Define specific heat capacity.

1. Water

Water is the most important resource for our survival next to air.

2. Composition of Water

Water contains hydrogen and oxygen as its constituent elements. Its molecular formula is ${\mathrm{H}}_2\mathrm{O}$.

3. Electrolysis

Water is broken down into its constituent elements by electrolysis. During electrolysis hydrogen and oxygen are obtained in the ratio $2:1.$

4. Properties of Water

(i) Water has a maximum density of $1 \mathrm{g}/\mathrm{cc}$ at $4°\mathrm{C}.$ At temperatures below and above $4°\mathrm{C},$ water has a density of less than $1 \mathrm{g}/\mathrm{cc}.$ This unique property of water helps in the survival of aquatic life in winters and summers.

(ii) Sea water contains many minerals and salts dissolved in it and so it is said to be saline.

(iii) Water freezes at $0°\mathrm{C}$ and boils at $100°\mathrm{C}.$

(iv) Water reacts with some reactive metals vigorously.

5. Water- A Universal Solvent

(i) Water is a universal solvent as it can dissolve many substances.

(ii) Water has dissolved gases which are used by aquatic life for respiration and photosynthesis.

6. Potable Water

Water that is used for drinking is called potable water.

7. Purification of Water

Water is purified by following steps:

(i) Sedimentation

(ii) Filtration

(iii) Sterilisation

8. Hardness of Water

Hardness of water is due to the presence of dissolved salts of calcium and magnesium.

9. Water Pollution

Water pollution is the result dumping untreated domestic solid waste and sewage, agricultural waste, industrial effluents into lakes, rivers, etc.