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Water is a precious natural resource The substance of water is the cause for the origin of life on earth.   65 percentage of the body parts of animals and plants contain water. Water is essential not only for sustaining life but also for agriculture, industry, energy production and transportation. We use water for different purposes in day-to-day life. We get water in large amount from different natural resources. Rain water, wells, backwaters, streams, rivers and oceans are different water resources. Water is a substance found in nature in all the three states of matter namely solid, liquid and gas.

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Water is one of the most plentiful and essential of compounds. A tasteless and odorless liquid at room temperature, it has the important ability to dissolve many other substances. Indeed, the versatility of water as a solvent is essential to living organisms. Life is believed to have originated in the aqueous solutions of the world’s oceans, and living organisms depend on aqueous solutions, such as blood and digestive juices, for biological processes. Water also exists on other planets and moons both within and beyond the solar system. In small quantities water appears colorless, but water actually has an intrinsic blue color caused by slight absorption of light at red wavelengths.

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An introduction to water video link
https://youtu.be/nSENolWbyYQ

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Boiling point of a substance is the temperature at which a liquid boils at normal atmospheric pressure. Water has a definite boiling point.   It is the temperature at which liquid form of water changes to it’s vapour state.

How can we determine the boiling point of water? Let’s check with an experiment,

Experiment : Take  little amount of water in a round bottomed flask or boiling tube. Arrange a thermometer in such a way that it kept immersed in water inside the boiling tube with the help of a  rubber cork with two holes. Connect a condenser to the other hole. Heat the round bottomed flask and record the thermometer readings at regular intervals. Note the point at which temperature remains constant.

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The boiling point of water is  100°C. Water exists in the liquid forThe boiling point of water is  100°C. Water exists in the liquid form at temperatures between 0°C and 100°C.But water changes to vapour at any temperature up to 100°C.Evaporation is the process by which a liquid changes to vapour. On heating water, the heat absorbed is utilized for rise in temperature and for evaporation.

Once water starts boiling, the temperature will not change becauseall the heat supplied is utilized for the change of state. Hence, all the excess heat supplied will be contained in the steam at the same temperature.

The effect of  Pressure  and Other substances

The boiling point of water gets affect by the presence of other substances  and pressure. In both the cases the boiling point of water increases.
Now can you explain why it is easy to cook in pressure cookers? Is it easier to cook food in altitudes?

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Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to a given mass of a material to produce a unit change in its temperature. Water has the ability to hold more heat (heat capacity) when compared to other substances. More than 65% of the body of all the organisms are filled with water , uses this advantage of water.

The following are some of the situations which make

use of the high heat capacity of water.

1. Water is used in radiators to regulate the heat in

2. automobile engines.

3. Water is used to cool hot objects.

4. Two thirds of the earth is water. This regulates the

5. temperature of the earth.

A video link showing high heat capacity of water :

https://youtu.be/aUfrhTUAGOw

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We all familiar with the freezing of water Freezing point of a liquid is the temperature at which it freezes to solid at normal atmospheric pressure.

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At freezing point water changes from its liquid state to solid state ie, ice The freezing point of water is 1000C

What happens to the density of water while freezing?

Almost all the substances on freezing decrease its size and shrinks. What about water? The case is just reverse in water.  As water changes to ice size increases, that means, When water is converted to ice its volume increases and the density

decreases.

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When atmospheric pressure goes below 4⁰C , due to anomalous expansion of water it's volume increases and density decreases. Water below 4⁰C raises to surface of water and covers the surface with ice. The aquatic life in water bodies at colder regions is sustained in water below this layer of ice.

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A video about anomalous behaviour of water :

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A video about anomalous behaviour of water :

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https://youtu.be/WbDbH121Nv4

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Have you ever seen certain insect moving on the surface of water ? how is it possible ?

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Small insects can move randomly on the surface of water. This is because of surface tension. Surface tension is the characteristic property of liquids. Water also possess surface tension.

Place a blade on the surface of water, what can you observe? The blade floats on water.

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The number of molecules in vapor form just above the surface of water is less than those on the surface. Hence the attraction towards the sides and the interior will be greater. Consequently, the liquid surface behaves like a stretched membrane. This behaviour causes surface tension.

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An interesting experiment shows the sirface tension of water, video link :

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An interesting experiment shows the sirface tension of water, video link :

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https://youtu.be/6ZRWLoXqgow

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What  is the relation between surface tension and surface area ? let’s   check with an experiment !

Tie a thread to a metallic loop, immerse it in soap water and create a soap film. Prick a portion of the film using a pin. What is the shape of the remaining portion of the film, what could you see? How the surface area changes?

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Surface tension acts in such a way so as to reduce the surface area of a liquid. For the definite mass of a substance, surface area is minimum for spherical shape. That is why liquid droplets assume spherical shape.

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Water is formed by the combination of the constituent elements, hydrogen and oxygen. Hydrogen and oxygen can be produced by decomposing water and water can be produced by combining hydrogen and oxygen

Electrolysis of Water
Water can be decomposed into its components by electrolysis.

Experiment:  Two test tubes filled with water are kept inverted over the iron nails without allowing any air bubbles to enter. Add a few drops of an acid to the water and then pass electricity through it from a battery. Collect the gases liberated in the test tubes. Mark the water levels in the test tubes after the experiment.

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The electrolysis of water give raises hydrogen and oxygen components The volumes of hydrogen and oxygen obtained in these experiments will be in the ratio 2:1. Water molecule contains hydrogen and oxygen atoms in the same ratio and hence it is represented as H2O.

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Which is the universal solvent? Yes ! It is water. Water has so many properties that make it as the universal solvent. When substances like sugar , vinegar , salt etc added to water it acquire the properties of the substances. That shows the ability of water to acquire the properties of other substances when dissolved in it.

Neutral solvent

Pure water has neither the properties of acid nor those of alkali. Hence it is called neutral solvent. But when water mixed with hydrochloric acid, it shows acidic properties and changes the colour of blue litmus to red. The mixture of water with sodium hydroxide shows basic properties and turns red litmus to blue.

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https://youtu.be/hKvCNkm-DGk

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Water acquires the colour of any coloured substance added to it. When red ink is added to water it turned red and adding copper sulphate and nickel sulphate give blue and green colour to water respectively.

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Since water can dissolve various substances and is widely used for preparing solutions, it is a universal solvent.

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Soft water and hard water

The water in which soap does not lather easily is called hard water.  Water in whichsoap gives lather readily is called soft water.

Reason for hardness
Rain water gets filtered and flows through the soil to reach well, rivers and sea. During this process, many substances present in the soil get dissolved in it. It has been found that mostly the salts of calcium and magnesium get dissolved in water

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https://youtu.be/F_lw6eSNtvA

https://youtu.be/jDgSohKVLio

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Temporary and permanent hardness

The hardness of water containing Calcium or Magnesium bicarbonates is removed during boiling. This type of hardness of water is known as temporary hardness. The hardness of water containing the chlorides and sulphates of calcium and magnesium is not removed even after boiling. This is permanent hardness.

Heavy water

Heavy water is water molecules containing deuterium, the isotope of hydrogen, instead of ordinary hydrogen atoms. This is used in nuclear reactors

Additional links related to hardness of water :

https://en.wikipedia.org/wiki/Hard_watr?wprov=sfla1

https://www.usgs.gov/special-topic/water-science-school/science/hardness-water?qt-science_center_objects=0#qt-science_center_objects

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Water and oxygen

How the aquatic creatures get oxygen to breath ?

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Gases like oxygen and carbon dioxide dissolve in water. Aquatic animals and plants make use of the oxygen dissolved in water. Water gets polluted as the amount of oxygen in it decreases.

The wastes dumped into rivers and ponds decay using the oxygen dissolved in them. This depletes the amount of oxygen and results in the destruction of aquatic life.

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Water pollution occurs when harmful substances—often chemicals or microorganisms, wastes, fertilizers, insecticides, detergents etc —contaminate a stream, river, lake, ocean, aquifer, or other body of water, degrading water quality and rendering it toxic to humans or the environment.

Causes of water pollution

1. Dumping of wastes in water resources

2.  Rampant use of fertilizers

3.  Excessive use of detergents

4.  Insecticides getting mixed with water

Video link related to water pollution :
https://youtu.be/h198sZXP7fU

Additional Site link related to water pollution :

https://www.britannica.com/science/water-pollution

Water scarcity

Scarcity of drinking water is one of the major issues faced by the mankind. Existing water resources are getting polluted day by day. This puts the very existence of life on earth in danger. So we have to save water for our future generation. Rain water harvesting, strict laws to protect water etc can be remedies of water pollution and water scarcity
Rain water harvesting :

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https://youtu.be/vSz9lRbl_0I

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1. What is the boiling point and freezing point of water at atmosheric pressure?

2. Why water steam causes more severe burns than boiling water at same temperature?

3. Is it difficult to cook food at high altitude? Why?

4. What is anomalous expansion of water ?

5. Write  the reason for spherical shape of rain droplets ?

6. Name the process by which water decomposes to its components ?

7. Define hard water and soft water ?

8. Name two substances that cause hardness of water ?

9. Water is a neutral solvent,  explain ?

10. Write a note on water pollution ?

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1) Water Water Everwhere...................1
2) Boiling point of water...................1-2
3) Heat capacity of water....................2
4) Freezing point of water................2-3
5) Surface tension of water..............3-4
6) Components of water...................4-5
7) The Universal solvent.....................5
8) Hardness of water.......................5-6
9) Water pollution...............................6
Questions........................................6-7