Page 182 - ICSE Chemistry 8

moisture from salts. Drying agents (also called desiccants) come
in various forms and have a wide range of applica ons in food,
pharamaceu cal, packing and electronic industries. They help in
sustaining a state of dryness in their vicinity. You must have seen silica
gel packets in your newly purchased leather and electronic items.

It may also happen at mes that some substances on being exposed
to air lose the water of crystallisa on thereby reducing in weight
and becoming amorphous. These substances are called effl orescent
Fig. 8.8: Silica gel, a desiccant
substances and the phenomenon is called effl orescence. This is
.
observed in Na SO 10H O which loses its water of crystallisa on on being exposed to air. Similarly,
2
4
2
.
.
FeSO 7H O loses its water of crystallisa on when exposed to air and Na CO 10H O loses 9 molecules of
4
2
2
3
2
water of crystallisa on on being exposed to air.
Take a Break!
Match the columns.
Column A Column B
1. True solu on (a) Mud in water

2. Suspension (b) Makes it diffi cult to see through smoke or fog

3. Colloid (c) Calcium oxide
4. Tyndall eff ect (d) Saline water

5. Hygroscopic substance (e) Emulsion

CHEMICAL PROPERTIES OF WATER

Let’s study the chemical proper es of water.

Thinking Fountain
Nature Why is pure water a compound
and not a mixture?
Pure water is neutral. It has no eff ect on litmus solu on.
Stability

Water is highly stable. It does not decompose Know Your Scientist
easily. However, when heated above 2000°C
John Ri er (1776–1810) confi rmed the composi on
or when electric current is passed through
of water to be two parts hydrogen and one part
it, it decomposes into hydrogen and oxygen
oxygen in 1800. This composi on was confi rmed
gases. when the amount of hydrogen and oxygen produced
by the electrolysis of water were measured by Ri er.
2H O Electrolysis 2H 2 + O 2 Water’s composi on was discovered in 1781 by Henry
2
Water Hydrogen Oxygen
Cavendish.

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