The composition of air

The air contains a number of gases. The amount of water vapour in the air varies from place to place, and day to day. For this reason, the proportions of the gases in the air are usually given for dry air.

Some of the gases in the air are elements: nitrogen (N₂), oxygen (O₂) and argon (Ar). But carbon dioxide (CO₂) is a compoun

Most compounds between non-metal elements are also molecular. Some examples are: water (H₂O), carbon dioxide (CO₂), nitrogen dioxide (NO₂) and ammonia (NH₃).

All of these substances have very strong covalent bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. 

Because the weak intermolecular forces break down easily these substances have low melting and boiling points. This means simple molecular substances are gases, liquids or solids with low melting points, and low boiling points. 

The animation below shows how the weak intermolecular forces between water molecules break down during boiling or melting:

Carbon dioxide in the air can dissolve in rain water to form carbonic acid, H₂CO₃.

CO₂ + H₂O 

H₂CO₃

Carbonic acid is a weak acid. It partially ionises to form hydrogen ions.

H₂CO₃

H⁺ + HCO₃^-

The hydrogen ions from carbonic acid give natural rain water a slightly acid pH value of 5.6. Over millions of years this very dilute acidic solution has been responsible for the formation of caves in areas of limestone rocks. Limestone is made of calcium carbonate, which reacts with acids.

calcium carbonate + acid

calcium salt + water + carbon dioxide

During the last century the rain water in some parts of the world has become far more acidic. This acid rain has been caused by the emission of pollutant gases such as sulfur dioxide. When coal is burned in electricity power stations, sulfur impurities form sulfur dioxide.

S + O₂

SO₂

The gas is also produced when fuels obtained from crude oil are burned. When sulfur dioxide is released into the air it reacts with water and oxygen to form sulfuric acid, H₂SO₄.

Sulfuric acid is a strong acid. It ionises completely to hydrogen ions.

H₂SO₄

2H⁺ + SO₄^2-

This gives rain water a pH below 5.0. Rain water that has this higher level of acidity can cause damage to buildings and statues, particularly those made of limestone. It can also reduce the growth of, or even kill, trees and crops. Acid rain may even lower pH of water in lakes, killing fish.

It is difficult to predict, even by using computer models, exactly what the mean temperature of the Earth will be in the future. However it seems sensible to limit global warming and protect the environment, through sustainable development and by reducing waste, wherever possible.

Scientists believe that the Earth was formed 

The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were small quantities of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans. 

So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up? 

The proportion of oxygen went up because of photosynthesis by plants. 

The proportion of carbon dioxide went down because: 

The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing.

Carbon dioxide is a greenhouse gas. It absorbs heat energy and prevents it escaping into space. This keeps the Earth warm enough for liquid water and living things to exist on it. Greenhouse gases are not a bad thing in themselves. But too much of them in the atmosphere leads to global warming, where the average temperature of the Earth rises quickly.

Carbon dioxide is produced by burning fossil fuels. Because of this, the amount of carbon dioxide in the atmosphere has increased, especially in the last one hundred years.

. Methane and ammonia reacted with the oxygen

. Denitrifying bacteria released nitrogen into the air

. The presence of oxygen allowed animals to evolve

It has taken a long time for it to be generally accepted that the increase in carbon dioxide in the atmosphere is a cause of global warming. There has been uncertainty in some of the data but this has reduced as more data has been collected. Computer models help us to predict that an increase of only a few degrees will cause changes to the Earth's climate. This is why it is important to attempt to combat global warming, for example by reducing our reliance on fossil fuels.

A mountain of household and industrial rubbish

Pollution is the addition of substances to the environment that may be harmful to living organisms. Population growth and increases in the standard of living cause more waste to be produced. If this waste is not handled correctly, it leads to pollution. The most obvious form of pollution is often simply just litter on the ground, but pollution can affect the air and water too.

Most rubbish is buried in landfill sites and not all of it comprises safe materials. Even common household items can contain toxic chemicals such as poisonous metals. Many smoke alarms contain radioactive americium. Industrial waste is also discharged onto the land. Many farmers apply pesticides to improve their crops, but these can damage living things. Toxic chemicals can be washed from the land into rivers, lakes and seas.

Oil spills cause a lot of harm to the environment, both at sea and on land

Water pollution is caused by the discharge of harmful substances into rivers, lakes and seas. 

pollutanttypical effectfertilisers | damages aquatic ecosystems
sewage | kills aquatic organisms and harms human health
toxic chemicals | kill aquatic organisms and harm human health

The resources on the Earth are limited. It is important that the things we do now do not make things difficult or impossible for future generations. To achieve sustainable development, we need to balance these things carefully:

Recycling is an important way to help us achieve sustainable development. We can recycle many resources, including:

Glass is easily recycled. It can be melted and remoulded to make new objects, such as bottles. The energy needed to do this is less than the energy needed to make new glass from its raw materials. However, we need to sort different coloured glass ready for recycling, and transport it to the recycling plant.

Just like glass, it takes less energy to melt and remould metals than it does to extract new metals from their ores. Aluminium is a valuable metal that melts at a relatively low temperature, and is particularly attractive for recycling. However, a lot of metals that we use are mixtures called alloys, and it can be difficult to sort different metals ready for recycling. 

Paper is not melted when it is recycled. Instead, it is broken up into small pieces and reformed to make new sheets of paper. This takes less energy to do than making paper from trees. However, paper can only be recycled a few times before its fibres become too short to be useful, and the recycled paper is often only good enough for toilet paper or cardboard. But it can be used as a fuel or compost instead.