, and trends in acid rain, both in Europe and North America and later in other geographic areas (Table 2). The original EMEP network has since the start over 40 years ago formed a broad atmospheric monitoring system. The originally established simple monitoring stations have over time been complemented with more advanced monitoring, and some stations are today advanced atmospheric chemistry platforms with continuous collection of a multitude of atmospheric parameters (Fig. 6). The EMEP database is nowadays widely used for a variety of scientific purposes including computation of long-term trends, exposure estimates, and as a basis for modelling. EMEP has also become a model for monitoring networks related to other geographical regions, conventions, and purposes. One example is the acid deposition monitoring network in East Asia (EANET). It is obvious that having a qualified centre for data collection and storage, standardisation, and intercalibration of methods has served the international policy system extremely well. Its open nature is part of the success. The financial support to EMEP, regulated through a separate protocol, has been fundamental for the development and progress of the monitoring activities.

Table 2 Long-term monitoring activities in relation to acid rain and other pollutants

Monitoring of air pollution effects in a systematic way under the Air Convention started a few years later than EMEP and was organised through so-called International Cooperative Programmes (ICPs). Separate programmes were set up for forests, waters, vegetation (primarily ozone), materials, and integrated monitoring. A separate ICP was set up for developing critical load methodologies and coordinating European-scale mapping activities (ICP Modelling and Mapping). The ICPs are of great importance for general understanding of the magnitude and geographical distribution of the effects and for showing how decreases in emissions have led to beneficial conditions in ecosystems and decreased material corrosion (Maas and Grennfelt 2016). Ecosystem monitoring is also important for the development and verification of ecosystem models. Since their start, the responsibility for the ICPs has been taken by different parties of the Air Convention (Table 2). The distributed responsibility has been of large importance for the establishment of networks of monitoring sites among the Convention parties, but the system has not had a stable financial support in the same way as for EMEP. This has resulted in the lack of a common source for easily accessible data or adequate resources for standardisation and intercalibration.

Monitoring and other data collection (i.e. emissions and critical loads) under the Air Convention are responsibilities of every country, and data are then used for the assessments on the Convention level as well as for the development of EU air pollution policies. The bottom-up process in data collection is important for the development of national expertise and, not the least, for the establishment of national policies. In this way, direct communication links between the science and the policy levels within countries have evolved.

Numerical modelling of atmospheric pollution is also a long-term commitment under EMEP. The atmospheric chemistry models are necessary for the understanding of the nature of transboundary transport but also to make budget estimates of the exchange of pollutants over Europe and North America, and later on a hemispheric scale. The Meteorological Synthesizing Centre West at the Norwegian Meteorological Institute together with the Eastern Centre in Moscow took the lead in this work. In addition to calculating transboundary fluxes, the centres are important for coordinating modelling efforts done by other groups, forming a basis for scrutinising models and support further modelling.

Field experiments and long-term studies-a way to understand processes and trends, and to visualise the problems

Some of the most important and reliable findings regarding acid rain and its effects on ecosystems emanate from long-term field experiments. These experiments, which are known from the sites where they are run, include Hubbard Brook (US), Solling (Germany), Risdalsheia (Norway) and Lake Gårdsjön (Sweden) (Fig. 7). The studies there have shown how acid deposition and the impact of other air pollutants have changed the ecosystems, but also how ecosystems respond to decreased emissions (e.g. Wright et al. 1988; Likens et al. 1996). A central feature in all these field experiments was the establishment of ion budgets, from which the chemical effects on acid deposition can be analysed and understood (Reuss et al. 1987).

In the intense research period during the 1970s and 1980s, a number of large-scale research programmes and experiments of temporary nature were set up, some of them in connection with the above-mentioned sites. The first research programme of some magnitude was the Norwegian programme "Acid precipitation-effects on forest and fish" (SNSF), which run between 1972 and 1980 (Overrein et al. 1981). At that time the scientific understanding was limited, and the programme received a lot of attention. The results were important for the general acceptance that long-distance transport of sulphur caused acidification of surface waters, with a serious die-off of fresh water fish populations (salmon and trout) as a main consequence. On the other hand, the studies on Norwegian forests did not give any significant evidence for acid rain effects. The SNSF project was a joint effort across disciplinary and organisational boundaries, with scientists mainly from the research institute sectors outside of traditional academia. This project served as a model for later research programmes and provided educational opportunities for a new generation of scientists working together on all aspects of the acid rain issue-emissions and their control, atmospheric transport and deposition, impact on ecosystems, health and materials, and finally development of pollutant-control policies.

The long-term field experiments served another important task. The sites became exhibition platforms, at which policymakers, experts, scientific journalists, and leaders of non-governmental organisations (NGOs) and others can be informed about the problem directly on site. During the most intense period in the 1980s and early 1990s, politicians and industry leaders, often directly involved in decisions on the highest levels, visited many of these experimental sites. For example, US congress members travelled across Europe to see and understand the issue in preparation for the 1990 amendment of the Clean Air Act.

Bridging concepts and approaches

Concepts developed, such as critical loads and similar approaches, formed links between science and policy, and were essential for the understanding and scientific legitimacy of the policy measures. These concepts also formed a basis for priority setting in agreements under the Convention and the EU, but also to some extent for national policies. Even "acid rain" can be considered as a bridging concept. While the acidity from sulphur and nitrogen compounds is threatening ecosystems through a chemical change, the expression also gives the impression of a threat to the life-giving rain, a fundamental necessity for life on Earth.

The quantification of transboundary fluxes was very important politically. The establishment of national budgets and so-called blame matrices formed the first bridging concept. The development of mathematical models to calculate source-receptor relations was a scientific challenge but when the annual tables were prepared showing the interdependence between countries with respect to atmospheric emissions and deposition, they served as an important basis for the need for common action. Anton Eliassen, the leader of the modelling centre at the EMEP Meteorological Synthesising Centre West (MSC-W) during many years (the Eastern center is in Moscow-MSC-E), was key to this development as well as for the communication of the results to policymakers.

As earlier mentioned, critical loads played an outstanding role for the development of the more advanced strategies leading to the Second Sulphur Protocol and the GP. Critical loads formed a successful link between science and policy that became crucial for the negotiations and agreements. The concept, first discussed in 1982, was taken from the original idea to application quite quickly during the 1980s. The Swedish expert Jan Nilsson was a key leader for the success of the concept, and the Nordic Council of Ministers played a unique role for forming the links between science and policy. Through a series of workshops involving both key scientists and key policymakers, the concept gained the legitimacy on which policies were developed. According to Jan Nilsson, it all started with requests from both industry and negotiators to have a sounder base for emission control, something that could express the long-term objectives for emission control policies. The concept was first met by scepticism, not least from scientists, but after a couple of workshops, the interest turned around and the concept became widely accepted (Nilsson 1986; Nilsson and Grennfelt 1988). When critical loads were included in the plans for the next rounds of the sulphur and nitrogen protocols in 1988, it changed the way the Air Convention operated.

The application of the critical loads concept has encouraged intense research over several decades where the main objective has been to find simple chemical parameters that can mimic the (often biological) real effects or effect risks. For lake acidification, where the effects of dissolved aluminium on fish often were chosen as the main biological effect, the acidity of the water, mostly expressed as acid neutralising capacity (ANC), is used (e.g. Henriksen et al. 1989; Forsius et al. 2003; Posch et al. 2012). For forests, where the toxicity of aluminium to tree roots is considered as critical, the Al ³⁺ to Ca ²⁺ ratio in soil water has become the main effect parameter (Sverdrup et al. 1990; de Vries et al. 1994).

Integrated assessment modelling (IAM) also has been a bridging concept. The idea of applying systems analysis goes back to the work at IIASA in the beginning of 1980s. A conceptual model was formulated by Joseph Alcamo, Pekka Kauppi, and Maximilian Posch for the interactions between emissions, their control (including costs), and the effects on ecosystems (Alcamo et al. 1984). Their work of bringing together the scientific knowledge to a comprehensive systems analysis tool formed a new way of framing environmental policies. Under the leadership of Leen Hordijk, the new idea was introduced to and accepted by the policy side, which had asked for more targeted methods for policies than simple percentage decreases in amounts of emissions. IAMs as a policy-supporting concept was then taken further by Markus Amann, who led the development of the more advanced RAINS (later GAINS) models that were used as a basis for the GP and later agreements (Amann et al. 2011). From the strategies strictly directed at ecosystem effects, the approach is now widened to include health effects, local air pollution impact, climate policies, and reactive nitrogen.

All the bridging concepts are to varying degrees dependent on underlying models, assumptions, and simplifications. For these to be accepted among policymakers, it is important to keep transparency and confidence in the underlying data and to scientifically evaluate and scrutinise them. This is particularly important for the IAMs, which are the final step in a chain of inputs (Fig. 8). The models have often been criticised, not least from industry and other stakeholders that are questioning the priorities that result from the IAM calculations. IIASA, as a provider of the model calculations, has, however, been transparent, and countries and stakeholders have always had the option to re-check data and take this into account when developing their own negotiation positions.

Forming science-policy credibility

In all interactions between science and policy, it becomes crucially important to maintain scientific credibility. The close involvement of scientists has been a signature of the Air Convention. Scientists have always had a role at the policy meetings, communicating results from basic scientific research over outcomes of monitoring and inventories to presenting options for control strategies. Scientists have in this way taken the responsibility to move scientific knowledge into the policy system and presenting results in a way that has been understandable and useful for the policy work. The role of the scientists has been as honest brokers, not that of issue advocates to follow the terminology of Pielke ( 2007). The leadership from the policy side and its sensitivity to changes in the underlying science and observations of new problems have also been important, and have resulted in repeated changes in the framings of the Air Convention to adapt to new situations: going from an initial framing around sulphur and acidification, through extension to eutrophication, human health, materials, crops, biological diversity, and finally to links to climate, urban air quality, and societal changes. A balanced interplay between the two communities has in this way been developed and maintained over time.

Another factor is the building of networks. The strong networks of scientists and policymakers pushed the politicians. The whole field of international diplomacy during these four decades of the Convention is built on incremental developments forming protocols of increasing capability to solve specific environmental issues by cutting emissions in a cost-effective way.

Future challenges

New approaches necessary

International air pollution control is by many considered as a success story. However, the success is in many ways limited to Europe and North America and a few additional industrialised countries (including Japan and Australia), where emissions of sulphur dioxide, nitrogen oxides, VOCs, and some other compounds have been decreased significantly (Maas and Grennfelt 2016). But even in the areas, where air pollution has been a top priority for several decades, air pollution remains a problem. Ecosystem effects, which were the main reason for the establishment of the Convention, are to some extent reduced, but the acidification effects of historical emissions will remain for decades (Wright et al. 2005; Johnson et al. 2018) and the emissions of ammonia have so far only been reduced by 20-30% in Europe and even less in North America. Looking at health effects, it is difficult to talk about success, when hundreds of thousands of inhabitants on both continents are predicted to meet an earlier death due to air pollution.

But the problem is even larger and more urgent when looking outside the traditional industrialised world. The focus is today on the large urban regions in the countries that are facing rapid population growth and industrialisation. Although large efforts now are being made to decrease sulphur emissions in China-the world's leading sulphur emitter-major challenges remain. In India and several other countries, sulphur emissions are still increasing. Estimates indicate that more than four million people die prematurely due to outdoor air pollution globally ( https://www.who.int/airpollution/ambient/health-impacts/en/). It is assumed that fine particles (PM2.5) are a main cause for the health effects. The new and great challenge is therefore to control air pollution in relation to health risks, in particular by decreasing exposure to the small particles.

There is, however, a risk that control measures will only to a limited extent focus on the right sources and the right measures. In Paris, several air pollution episodes with high concentrations of particles have occurred during recent years. At first, these episodes were considered to be caused essentially by local emissions. More thorough analysis has, however, shown that they were to a large extent caused by regional emissions and buildup of high concentrations over several days when urban emissions of oxides of nitrogen from traffic mix with ammonium emissions from surrounding agricultural areas to form particulate nitrate. Similar situations are also often encountered in urban regions in developing countries, e.g. by agricultural waste burning, and need to be considered. Air pollution problems are, as previously mentioned, also linked to intercontinental and hemispheric scales.

It is also obvious that the research communities within air pollution and climate change need to work more closely together. Health aspects are of importance both from air pollution and climate change perspectives, and heat waves carry poor air quality as winds are often very low and the atmospheric boundary layer stagnant. During heat waves, the soil and vegetation dry up and increase the likelihood of fires, which also can cause severe air pollution, as seen in wildfires around the world (e.g. California in 2018).

Despite the large progress in atmospheric and air pollution science, basic questions still need further investigations to develop the best policies. Such areas include a better understanding of health effects from air pollution, nitrogen effects to ecosystems, and air pollution interactions with climate through carbon storage in ecosystems and impacts on radiation balances. Modelling is a scientific area where much progress has been made and where increased computer power, as in climate change research, has allowed integration of atmospheric chemistry into the climate models formulated as Earth system models, coupling the atmosphere, ocean, the land surface, cryosphere, biogeochemical cycles, and human activities together. This has allowed studying air pollution and climate change simultaneously. The modelling approach can be further developed when observations are designed to map Earth system component boundaries to understand and quantify the flows and interactions between different compartments, including terrestrial and aquatic ecosystems. Air pollution should be an integrated part of such models. In this context, global-scale concepts such as "planetary boundaries" and "trajectories of the Earth system vs. planetary thresholds" have been developed (Rockström et al. 2009; Steffen et al. 2018).

Solutions are available; driving forces and investments are lacking

In 2016, the Air Convention launched a scientific report "Towards Cleaner Air", in which the actual air pollution situation within the UNECE region was updated (Maas and Grennfelt 2016). The report also presented future challenges and ways forward to solve the air pollution problems. It also showed that solutions are available for most of the identified problems at affordable costs below the health and ecosystem benefits of the control actions.

Even if solutions are available, many parts of the world are facing large problems in implementing them. There are several reasons, but often there is a lack of knowledge and resources. This is particularly true in many developing countries. Another reason is the lack of political interest. Air pollution is still not of top priority among politicians, even if there is overwhelming evidence that air pollution is one of the most common causes of shortened life expectancies. Another reason may be that other interests (e.g., industry and agriculture) are forming strong lobbying forces delaying actions.

Air pollution is a problem that cannot be seen in isolation. Future policies need to take into account climate change and climate change policies. Whereas some air pollutants-in particular black carbon particles-contribute to warming, others, including sulphate particles, tend to cool the climate. A reduction in sulphur dioxide emissions, although highly desirable from health and ecosystems perspectives, will therefore contribute to warming. On the other hand, a reduction of black carbon will be a win-win solution. It is also important to see air pollution control in the perspective of sector policies, such as energy, agriculture, transportation, and urban planning in order to meet the challenges to decrease air pollution problems.

Internationally coordinated actions and infrastructures are keys for success

The perspective of international cooperation on air pollution is changing. Policy development is no longer limited to long-range transport in line with that developed under the Air Convention. The ranking of air pollution as a top ten cause of premature deaths in the world has given high priority to the issue within fora such as the WHO and UN Environment. Both organisations have adopted resolutions calling for actions (WHO 2015; UN Environment 2017). Additional initiatives are taken by other organisations, such as the World Meteorological Organisation (WMO), the Climate and Clean Air Coalition (CCAC), and the Arctic Monitoring and Assessment Programme (AMAP). WMO is particularly important as a global technical agency for weather and climate observations, research and services, and it is rapidly developing its regional and global capacities in Earth system observations, modelling, and predictions to the benefit of mitigating a range of environmental threats and for global use. The research is done in large programmes like Global Atmosphere Watch (GAW) and the World Weather Research Programme (WWRP). Even if the starting point and modes of action can be different, all initiatives are aiming for the same goal, cleaner air. It is also worth mentioning the initiative taken by the International Law Commission, under which a proposal for a Law for the Protection of the Atmosphere has been prepared ( http://legal.un.org/ilc/summaries/8_8.shtml) but in the current international atmosphere there is a lack of political support to implement it. Our hope is that the situation will change soon-the initiative is too important to fail.

The UN has put forward a very strong agenda in order to reach the Sustainable Development Goals (SDGs), and air pollution is an integral part of several of the SGDs, like goal No 2: No Hunger, No 3: Good health and well-being, No 6: Clean Water, No 7: Affordable and clean energy, No 9: Industry, innovation and infrastructure, No 11: Sustainable cities and communities, No 13: Climate action, No 14: Life below water, No 15: Life on Land, No 16: Peace and Justice, and No 17: Partnerships for the Goals. The approach taken to develop multiple pollutant-multiple impacts protocols under the Air Convention can serve as important learning ground to meet the ambitions of many of the SDGs. Air pollution plays an integral role in the evolution of the food production and ecosystem services, the health of the population, the shape of the energy and transportation systems, and the availability of clean water. Climate change is a very significant common and cross-cutting factor.

The Air Convention has taken some steps in promoting air pollution on a wider scale. Due to its long history and well-developed structure, it has taken a role of making sure that international organisations having air pollution on its agenda are aware of each other and to invite to further collaboration and development. Initiatives are taken both within the formal Convention structure and through dedicated workshops (UNECE 2018; Engleryd and Grennfelt 2018). The approach developed under the Air Convention, which has proven successful in linking scientific evidence, monitoring, and integrated assessment modelling directed towards cost-effective solutions, may also serve as a working model for environmental problems in other fields.

These new international initiatives have a strong emphasis on policy development. The experience from the 50 years of international air pollution development is the value of well-defined scientific objectives and activities supporting policy. The increased interest from WHO and UN Environment is welcome and there are expectations of an active role from these organisations in combatting the situation in many parts of the world. However, for these organisations, air pollution is just one of several priority areas, and priorities may change. Further, none of these organisations are likely able to set up advanced infrastructures with respect to emission inventories, monitoring, and research. Here WMO needs to live up to its mission and capitalise on global research and development efforts and improve the global operational capability to observe, analyse, and forecast the development of the Earth system and its components, air pollution being an important part. This is in line with the WMO strategic plan and with fast growing capabilities in some countries and in global centres like The European Centre for Medium Range Weather Forecast (ECMWF). WMO, through GAW, is also developing a research-driven operational system (IG3IS) for top-down determination of greenhouse gas emissions, to complement the usual bottom-up-based inventories where emission factors and fuel consumption or production statistics form the basis for the emission estimates ( https://library.wmo.int/doc_num.php?explnum_id=4981). The Air Convention and the science support for the policy work there has been a model for the WMO ambitions on a global basis. However, current investments in these new capabilities are not enough to get the societal return they would offer.

Therefore, we see a need for developing long-lasting infrastructures that can continuously develop science-based control policy options, potentially as part of a wider network of global observatories for comprehensive monitoring of interactions between the planet's surface and atmosphere (Kulmala 2018). Such a network should be able to support policies from local to the global levels. The challenge is how to organise and raise resources for scientific support on a wider scale. Financial institutions such as the World Bank and/or regional banks may step in and make sure that control measures and investments are made on a sound basis with respect to global air pollution.

There is also a need to mobilise new generations of scientists, scientists that are willing to cross boundaries and focus on thematic problems and to build legitimacy among policymakers (e.g. Bouma 2016). Today we have more developed and stronger political institutions to handle environmental problems, which may make it harder for scientists and individuals to influence and make a difference. It is also important to mobilise new generations of dedicated policymakers. Unfortunately, we also see that politicians often are questioning science and seeing science as just a special interest. Public awareness may be a key for forming stronger interests and put pressure on decision-makers. During the acid rain history, NGOs played an important role in driving the awareness at a wider scale than local or national actions and could be important for a more global movement towards cleaner air. We also see the need for a deeper responsibility not only from politicians but also from industry. The so-called "diesel gate" exposed the cynic view from parts of the industry to peoples' health, which hopefully will not occur in the future. Instead we hope that it was an eye-opener and that industry instead can play a role as a forerunner and a positive power for a cleaner atmosphere.

Final remarks

The Acid Rain history taught us that when science, policy, industry, and the public worked together, the basis was formed for the successful control of, what was considered, one of the largest environmental problems towards the end of the last century. We learnt from experience that science-based policy advice worked well when the best available knowledge was provided, and used to understand the specific problems, generate, and evaluate the policy options and monitor the outcomes of policy implementation.

However, the world does not look the same today, and we cannot just apply the ways the international science community worked together then on today's problems. But there are lessons to be learnt. Most important is the building of mutual trust between science advisers and policymakers, and that both communities are honest about their values and goals. In this way, a fruitful discussion around critical topics within society can be formed. The advice works best when it is guided by the ideal of co- creation of knowledge and policy options between scientists and policymakers (SAPEA 2019).

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Acknowledgement

Open access funding provided by The Swedish Environmental Protection Agency. The Symposium could not have been arranged and this paper written without financial support from the Nordic Council of Ministers, the Swedish Environmental Protection Agency, the Mistra Foundation, and other organisations and institutes in the Nordic countries. We are also grateful to all participants of the Symposium and their contributions with background material for this paper. We also thank the two anonymous reviewers of the manuscript. Their comments greatly improved the quality of this paper.

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Affiliations

1. IVL Swedish Environmental Research Institute, PO Box 53021, 40014, Gothenburg, Sweden

Peringe Grennfelt

2. Swedish Environmental Protection Agency, Virkesvägen 2F, 10648, Stockholm, Sweden

Anna Engleryd

3. Finnish Environment Institute, Latokartanonkaari 11, 00790, Helsinki, Finland

Martin Forsius

4. The Norwegian Meteorological Institute, P.O. Box 43, Blindern, 0313, Oslo, Norway

Øystein Hov

5. Department of Meteorology, Stockholm University, 10691, Stockholm, Sweden

Henning Rodhe

6. Department of Forestry and Environmental Resources, NC State University, 5211 Glenhope Court, Cary, NC, 27511, USA

Ellis Cowling

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Correspondence to Peringe Grennfelt.

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# data[].attributes.attachment_link.title
Acid rain and air pollution: 50 years of progress in environmental science and policy

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Because of its serious large-scale effects on ecosystems and its transboundary nature, acid rain received for a few decades at the end of the last century wide scientific and public interest, leading to coordinated policy actions in Europe and North America.

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Paper on how teaching acid-base should be changed based on NoS

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Abstract

We propose explicit and implicit approaches for the teaching of acid-base chemistry based on research into the history and nature of science (NoS). To support these instructional proposals, we identify four rationales for students to understand acid-base processes: daily life, socio-scientific, curriculum, and history of science. The extensive bibliography on misconceptions at all educational levels justifies the need for a change from the usual pedagogical approaches to teaching the acid-base domain (traditionally involving conceptual-focused teaching) to a deeper and more meaningful approach that provides (implicitly or explicitly) a chance to reflect on how scientific knowledge is constructed. Controversial moments in science from 1923, when three researchers (Bronsted, Lowry, and Lewis) independently enunciated two theories from two different paradigms (dissociation and valence electron), underpin our first sequence with an explicit NoS approach for both lower secondary school and upper secondary or university levels. Our inquiry teaching cycle promotes the transformation of a hands-on activity (using cabbage as an indicator) into an inquiry, and subsequently, we use an historical model to propose a sequence of activities based on the modeling cycle of Couso and Garrido-Espeja for lower secondary school. Finally, we identify some implications for a model-focused teaching approach for upper secondary and university levels using more sophisticated models.

Introduction

Researchers in the area of the nature of science (NoS) often provide recommendations for teachers. It is usually suggested, directly or indirectly, that teachers should improve their knowledge about what science is and how it is constructed, so that they can transfer this knowledge to the classroom, transforming it into sequences of activities for their students. For example, Nouri et al. ( 2019) recommend a well-designed history of science (HoS) intervention to convey essential lessons about the NoS consensus described by McComas ( 2006) such as science depends on empirical evidence; cultural, political, and social factors influence science; or science has a tentative or fallible nature.

Many authors identify multiple potential benefits for learning NoS through such approaches: teaching scientific methods, challenging myths related to how science works, and differentiating between idealized scientific laws and observations (Niaz 2009). However, they also highlight that research involving rationales and strategies for teaching HOS is scarce and Nouri et al. ( 2019) recommend expanding science teacher educators' rationales for teaching HOS to inspire a broader array of orientations and teaching strategies. They also suggest paying special attention to instructors' orientation towards teaching HOS which may have an impact on their effectiveness.

Such recommendations usually arise from studies focusing on the benefits of NoS for students and research on what teachers think, their beliefs about NoS, or in a less declarative way and close to their educational reality, the connection (or otherwise) between this knowledge and what the teacher really does in their classes (Leden et al. 2015). One of those recommendations involves the design of NoS classroom activities: explicitly and implicitly (Duschl and Grandy 2013) and using reflective approaches to NoS teaching and learning. These approaches open up the range of possibilities for teachers. For example, proposing explicit general activities, linked (or not) to a specific issue of science content, to develop students' understanding of an aspect of the NoS consensus view (Lederman 2007; McComas 2006). Such an activity might involve the use of a mystery box to help students to learn about observation, interpretation, and argumentation (Cavallo 2007; Rau 2009). Another example involves scientific practices, such as the National Research Council's ( 2000) inquiry into the problem of the tsunami on the US West Coast or "Mrs Graham's" class which tackled the problem of leafless trees with explicit reflection such as metamodeling learning progression (Schwarz et al. 2009) on how science is built. In this paper, we use Garrido-Espeja and Couso's ( 2017) definition of a model which is a "small number of big or core ideas (Harlen 2010) that have the potential to explain a lot of different phenomena (Izquierdo-Aymerich and Aduriz-Bravo 2003), such as the particle model of matter" or, indeed, the chemical change model.

Implicit and explicit NoS teaching approaches (Duschl and Grandy 2013) have a place in the high school science curriculum (12-18 years old) because an initial study of what science is is often included. At the same time, in chemistry courses, some topics such as atomic structure, the periodic table, or acid-base are often introduced through their historical developments. This history of chemistry topics present in curricula allows the design of authentic scientific practices (implicit approach Burgin and Sadler 2016).

The content overload in the Spanish science curriculum forces some teachers to dispense with developing the initial lesson about what science is and how it is built or with spending more time in deepening these NoS aspects when working on some historical development present in the curriculum, i.e. atomic structure, periodic table, etc. Thus, before deciding the approach on explicit or implicit teaching, the first teacher decision is to develop or omit this initial NoS lesson and the second is to decide whether to deepen or not the historical developments present in the curricula.

We turn now to address the issue faced by teachers: how to translate these NoS teaching approaches to sequences of activities on a specific topic? In this theoretical article examining teaching practice, we want to focus on the historical development of acid-base theories (Arrhenius, Bronsted-Lowry and Lewis) to analyse the steps to follow to design sequences of activities for different NoS approaches.

Objectives and Purposes

The main objective of this paper is to translate the explicit and implicit NoS approaches using the historical development of acid-base domain into activity sequences that can be used as a reference by teachers. In the next section we will outline the importance of teaching the topic of acids and bases because we understand that the first decision for a teacher is whether to spend time on the historical development of the acid-base domain present in chemistry curricula at secondary level, high schools and university level (in analytical and inorganic chemistry subjects).

Finally, we discuss how to design sequences of activities. We examine conventional teaching approaches to the topic and its consequences in terms of students' alternative conceptions and their difficulties to transfer and apply knowledge and to recognize acid-base models' limits of applicability. In this section, we will use research results (our own and others) from assessments of high school students, university students, both undergraduates and postgraduates, and pre-service teachers to show the common acid-base teaching (concept-focused teaching) approach and its consequences.

These discussions and an acid-base historical development (timeline in Section 4) will help us to analyse the current situation in order to scaffold the design of sequences of activities utilizing different NoS approaches:

1. 1.
We will propose NoS sequences with an explicit approach through controversial moments of the acid-base historical development for both lower secondary school and upper or university levels;

2. 2.
We will transform a hands-on activity (using cabbage as an indicator) into an inquiry for lower secondary level (Section 7.1) and from it;

3. 3.
We will propose a modeling sequence based on the modeling cycle of Couso ( 2020) and Couso and Garrido-Espeja ( 2017) using an historical model (Erduran 2001) for the same level (lower secondary school);

4. 4.
We will identify some implications about a model-focused teaching for upper secondary and university levels using more sophisticated models.

Finally, we will discuss the change in teachers' awareness of a model-focused teaching approach that extends and gives meaning to the usual concept-focused teaching. In short, in this paper, we are constructing a science teaching learning progression in a theoretical manner (Schneider and Plasman 2011) to build models using the history of acids and bases as a theme which could be used as a reference by teachers in their professional practice.

Why Teach the Topic of Acid-Base?

Acid-base chemistry is a traditional topic in high schools and universities. Key concepts are presented in analytical and inorganic chemistry subjects in virtually all Spanish and British universities. Acid-base processes also appear in other subjects such as ionic equilibria and chemistry lab work. In these subjects, they are often referred to as "acid-base reactions", "acid-base titrations", "ionic solutions", and "acid-base theories".

We have identified four categories of rationales why students need to understand acid-base processes: daily life; socio-scientific; curriculum; and HoS argument. We now discuss each in turn, briefly.

Acids and bases are commonly recognized by students and the general public. People know about acidic sweets, stomach acidity, antacids, etc. (Cros et al. 1986, 1988). Words such as "acid" and "neutral" are used in some TV advertisements ("Fairy is neutral and protects your hands"; "Johnson's pH 5.5 has natural pH"). Nevertheless, understanding of acid-base concepts is still limited. Furthermore, the use of scientific concepts is increasing and highlighted in advertisements to show products both as beneficial or a matter of trust despite misunderstanding of scientific concepts (Jiménez-Liso et al. 2000). Thus, some acid-base contents are necessary to understand the phenomena encountered in daily life.

Pseudoscientists take advantage of the lack of awareness of the population about scientific expressions using it to promote "scientific credibility" to their unfounded proposals of health and home remedies. Poor science is common in advertisements about cosmetics and cleaning "with pH" products, all sorts of diets and foods that reduce acidity in your body to prevent or treat cancer, etc. Those adverts can serve us as a context to raise socio-scientific controversies (Evagorou and Osborne 2013; Sadler and Zeidler 2009) about medicalization (Domènech Calvet et al. 2015) or alternative treatments (Uskola Ibarluzea 2016).

Although acids and bases are encountered in students' daily life (and social networks), they are rarely taught well at primary school level. For example, in the Spanish primary school curriculum, chemical reactions are only exemplified through oxidations, combustions, and fermentations, with no mention of acid-base reactions. However, if combustion and oxidation are the most used examples of chemical changes, they lead to the establishment of some alternative conceptions such as "all chemical changes are irreversible" (Stavridou and Solomonidou 1998) or "mass is not conserved in chemical reactions" (Stavy 1990). Not surprisingly, some alternative ideas held by students closely match ideas held by people studying science many centuries ago (Wandersee 1986).

The curriculum rationale could be reinforced by the history of science rationale: the knowledge of historical models (Justi and Gilbert 1999a) and the context in which they were formulated could improve understanding of acid-base models, their limitations, and, as a consequence, the conditions required to select each model (Erduran 2001).

Taken together, these arguments justify why teachers need to develop acid-base content in their chemistry curriculum. In the next section, we justify why current acid-base teaching might be changed in order to improve understanding of scientific content.

Acid-Base Historical Development

Historical aspects of acid-base domain could constitute an educational resource of great relevance to prevent students seeing science as a finished product and to appreciate how some theories and explanations are provisional. For instance, the acid-base historical development would allow teachers to discuss with their students the limitations of each of the acid-base theories and why they were used in the past or still are used (Alvarado et al. 2015).

In this paper, we use three famous acid-base theories (Arrhenius, Bronsted-Lowry and Lewis), although the historical development of the acid-base domain is as long as the history of chemistry itself. Figure 1 represents a timeline of the acid-base domain showing links with the chemical change models presented by Justi and Gilbert ( 1999b) and also with some acid-base models reviews (de Vos and Pilot 2001) or history of science books (Taton 1957, 1959; Taton and Goupil 1961).

The historical development of a domain is usually introduced to students in a very condensed manner in order to focus on the last, the most useful or the longest surviving acid-base theories. In this first part of the paper, we only use acid-base "theories" as it is commonly called, but from Section 6, we will be called acid-base "model" to focus on the explanatory and predictive power of the models. This timeline could be a good illustration of the historical development of ideas which is broader than that usually presented to the students. On the left side of the timeline, we use the term pre-model (Justi and Gilbert 1999a) to indicate that an acid-base classification does not have to be explanatory.

In order to understand scientific models, we need to appreciate that they have been constructed to explain and predict phenomena, so the models are more than a descriptive account of the material world. In this sense, acid-base historical models are a good opportunity to understand how change has taken place in the scientific models over time.

Many of the situations where people encounter science involve the use of scientific knowledge, alongside other forms of knowledge, to reach decisions about action. This is often the case for lay people, who typically find science through media portrayals of socio-scientific issues, or through consultations with experts such as medical practitioners. Lay views of science tend to portray such issues as being easily resolved through simple empirical processes (e.g. Driver et al. 1996). This position, however, is often not sustainable, as illustrated by the following examples.

An example of science in the media is the case of enhanced global warming as a result of increased levels of carbon dioxide in the atmosphere due to the combustion of fossil fuels. It is not difficult to find widely different predictions in the media about the likely environmental impact of the burning of fossil fuels. These differences in predictions are based upon the application of models of the atmosphere. Resolving those differences involves a complex interplay between models, empirical evidence and methodological expertise. Understanding how such differences arise, and why they cannot be resolved quickly and simply, involves understanding something about the use of models.

It is not only lay people who encounter science in situations that are characterized by uncertainty. Many experts will be faced with situations where scientific knowledge has to be drawn upon, alongside other considerations, to inform decisions about action in novel situations. A sad and recent example is the current scientific/political/cultural environment with COVID-19. The academic literature now includes several accounts of how experts have to create new knowledge in order to answer questions that emerge in specific situations. Another, much older example, is Brian Wynne's account of how experts had to develop new knowledge about the impact of pollution following the Chernobyl accident impacting upon the milk produced by sheep which grazed on the Cumbrian fells (Wynne 1989). In order to appreciate why the available scientific knowledge may be inadequate to inform action in specific, local conditions, such experts need to understand something about the nature of models in science. So, to summarize, it is necessary to teach models at university level to make the following points:

In order to have a sophisticated understanding of the conceptual content presented to them in chemistry degree programmes, students need to have some understanding of how the models are built.

In addition, if the students develop this understanding of the nature of models, it may enable them better to understand situations involving uncertainty, whether as educated citizens, or if they go on to become professional scientists, in their professional practice.

These general arguments about teaching models can be exemplified in the case of acid/base models.

Conventional Acid-Base Teaching

Conventional chemistry teaching might begin with questions such as "What is an acid?" or "What is a base?"; "What happens when an acid is added to a base, and vice versa?"; "What does pH mean?"; "Is it always possible to reach pH=7 when an acid is added to a base?" Conventional teaching shows some concepts from Arrhenius and Bronsted-Lowry's theories that we summarized in Fig. 2.

It is possible to recognize some differences between both theories in relation to considering acids and bases as substances (Arrhenius) or as the conjugated acid-base pairs (Bronsted-Lowry). As is normally mentioned, Arrhenius', Bronsted-Lowry's, and Lewis' theories are usually presented together (Tarhan and Acar Sesen 2012). Acid-base theories are introduced in the style of a short story without any connection with the phenomena they want to explain or with the historical problems that inspired them.

By teaching these acid-base theories together we expose the combination of the acid-base concepts (acid, base, neutralization, etc.) of the three theories without appreciating a significant advance between them, which for many students could mean only a terminological change. Teachers (and textbooks) usually say that "Bronsted-Lowry extend the definition of acids and bases" (Nyachwaya 2016, p.510) given by Arrhenius. In this sense, Arrhenius', Bronsted-Lowry's and Lewis' models are often presented several times in chemistry programmes, introducing some inconsistencies in their presentation that lead to "hybrid models" (Justi and Gilbert 1999a) and some concepts and their definitions are mixed in two or more models (Gericke and Hagberg 2010).

The science education literature is replete with examples of the consequences for students' learning of this typical way of teaching acid-base content focused on the definition of its concepts and with two or three theories introduced simultaneously. In the next sections we will use a review of research results (our own and others) on the understandings of high school students, university students (both undergraduates and postgraduates), and pre-service teachers in order to design some proposals focusing on two approaches, one NoS explicit and the other NoS implicit (Sections 6 and 7 respectively).

Consequence I: Students' Alternative Conceptions

There have been a number of studies into students' misunderstandings of acid-base phenomena (for example McClary and Bretz 2012; Nyachwaya 2016). Many students have difficulties in learning acid-base concepts and the presence of alternative conceptions (Hoe and Subramaniam 2016) can interfere with their understanding. For instance, students think that acids alone are associated with corrosiveness (Demircioğlu et al. 2005; Hoe and Subramaniam 2016; Özmen et al. 2009) and are more dangerous and reactive than bases (Hoe and Subramaniam 2016; Nakhleh and Krajcik 1994; Sheppard 2006). They also think that rain water in an unpolluted area is neutral or the solution formed after adding an acid and a base is always neutral (Banerjee 1991; Hoe and Subramaniam 2016; Scerri 2019; Schmidt 1991). As Quílez ( 2019) points out, many of these misunderstandings come from students' terminological difficulties. Therefore, students do not understand why the degree of acidity or basicity of two acidic or basic solutions is different, although they have the same concentration (Alvarado et al. 2015). Moreover, a superficial correlation of chemical structures with acidity or basicity may explain why students believed that compounds containing H will produce H⁺ and compounds containing OH will produce OH − (Demircioğlu et al. 2005; Hoe and Subramaniam 2016).

Besides, the belief that a stronger acid is either the one that produces a higher hydrogen ion concentration, or that has more H in the formula, or an acid with a higher initial concentration (Demircioğlu et al. 2005; Hoe and Subramaniam 2016; Özmen et al. 2009; Ross and Munby 1991), reveals that students do not apply correctly the definitions of strong and weak to acids and bases (Garnett et al. 1995; McClary and Bretz 2012) and both equilibrium and incomplete dissociation of acid and bases are not considered. Similar problems are found with students' understanding of bases (Hoe and Subramaniam 2016).

Another issue is that students do not totally differentiate between the terms acidity and pH (Alvarado et al. 2015); they do not consider pH is providing information about both the H⁺ and OH − concentrations (Garnett et al. 1995), and show either a lack of consideration about the influence of variables such as the temperature or the solvent, using strength and concentration as if they were synonymous or having problems when they must differentiate between an acid-base reaction and a neutralization reaction (Alvarado et al. 2015).

Many of those alternative conceptions are consistent with students using the theory in other contexts to which Arrhenius proposed it. In the next subsection, we are going to look deeper into the difficulties linked to transferring knowledge to new situations or to recognizing the limits of each of the acid-base theories.

Consequence II: Difficulties in Transferring and Applying Knowledge

Based on our own results in the Spanish context (Author 2000), the consequences of acid-base conceptual teaching for both undergraduate and postgraduate chemistry degree students become evident due to (a) students' difficulties to transfer knowledge and (b) the problems recognizing the limits of applicability of acid-base theories:

(a)
Transference of knowledge to new situations. Despite having been taught about acid-base concepts many times, undergraduate university students ( n  = 450) from three Spanish universities showed weaknesses in their abilities to recognize an acid-base process and the proportion giving the correct answer decreased when the complexity of the theory applied increased:

Most students (78%) recognized a proton transfer process as Bronsted's model.

26% recognized the autoprotolysis of solvents (SO ₂) as an acid-base process.

Only 12% considered the electron transference as in Lewis' process.

Less than 2% of the students applied all three theories.

(b)
Applicability of acid-base models. As a consequence of the previous result, it was expected that most of the graduates in their secondary education teacher Civil Service examination would cite Arrhenius', Bronsted-Lowry's, and Lewis' models (data from research on 50 exams). Nevertheless, in approximately 15% of the cases, the description was wrong. Only 52% of the candidate teachers identified the boundaries of Arrhenius' model, three-quarters made no comment on the limitations of Bronsted-Lowry's theory, and none recognized that there might be limitations in Lewis' theory and what they explicitly consider is the currently accepted one (Jiménez-Liso 2000), similar to the results founded by Yalcin ( 2011) with Turkish candidate teachers. Previous omissions of the limits of applicability of the different acid-base theories are worrying as regards that, if they passed this exam, they would be qualified as secondary education teachers in Physics and Chemistry. They do not usually follow any continuous professional training as teachers, a fact that also occurs in other countries such as England, France, Finland, and Cyprus, and could affect the quality of teaching and the improvement of the education system (Evagorou et al. 2015).

Some Spanish university students explained that an electron transfer process (Lewis' model) or an autoprotolysis of solvents (SO ₂) is not an acid-base process, for example, "It is not an acid-base process", "It is a redox process", or "It is not an acid-base process because there isn't H⁺ or OH⁻ or H ₃O+". Thus, as in many occasions mentioned by other authors (Drechsler 2007; Drechsler and Van Driel 2009; Zoller 1990), the Bronsted-Lowry acid-base process is more recognized by students than other acid-base models. For us, this result is evidence that the university students did not transfer their knowledge about acid-base acquired to new situations, for example, the Lewis' acid-base electron transferences.

Teachers must have knowledge about teaching scopes and limitations of different acid-base models; nevertheless, most of them had not developed teaching strategies for this issue and only a few teachers said that they usually discussed the use of models of acids and bases in their teaching (Drechsler and Van Driel 2008). Although they recognized some difficulties of the students such as confusion between models, only a few emphasized the different models of acids and bases (Alvarado et al. 2015; Drechsler and Van Driel 2008). Moreover, despite some teachers believing that most students do not understand the use of models, they tried to teach it anyway in order to help the best students in their learning, hoping other students understand that simple models are not the whole truth (Drechsler and Van Driel 2008).

Consequence III: Misunderstanding About the Development of a Historical Model

By presenting two or three theories together, the Lewis and Bronsted-Lowry definitions are just that-definitions-and the validity of one of them does not automatically negate the other (although it may expand the set of substances which are classed as acids). The conceptual-focused teaching mentioned above is insufficient because it comes from a purely theoretical perspective without any kind of application, reduced to the definitions instead of containing a clear explanation of their development (Cid Manzano and Dasilva Alonso 2012) and of the problems or phenomena that gave rise to new ideas.

The three favourite acid-base theories are presented as a collection of "agreed upon facts", so students memorize them without questioning their relationship with other scientific knowledge (Justi and Gilbert 1999a), and focusing on the products rather than on the processes of science.

There is clear evidence that many problems learners have arisen from acid-base theories confusion. When several theories of acid-base are presented together to students the scope to produce confusion is expanded, above all if most learners have a very limited notion of the role of models in science (Driver et al. 1996; Grosslight et al. 1991; Taber 2001).

The role of a model in science is related to developing a scientific understanding of some phenomena, explaining them and predicting other related phenomena, then applying the new knowledge to novel situations or contexts (Izquierdo-Aymerich and Aduriz-Bravo 2003; Oh and Oh 2011). So, the reason for explaining three (or more) acid-base theories together is not related to scientific understanding of the phenomenon. The reasons for introducing the three most used acid-base models together appears to be twofold: firstly, conceptual survival (a concept from past chemical curricula that is retained in modern chemical curricula) and, secondly, to show the history of a chemistry concept in a narrative manner. Thus, the emphasis is focused on the differences between each concept, a fact that does not promote a proper understanding of science, instead of comprehending the conditions when the models were built and, consequently, the limitations they have. No advantage is taken of the opportunities to get the students to reflect on the nature of science through the history of acid-base theories. On the contrary, they usually develop a distorted image of science itself and of how it is carried out.

Before we discuss teaching acid-base models in the chemistry curriculum, it is necessary to clarify terminology. Acid-base concepts, definitions, theories, and models are often used as synonymous. Students' mistakes often arise due to the ambiguous use of the terminology (Jiménez-Liso and de Manuel Torres 2002). To avoid this difficulty, we have adopted acid-base models as the correct terminology to refer to the models that explain and predict phenomena proposed by Arrhenius, Bronsted, and Lewis, because we understand that the theories in which they are included are the ionic dissociation theory, solvents theory, and valence electron theory respectively.

Considering that curriculum materials shape teachers' practice and characteristics (as their knowledge or beliefs) and students' opportunities to learn in science (Davis et al. 2016; Pareja Roblin et al. 2018), in the next section, we try to identify activity sequences, firstly with an explicit NoS approach using the timeline of the historical development acid-base models.

Explicit Approach to NoS Teaching

As Burgin and Sadler ( 2016) mentioned, the prevalent model for teaching NoS in school has been referred to as the explicit/reflective approach (Lederman 2007). In this approach, the priority object of study is to teach the great consensus about the nature of science (tentativeness, creativity, ...) to avoid the main distorted views of science. Typical activities using this approach are discussions about a "paper towel investigation", about the "card exchange" (Cobern and Loving 2002), or about historical cases (readings or movies) (Aduriz-Bravo and Izquierdo-Aymerich 2009; Moreno et al. 2018) and scientific errors (Kipnis 2011) as a particular historical case or some historical controversies (Niaz 2009). All of these strategies for teaching are linked to some rationale, to educational purposes (Nouri et al. 2019). The acid-base timeline (Fig. 1) could link with the next curriculum purposes for specific educational levels (Table 1).

Table 1 Opportunities to improve the understanding of scientific content using acid-base historical models (or pre-models)

More interesting than improving acid-base content understanding are the opportunities to advance the understanding of nature of science content using certain moments of the acid-base timeline, for example, the story of what happened with acid-base models in 1923.

In 1923, Bronsted and Lowry proposed their explanations about acid and base behaviour. Both knew Arrhenius's model ( 1903) and a less famous one today: the solvent-solute model proposed by Franklin ( 1905). Some 21 years later, two researchers independently proposed a particular case of solvent model (proton model) where the water acts as solvent and its autoprotolysis as definition of acids (proton donor) or bases (accept protons).

We can use the original papers from Bronsted, Lowry, and Lewis to help them to answer the next questions that we can ask to our high school students (or university students):

Why does emerge a limited model (proton model, Bronsted-Lowry model) after a broader model (Franklin model)?

We want to scaffold "the epistemic value of simplicity, referred to as Ockham's razor, meaning that the simplest applicable model is the most elegant and the best" (Rollnick 2019, p. XIV). The solvent model proposed by Franklin ( 1905) was known by Bronsted and Lowry but they only used the water as solvent, so for solving their problems, they did not need a broader model and they specify it on a model more simple but more useful. In fact, it is the most widely used and known today because most acid-base reactions are aqueous.

A Danish researcher (Varde, Denmark) (Bronsted 1923) and the same year that (Lowry 1923) from Bradford (UK) propose the same proton model, how do you think it was possible for two researchers in different countries (without knowing each other) to propose an identical model?

Perhaps in our digital era this scenario is unthinkable: Two researchers producing identical researches without any previous contact, but in 1923, they heard from each other when they read the papers already published in two different journals. The scientific community recognized the merit of both of them and, thereafter, their model was named Bronsted-Lowry. What circumstances led both to propose the same theory? Bronsted ( 1923) started from the dissociation electrolytic theory of Arrhenius, which initially does not call into question his idea of acid (A →B + H⁺) and for which he tries to find a better definition of base: "It is the purpose of the present small contribution to show the advantages that come from a modified definition of a base" (Bronsted 1923, p. 718), specifically the difficulties in explaining the basicity of ammonia:

If we accept scheme (NH ₄⁺ + OH⁻ <===> NH ₄OH), as a suitable expression for characterizing bases, we will be forced to give a special definition of a base for each special solvent. However, in principle, acid and basic properties are independent of the nature of the solvent, and the concepts of acids and bases are in fact of such a general character that we must consider it a necessary requirement of these concepts in general to formulate a pattern independent of the nature of an arbitrary solvent (Bronsted 1923, p. 719)

and ends by concluding:

The equilibrium formulated in scheme (1) between hydrogen ion and the corresponding acid and base can be called a simple acid-base equilibrium. By mixing two simple systems, a double acid-base system and an acid-base equilibrium result that can always be formulated as follows:

$$ {\mathrm{acid}}_1+{\mathrm{base}}_2<===>{\mathrm{acid}}_2+{\mathrm{base}}_1 $$

This equilibrium includes a number of important reactions such as neutralization, hydrolysis, indicator reactions, etc. (Bronsted 1923, p. 728)

On a different path, Lowry ( 1923) knew the electron valence theory of Lewis ( 1923) and relied on it to distinguish two types of chemical affinity (polar and non-polar) and their links in organic and inorganic substances, which led to the need of proposing H ₃O⁺ as what is exchanged in acid-base reactions overcoming Arrhenius H⁺ proposal, the difficulty of the basic character of NH ₃ and the relative character of strong or weak acids depending on which substance they react with (Fig. 3).

With the previous knowledge, students are willing to answer one last question: How do you think two models emerged in 1923 from three different people working independently and in different paradigms (proton or electron paradigms)? 1923 was a good year for the acid-base historical development. Lewis ( 1923) also raised his electron model (Fig. 4) under a totally different paradigm (based on his electron valence theory) and to solve a problem not contemplated by his contemporaries: acid-base behaviour in reactions without solvent, for example, in gas reactions.

Discussing with upper secondary students (or university students) these acid-base moments of a broad timeline, we could challenge the accumulative-linear and erroneous image of science.

For university chemistry or geology degree students, similar questions could be posed using the 1939 and Lux-Flood models for reactions in high pressure such as on geological process (without any dissolution). And also for university chemistry students (pre- and post-graduated), another interesting controversy could be the qualitative and quantitative chemical approaches (Chamizo 2018) between Pearson ( 1963) who proposed empirically his hard and soft acid-base model and Drago ( 1973).

Implicit Approach to NoS Teaching

Inquiry-Based Teaching Proposal

Barrow ( 2006) described inquiry firstly as an epistemic practice (Kelly 2008), secondly, as scientific skills that students should develop, and finally as a teaching approach. We remain with this last meaning of inquiry to propose an instructional sequence of activities. As there are a multitude of research proposals (Pedaste et al. 2015), we have specified our teaching approach in a cycle (Fig. 5 in orange) to connect it with the modeling cycle (Fig. 5 in green) proposed by Couso ( 2020) and Couso and Garrido-Espeja ( 2017).

In the acid-base domain, reactions can be followed with indicators from daily life such as red wine. When we use the red cabbage as an acid-base indicator, we generally emulate Boyle's descriptive pre-model in order to recognize the acid-base nature of some daily life products. In this way, we create (as Boyle did) a classification of acid, neutral, or basic substances.

We transform these hands-on activities about the acid-base classification to an inquiry-based teaching where the steps will be easily recognizable by our students so that they become aware of how they have learned (learning and emotions self-regulation) and, therefore, can make an explicit debate about the phases of the inquiry, how they help to learn, and what emotions they felt during this sequence (step 7 in cycle orange, Fig. 5).

To do this, we begin with a familiar problem (chewing gum TV advertisement stops the acid attack, strengthens the tooth enamel and helps to keep your teeth strong and healthy while in the image we can show that raise the pH of the mouth to prevent the formation of cavities (Jiménez-Liso et al. 2018) that engages students to explain their personal ideas: chewing gum is the opposite of acids generated by food in the mouth, the TV ad does not tell the truth and the gum does nothing, warms and destroys the acids, more saliva is generated or the gum traps the remains of food. The key moment in this sequence is the students' proposals for designs of experiments that allow them to find evidence to confirm or reject their hypotheses.

The experimental designs raised by our students facilitate the discussion about the usefulness of the designs (what did they measure? With what did they check?). For instance, some students proposed to put some food in a glass with water (to simulate the mouth) along with the gum and measure with pH paper, to which another group responded that they did not check the "before" and the "after" adding chewing gum, that is, the effect of chewing gum. Others suggested sucking pH paper after eating and again after chewing gum (López-Banet et al. 2021).

Taking measurements with a pH meter can be a conflict for the students with their expectations, both because the chewing gum does not raise the pH of the acid-dissolution (mouth simulation) and neither does adding water (dilute). This opens the option of deepening the mathematical conflict that involves a linear scale (pH values of 1-14) versus a logarithmic scale (which means pH) asking how much water would be necessary to raise the value by one point (López-Banet et al. 2021).

However, as Osborne ( 2014) mentioned, hands-on activities, such as the acid-base classification using red cabbage indicator, are not normally accompanied by an interpretation or explanation of the phenomena. In our inquiry-based sequence, students built essential descriptive knowledge (acid-base reactions vs dilution with water or saliva) so that they now recognize the need to seek an explanatory model perfect to start the modeling cycle (Fig. 5).

Cycles of inquiry (in orange; Jiménez-Liso et al. 2019 and Jiménez-Liso 2020) and modeling (in green; Couso 2020; Couso and Garrido-Espeja 2017) of teaching sequences and their aims of instructions (phases of the scientific practices)

Model-Focused-Teaching Proposal for Lower Secondary Level

The use of red cabbage as an acid-base indicator, often carried out by students aged up to 16 years old, is not accompanied by its possible explanation using models, keeping the explanation for higher levels (16-18 years old and university level). In this sense, the first introduction of an explanatory model is presented for 16-18-year-old students and, generally, Arrhenius or Bronsted-Lowry's model is used to present acid-base processes disconnected from those activities carried out (or not) during previous years (Jiménez-Liso et al. 2010). Therefore, if we focus the contents exclusively on the phenomenon and the identification of substances, we are only increasing the students' experiential field, but not their ability to explain phenomena they observe or to foresee what is going to happen in new situations.

Some hands-on activities about properties of acids and bases emphasize the teaching and learning of chemical knowledge through models and modeling by the formulation, evaluation, and revision of chemical models. When we ask students to express what they think happens "inside" by adding a base to an acid and observing changes in the colour of the indicator or changes in pH values, their initial models are unsatisfactory for some explanatory reasons (steps 1 and 2 in modelling cycle; Fig. 5 in green; Couso 2020; Couso and Garrido-Espeja 2017). Students may have difficulties when it comes to expressing these initial models through drawings, most often represent non-explanatory circles, and only some of them point differently to acids or bases indicating that it is acidic when acids "predominate" over the bases and vice versa (López-Banet et al. 2021). Despite these difficulties of the students in explaining "what happens inside", we cannot consider these initial models as students' alternative conceptions described in Section 5.1 for two reasons: firstly, students' alternative conceptions were the product of punctual and "academicist" knowledge and, secondly, the difficulty for the initial models to be explanatory is the initial step to become aware of the need to build a model, that is an idea that helps explain a phenomenon (change of colour of acid-base reactions with indicator) and to predict new ones (for example the bubbles when we add bicarbonate to the vinegar).

Students are expected to relate properties of a substance to its shapes, in a similar way to Nicolas Lemery's model (Erduran 2007; Erduran and Kaya 2019). This seventeenth-century scientist explained that acids consist of keen particles that prick the tongue when they are tasted, differing both in length and in mass from one another. On the other hand, alkalis have pores where the acid points entering into do strike and divide them when oppose the motion of acids. So the difference of the points in acid substances is the cause why some acids can penetrate and dissolve well certain sort of mixts (Lémery 1697). Our version of this model is a PACMAN model (Jiménez-Liso et al. 2018) suggested sometimes by some of our students. They are able to reason as ancient scientist used to and to build their own explanations in a similar way about what happens in a microscopic level by means of descriptions of the reality (macroscopic level) and their intuitive thoughts.

This anthropomorphic model is already useful for students because it explains acid-base phenomena but it needs to be refined (steps 3 and 4 of the modeling cycle) because it does not serve them to explain a well-known experiment: why the balloon is inflated by adding bicarbonate to the vinegar. When students must construct a model to explain this precise knowledge of reality (what happens with the balloon), they introduce partial modifications to their useful models (Lemery or PACMAN model with triangles as acid) such as "bow ties" that fly when the PACMAN eats the triangles, and they argue on its validity (or not) according to the descriptive knowledge they already have. This process leads them to identify the insufficiency of their initial models, the useful of PACMAN model and its limits, and the need for refinement to explain the production of gases in acid-base processes. Figure 6 shows other alternative model based on the fighting idea to form a structure together that explain gases formation in an acid-base reaction, done by other students.

As it is necessary to help students to comprehend the nature of models, a possible strategy could be introducing an explanation similar to that one mentioned as past scientists did. The activities previously mentioned encourage pupils to express their own ideas, giving opportunities to evaluate and restructure them, in order to pass from their initial to more scientifically valid conceptual schemes ones. For instance, pupils draw representations trying to explain the way they perceive some common substances and they described their models in class to share their ideas, as drawing "bubbles" in acids and less bubbles or no bubbles in the base substances (Erduran 2003).

Lemery, PACMAN or fighting models are very anthropomorphic. However, these models allow quick connection with the chemical formulation and the Arrhenius model (Fig. 7 from Jiménez-Liso et al. 2018).

Correspondence between the PACMAN model elements and chemical formulation (Jiménez-Liso et al. 2018)

Connected Key Aspects of Acid-Base Model-Focused Teaching for Upper Secondary Level (or University Degrees)

As Couso ( 2020) mentioned, a model-focused teaching would be to put students in the situation of building themselves "adequate enough" explanations, in other words, to construct school-based scientific models to describe the behaviour of the world and to comprehend how it works (Aduriz-Bravo and Izquierdo-Aymerich 2009; Izquierdo-Aymerich 2000). Instead of learning the models as the result of the scientific activity, it would be enough to focus on some specific big ideas (Harlen 2010) or key ideas (National Research Council 2012) that have the potential to explain a lot of different phenomena (Izquierdo-Aymerich and Aduriz-Bravo 2003). Thus, a model-based teaching approach offers instructional strategies for improving conceptual learning in science education (Shen and Confrey 2007) and permits students go beyond the idea of ​models as reproduction, allowing them to reach the vision that the relationship between model, experiment, and reality is dynamic and evolutionary (Tasquier et al. 2016).

In order to build a school science using more sophisticated models for upper secondary or university levels, as in our case, we talk about the model associated with phenomena using the concrete term "key connected aspects", which should emphasize:

The purposes of each model: for example, Arrhenius' model is an explanation based on the classification of substances in acids or bases and their reactions, Bronsted-Lowry's model is based on equilibrium, and Lewis' model focus on a different paradigm, the electron theory. We want to emphasize on this idea because it changes the acid-base view, from the conceptual-focused teaching (Fig. 1) because we defined acid that contains H⁺ (Arrhenius) or that donates H ₃O⁺ (Bronsted-Lowry) to a new view with an explanatory power of both models, which in the case of Arrhenius explains reactions between substances and Bronsted-Lowry explains equilibrium, balances, and, therefore, their reversibility (Fig. 8), as we will see below.

Acid-base characteristic from each scientific model: our perceptions about acid-base definitions given in Fig. 1 change from acid-base as substances to the absolute acid and base properties based on their chemical composition in Arrhenius' model (Fig. 8, left), from the acid-base pairs conjugated to the relative properties of substances in Bronsted-Lowry's model (Fig. 8, right), or from acid-base as accept-donor pair of electrons (the electron paradigm) to its possibilities to explain solvent absence in Lewis' model. These decisive acid-base characteristics are connected to the models' educational purposes through a simplification of the historical scientific consensus models and it explains why some historical models can still be used to explain some phenomena (Table 1). The comparison and contrast of these key features, the nature, and purposes of models can be addressed in the teaching.

Scope, boundaries, and explanatory power: for example, Bronsted-Lowry's model can explain not only the reason why a reaction between a strong acid and a weak base produces a pH < 7 solution without using hydrolysis concept but also that the reaction between a base and water is possible and that two acids (one stronger than other) can react (however a weak acid, according to Arrhenius' classification, reacts like a base). These explanations are not possible using Arrhenius' model.

It would not be prudent to discard a model that is easy to understand and is well applicable in many cases (Ockhams razor (Rollnick 2019). For instance, many chemical reactions occur in aqueous solutions because many compounds have hydrogen or hydroxide ions. Thus, teaching the Arrhenius concept is important for the purpose of promoting the recognition of the meaning of the acid-base characteristic from this scientific model in science learning. However, for this purpose, the introduction of new concepts needs to be followed in order to overcome the limitations of the Arrhenius concept (Paik 2015). On the other hand, the key ideas from Bronsted-Lowry model emphasize five concepts: equilibrium, reversibility, simultaneous, and the relative strength of acids and bases, both in aqueous and non-aqueous solvent (Fig. 8). When acid-base reactions occur without solvent, for example, gases reactions, neither Arrhenius' model nor that Bronsted-Lowry's model serves to explain them, so we need other models such as Lewis' electron valance model or Lux-Flood model for geological hard pressure acid-base reactions.

Conclusions

The arguments put forward in this paper might convince teachers to deepen their teaching of acid-base processes, at all possible educational levels, by taking advantage of the presence of historical development in upper secondary and to cover the need to advance it to primary or lower secondary levels by the arguments involving the presence of acids and bases in our daily lives and on solving socio-scientific issues about health or the environment.

The extensive bibliography on alternative conceptions at all educational levels (including teachers and candidates to be) justify the need for a change in the usual way of presenting it that focuses on the presentation of definitions on acid, base, theirs reactions, pH, etc., in two or three "theories" presented together. This fragment of the history of science that survives in the current curriculum (in upper secondary and chemistry degree, university level) offers a very good opportunity for NoS teaching without overloading the already extensive and concentrated chemistry curriculum.

Thus, the main aim of traditional acid-base teaching is to learn the main concepts by means of conceptual-focused teaching and it is very far from making sense to the students, because it makes them look at the bricks and not in their usefulness as part of a larger and more beautiful castle (meaningful) and useful. Inquiry-based teaching (Section 7.1) and the model-conceptual teachings that were exemplified in this paper (Sections 7.2 and 7.3) provide implicitly a chance to reflect on how science is constructed.

Whereas the conceptual-focused teaching only explains definitions and emphasizes on descriptions about behaviours (not always coordinated), model-focused teaching emphasizes on explanations, interpretations, and predictions (Stefani and Tsaparlis 2009). In this way, students should learn to use each model within its application domain to address different phenomena. Applicability of acid-base models should be better understood, and knowledge of acid-base models would be transferred to new situations, for example, to recognize a new process as an acid-base reaction.

In this paper (Section 7.3), we proposed that considering models include key ideas connected in a particular way they provide coherence to concepts. Both approaches are in conflict with each other, so this dual treatment is discussed: teaching isolated ideas in a conceptual-focused teaching or the relationship between connected key aspects through a model-focused teaching.

We have attempted to show the differences between NoS teaching approaches through several sequences of activities. First, one sequence with an explicitly NoS approach, as Lederman ( 2007) points as desirable, and then, three implicit approaches, similar to Duschl and Grandy's ( 2013) recommendations. These four sequences can help teachers to perceive the potential results of choosing one of those treatments in acid-base lessons, according to their own teaching goals. Also, this concretion in sequences of activities, which is the fundamental tools for teachers to teach, can encourage them to teach acid-base models in a way closer to the recommendations of NoS researchers.

As we pointed out in the introduction, by specifying the implicit-explicit debate in several sequences of activities, we are also offering, for science teacher training, a theoretical learning progression. Pre-service or in-service teachers in training could live inquiry and modeling sequences since Lemery to the more sophisticated models and it allows to place the implicit sequence one after this lineal progression to make explicit the awareness of how the science is built. Finally, as an agenda for future work, we could follow the steps outlined in this paper in order to develop an evaluation study about the efficiency of consensus NoS understandings of each implementation of our implicit, explicit-IBSE, explicit-modeling sequences, using frameworks such as Burgin and Sadler ( 2016).

Acknowledgements

This work would not have been possible without the inspired discussions with Esteban De Manuel (Rut's PhD supervisor) and John T. Leach and the participation of IES Murgi students, their teachers Lucía, Isabel and Carmen.

Funding

This work has been partially financed by the projects EDU2017-82197-P and PGC2018-097988-A-I00 of the Ministry of Science and Innovation (MCI) of Spain, the State Research Agency (AEI) and the European Regional Development Fund (FEDER), and with a visiting scholar in the University of Exeter PRX19 / 00364 of the Ministry of Education of the Government of Spain.

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Affiliations

1. Sensociencia Team, CEIMAR-University of Almeria, Almeria, Spain

Maria Rut Jiménez-Liso

2. Department of Science Education, University of Murcia, Murcia, Spain

Luisa López-Banet

3. CRISTEME, University of Exeter, Exeter, UK

Justin Dillon

Corresponding author

Correspondence to Maria Rut Jiménez-Liso.

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Changing How We Teach Acid-Base Chemistry

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We propose explicit and implicit approaches for the teaching of acid-base chemistry based on research into the history and nature of science (NoS). To support these instructional proposals, we identify four rationales for students to understand acid-base processes: daily life, socio-scientific, curriculum, and history of science.

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Investigate reactions between acids and bases; use indicators and the pH scale

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Investigate - Observe, study, or make a detailed and systematic examination, in order to establish facts and reach new conclusions.

Use - Apply knowledge or rules to put theory into practice.

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Uses of acids and bases for students with farming backgrounds.

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Soil pH is the measure of the sourness or sweetness of a patch of ground. The pH scale commonly in use ranges from 0 to 14, increasing with rising alkalinity and decreasing with rising acidity. Extreme readings in either direction are usually undesirable. If a reading falls on the middle part of that scale (numerically equal to 7), that means the soil is as neutral as Switzerland and you most likely will not have trouble growing the majority of plants suitable for your region there (all else being the same).

So basically, the ground in which you garden can fall into any of the following three categories:

Acidic soil pH

Alkaline soil pH

Neutral soil pH (neither acidic nor alkaline)

Soil pH is not fixed; you can take measures to alter it. If soil pH needs to be lowered (that is, the earth is not acidic enough), apply commercial fertilizers containing sulfur/ammonium-N. Ammonium sulfate is a fertilizer that will lower pH. If soil pH needs to be raised (that is, the earth is not alkaline enough), apply lime.

Fortunately, the web has online calculators to help you determine how much lime or sulfur you would need to apply, given the size of the area where you are attempting to raise or lower soil pH.

Kits are available at some garden centers or home improvement stores for testing your soil pH. Alternatively, you can send in a sample of your soil to your county extension office; they can perform the test and inform you of the results.

Why is soil pH important?

Now that we have gotten the basic information out of the way, let's dig deeper into what "soil pH" means --that is, what its true significance is to those who garden and landscape.

Soil pH is not, itself a nutrient, but it relates to plant nutrition. That is because it governs the availability of nutrients to plants. Particular nutrients that a plant needs can exist in the ground in abundance, but if they are not made available -- due to conditions that are too alkaline, for example -- they will do the plant no good. To be available, the nutrients must be soluble. Soil pH level affects this solubility.

Fun Fact

Plants That Like a Low Soil pH (Acidic Ground)

There are many acid-loving plants, including:

Plants That Like a High Soil pH (Alkaline Ground)

The fact that there are plants that will thrive in acidic ground is common knowledge in the gardening and landscaping communities. What is less widely known is that there are also recommended plants for alkaline soils (although in some cases the plants in question simply tolerate alkaline ground -- they do not necessarily prefer it). Examples are:

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Soil pH is the measure of the sourness or sweetness of a patch of ground. The pH scale commonly in use ranges from 0 to 14, increasing with rising alkalinity and decreasing with rising acidity. Extreme readings in either direction are usually undesirable.

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Article + video about acid rain to help students understand acids and bases in nature.

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Acid rain describes any form of precipitation that contains high levels of nitric and sulfuric acids. It can also occur in the form of snow, fog, and tiny bits of dry material that settle to Earth. Normal rain is slightly acidic, with a pH of 5.6, while acid rain generally has a pH between 4.2 and 4.4.

Causes of acid rain

Rotting vegetation and erupting volcanoes release some chemicals that can cause acid rain, but most acid rain is a product of human activities. The biggest sources are coal-burning power plants, factories, and automobiles.

When humans burn fossil fuels, sulfur dioxide (SO ₂) and nitrogen oxides (NO x) are released into the atmosphere. Those air pollutants react with water, oxygen, and other substances to form airborne sulfuric and nitric acid. Winds may spread these acidic compounds through the atmosphere and over hundreds of miles. When acid rain reaches Earth, it flows across the surface in runoff water, enters water systems, and sinks into the soil.

Effects of acid rain

Sulfur dioxide and nitrogen oxides are not primary greenhouse gases that contribute to global warming, one of the main effects of climate change; in fact, sulfur dioxide has a cooling effect on the atmosphere. But nitrogen oxides contribute to the formation of ground-level ozone, a major pollutant that can be harmful to people. Both gases cause environmental and health concerns because they can spread easily via air pollution and acid rain.

Acid rain has many ecological effects, especially on lakes, streams, wetlands, and other aquatic environments. Acid rain makes such waters more acidic, which results in more aluminum absorption from soil, which is carried into lakes and streams. That combination makes waters toxic to crayfish, clams, fish, and other aquatic animals. ( Learn more about the effects of water pollution.)

Some species can tolerate acidic waters better than others. However, in an interconnected ecosystem, what affects some species eventually affects many more throughout the food chain, including non-aquatic species such as birds.

Acid rain and fog also damage forests, especially those at higher elevations. The acid deposits rob the soil of essential nutrients such as calcium and cause aluminum to be released in the soil, which makes it hard for trees to take up water. Trees' leaves and needles are also harmed by acids.

The effects of acid rain, combined with other environmental stressors, leave trees and plants less healthy, more vulnerable to cold temperatures, insects, and disease. The pollutants may also inhibit trees' ability to reproduce. Some soils are better able to neutralize acids than others. But in areas where the soil's "buffering capacity" is low, such as parts of the U.S. Northeast, the harmful effects of acid rain are much greater.

Acid deposits damage physical structures such as limestone buildings and cars. And when it takes the form of inhalable fog, acid precipitation can cause health problems including eye irritation and asthma.

What can be done?

The only way to fight acid rain is by curbing the release of the pollutants that cause it. This means burning fewer fossil fuels and setting air-quality standards.

In the U.S., the Clean Air Act of 1990 targeted acid rain, putting in place pollution limits that helped cut sulfur dioxide emissions 88 percent between 1990 and 2017. Air-quality standards have also driven U.S. emissions of nitrogen dioxide down 50 percent in the same time period. These trends have helped red spruce forests in New England and some fish populations, for example, recover from acid rain damage. But recovery takes time, and soils in the northeastern U.S. and eastern Canada have only recently shown signs of stabilizing nutrients.

Acid rain problems will persist as long as fossil fuel use does, and countries such as China that have relied heavily on coal for electricity and steel production are grappling with those effects. One study found that acid rain in China may have even contributed to a deadly 2009 landslide. China is implementing controls for sulfur dioxide emissions, which have fallen 75 percent since 2007 -but India's have increased by half.

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The fossil fuels that humans burn for energy can come back to haunt us as acid rain.

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CW4, NoS1, NoS4, NoS7, Nos10, PW3

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Using litmus paper to see if a solution is an acid or a base.

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Using universal indicator to check for the strength of a substance.

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Acids & Bases Activity using Universal Indicator Solution

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Shows neutralisation happening in the presence of an indicator to see the pH change

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Shows how bases can also dangerous.

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Coke Cans in Acid and Base - Periodic Table of Videos