ICSE Class 7 Chemistry Chapter 5 Water

Water

In This Chapter You Will Learn

Importance of water

Potable water and its need

Water - a compound

Hard water and soft water

Effect of water scarcity on plants

Sources of water and water cycle

Purification of water to obtain potable water

Properties of water

Water pollution and control

Water management

Molecular Formula

\(\text{H}_2\text{O}\), relative molecular mass-18 amu. One molecule of water contains two atoms of hydrogen and one atom of oxygen.

Chemical Name

Dihydrogen monoxide.

Table 5.1 - Percentage Proportion of Water in Living Things and Common Eatables

5.1 Importance of Water

Water is one of the most essential substance for the existence of life. Since life on the earth began in the oceans, and since no living thing can survive without water, it is rightly called the source of life.

Water forms a large part of the body mass of all living organisms - 90% of human blood is water. Water has the ability to dissolve a number of substances. Therefore, it serves as the liquid medium in which all reactions within the living body take place.

Fruits and vegetables contain water in them. Even dry-looking substances like wood, peas, beans, grams, etc., contain some amount of water.

Water is Important

1. For everyday uses like cleaning, bathing and washing.

2. For drinking in order to satisfy our biological needs.

3. For growing crops.

4. For various manufacturing processes.

5. For generating electricity.

6. For providing nutrients to aquatic plants and animals.

7. It also helps to control the earth's climate.

8. Water regulates body temperature by the process of sweating and evaporation.

9. Water provides medium for all biochemical reactions inside our bodies to take place.

10. It is essential for the germination of seeds, growth of plants and in photosynthesis.

11. Water inside the bodies of animals/humans dissolves minerals, gases and many products of digestion and carries them at place of need in the body.

Teacher's Note

Water is essential in our daily life for drinking, cooking, and hygiene, making its importance impossible to overstate.

5.2 Sources of Water

Water is widely distributed on the earth. It covers nearly three-fourths of the earth's surface.

Water exists in all the three states of matter, i.e., solid as ice, liquid as water and gas as water vapour.

Under ordinary conditions, water is a liquid, and in this state it is present in oceans, seas, rivers, lakes, ponds, etc. It is also present under the ground.

A considerable amount of water is found in the form of ice or snow in very cold regions of the world, viz. Arctic, Antarctica, and the high mountains. Frost and hail are the other forms of frozen water.

In the atmosphere, water is present in the form of water vapours, which condenses to form clouds, fog, mist, etc.

Sources of Water

Water sources are divided into three main categories:

Surface water (oceans and seas, rivers and lakes)

Underground water (springs, wells)

Above surface water (rainwater)

1. Oceans and Seas

Oceans and seas are the largest sources of water, covering nearly 71% of the earth's surface. They contain a very high proportion of dissolved substances, mainly common salt. Therefore, ocean and sea water are the most important sources of salt, but they are not fit for direct consumption.

2. Rivers and Lakes (These are called Surface Water Sources)

Water in rivers and lakes comes from rain and melting snow. It also contains dissolved impurities like salts, and suspended impurities like clay, sand, twigs and living organisms (germs). Rivers and lakes are the most suitable sources of water for domestic and industrial use. But this water needs to be purified before consumption, since it carries germs that can cause diseases.

3. Springs and Wells

These are sources of underground water. This is the water that percolates through the upper layers of the earth's surface and gets collected on the solid rocks beneath. This water is next only to rainwater in its purity. It contains dissolved impurities, but it has no suspended impurities or germs, since it is filtered through the different layers of soil.

The taste of spring water differs from place to place. Sometimes this water is rich in minerals, which have medicinal properties.

4. Rainwater

It is the purest form of natural water, and it is formed naturally by evaporation followed by condensation of water vapour in the atmosphere. Rainwater dissolves the gases present in the air, and also collects dust particles, as it falls. Some of the important gases dissolved in rainwater are carbon dioxide, oxygen, nitrogen and sulphur dioxide. Rainwater also carries smoke and germs present in the air. Therefore, the first shower of rain contains impurities, but later showers are free from impurities, and therefore safe for drinking.

A major portion of water on the earth is present in the form of snow/ice. Most of the fresh water is frozen in glaciers and polar ice caps.

Teacher's Note

The water cycle ensures that fresh water reaches us through rain, which we can collect and use for various purposes.

5.3 Water Cycle

Water is considered a renewable resource on the earth. Water is a substance that is capable of existing in all the three states of matter. From one state it changes into another rather easily.

The change of water from one state to another occurs all the time in nature. It involves the process of evaporation, condensation and precipitation (rainfall). This process of interconversion helps in balancing the amount of water both in the atmosphere and on the earth's surface, and is known as the water cycle.

When water vapours cool rapidly well below 0°C, they freeze into snow which falls as snow flakes.

Water is added to the Atmosphere by the following Natural Processes

1. Evaporation from oceans, seas, rivers, ponds and lakes takes place due to the heat of the sun.

2. Burning of most fuels, mainly fossil fuels.

\(2\text{C}_2\text{H}_{10} + 13\text{O}_2 \xrightarrow{\text{heat}} 8\text{CO}_2 + 10\text{H}_2\text{O} + \text{energy}\)

OR

\(\text{Fuel} + \text{Oxygen} \xrightarrow{\text{heat}} \text{Carbon dioxide} + \text{Water} + \text{Energy}\)

3. Release of water due to respiration by human beings, and animals \([\text{C}_6\text{H}_{12}\text{O}_6 (\text{glucose}) + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}]\)

4. Transpiration by plants.

Water that is released into the atmosphere by these processes rises up in the form of water vapours. These water vapours cool down as they rise, and form clouds in the upper regions of the atmosphere. Clouds contain tiny droplets of water. As they move up and further cool, these droplets cling to each other and form bigger drops, which fall down as rain. Rainwater gets collected again in seas, rivers, lakes, etc. This cycle goes on and on, therefore, it is called the water cycle.

Teacher's Note

The water cycle continuously recycles water in nature, ensuring that water is available for life on Earth in various forms.

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