ICSE Class 7 Chemistry Chapter 05 Water

Water

Next to air, water is the most essential substance for our survival, and indeed for life on earth. It makes up seven-tenths of our body.

In ancient times, it was considered an element, but now it is established that water is a compound of hydrogen and oxygen (combined in the mass ratio 1 - 8).

Sources Of Water

1. Large Water Bodies Water bodies like rivers, lakes, oceans and the polar ice caps constitute large reservoirs of water, the oceans being the largest. In fact, water covers three-fourths of the earth's surface.

2. Groundwater Groundwater is also an important source of water, and is pumped out when required.

3. Clouds Clouds are yet another source of water.

4. Fruits and Vegetables Water makes up about 80-90% of fruits and vegetables.

An Essential Part Of All Living Things

A large part of the body weight of all living beings is made up of water. All chemical reactions in living organisms take place in the presence of water. Water either takes part in a reaction of this kind or acts as a medium for it. This will be clear from the following examples.

1. Photosynthesis requires water as a reactant.

\[6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{sunlight}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\]

(carbon dioxide) (water) (chlorophyll) (glucose) (oxygen)

2. Complex molecules like proteins and vitamins are also synthesised in plants in the presence of water.

3. The digestion of food inside our body requires water as a medium.

4. The transport of some essential substances, like blood in animals or sap in plants, requires water as a medium.

5. Some organisms live in water. In fact, the earliest life-forms were probably simple organisms living in water.

Table - Some Important Uses of Water

Teacher's Note

Water is essential for life - we see this every day when we drink, cook, and clean. Understanding why water has these special properties helps students appreciate why life depends on it.

The Balance Of Water On Earth

Water is required in large quantities for agricultural, industrial and household purposes. Yet the total amount of water on our planet remains constant. Let us see how.

Large volumes of water evaporate from water bodies like rivers, lakes and seas into the atmosphere. The water vapours condense on dust particles and form clouds. The clouds drift from one place to another and finally precipitate in the form of rain or snow. A part of the rainwater seeps through the soil and the rest flows into rivers. On melting, some of the snow also seeps through the soil, while the rest flows into rivers.

The water vapours of the atmosphere also form snow over high mountains. And on melting, the mountain snow finds its way into rivers, which finally merge with seas or oceans.

In this way, water changes from one form to another continuously, but the total amount of water remains constant. This is called the water cycle.

Teacher's Note

The water cycle demonstrates how water moves between the earth and atmosphere. Students can observe this cycle during the rainy season and understand why water is recycled naturally.

Potable Water

Water used for drinking must be clean, i.e., free from the following substances.

1. Suspended particles like mud and sand (so the water must be transparent)

2. Excess of soluble salts

3. Pesticides and fertilisers that find their way to water bodies from farms

4. Any harmful chemicals disposed of by industrial units

5. Harmful microorganisms that cause water-borne diseases like typhoid, cholera and dysentery

Fresh groundwater is generally fit for drinking, but water directly taken from a river, lake or sea is not. You will learn later that the water of a river or a lake contains disease-causing substances. And sea water is salty, and hence not fit for drinking.

Water used for drinking is called potable water. (In Latin, pot means drink.)

How The Need For Potable Water Is Met

With the growing population of the world, the need for potable water is also increasing every day.

A part of the need is met by groundwater. But groundwater is not available in plenty at a reasonably small depth in every area. Moreover, when too much of it is pumped up, its level (called the water table) goes down. When this happens, no more water can be drawn from that source. Hence, water is mostly drawn from rivers and lakes and purified to make it potable.

Purification Of Water On A Large Scale

Water is purified on a large scale in establishments that are known as waterworks. Usually, the following steps are taken (Figure 5.1) to purify water.

1. Pumping Water is pumped from the source to large tanks in the waterworks.

2. Sedimentation Water is allowed to stand in large tanks, called settling tanks, for a few hours. Mud, sand and other suspended particles settle down. The addition of a small quantity of potash alum (K2SO4 - Al2(SO3)3 - 24H2O) helps the suspended particles to settle down quickly. The solid that settles at the bottom of the tank is called sediment. That is why these tanks are also called sedimentation tanks.

3. Filtration The water from the sedimentation tank is filtered through layers of sand and gravel. Filtration is also done through activated charcoal, which affords better-quality water. Activated charcoal is a special type of charcoal made by heating ordinary charcoal in vacuum. This charcoal holds the impurities strongly.

4. Chlorination The filtered water is now treated with chlorine gas. Chlorine kills germs - harmful bacteria - and destroys many other harmful substances present in water.

5. Supply Finally, the water is pumped into the supply system.

Remember that water is not supplied only for drinking, but for all household purposes.

Purification Of Water At Home

It is advisable that water meant for drinking be purified at home also. This is done by the following processes.

1. Boiling and Filtering Water is boiled to kill the microorganisms and allowed to stand for some time. Some solid may also settle down. The clear water is decanted off and filtered through ceramic candles. Ceramic candles have fine pores which allow only the water to pass through, but not the suspended particles.

2. Radiating Water With Ultraviolet Rays Ultraviolet lamps radiate high-energy rays which kill germs.

Nowadays, domestic water purifiers are available, which purify water in three stages. Water from the tap enters the purifier, where it gets filtered first through a candle and then through activated charcoal (carbon). The filtered water is finally irradiated by ultraviolet radiation to render it free from germs. This water is safe to drink.

3. Using Bleaching Powder Or Potassium Permanganate In rural and backward areas, water is lifted in buckets from wells. Well water gets contaminated, especially during rains, and it is generally disinfected by mixing some bleaching powder. The bleaching powder slowly reacts with water to give chlorine, which kills the germs.

\[\text{CaOCl}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{Cl} \uparrow\]

(bleaching powder) (water) (calcium hydroxide) (chlorine)

Well water is also disinfected by using potassium permanganate (KMnO4). The reaction is complicated.

Even after treating it with bleaching powder, the water should be boiled to make it potable.

Teacher's Note

Water purification at home and in waterworks involves multiple steps to ensure safety. Students can learn about this by observing local water treatment plants or demonstrating filtration at home.

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