State the sources of water (a) on the earth’s surface (b) below the earth’s surface.

(a) Sources of water on the earth’s surface are — 

1. Snow, frost — Snow and frost are the natural forms of water present. They are found in the solid state. 

2. Rain water — The purest form of natural water almost free from impurities is rain water. Rain water may dissolve oxygen, nitrogen & carbon dioxide gas forming weak acids. In industrial regions, nitrogen dioxide and sulphur dioxide evolved may dissolve in rain water forming nitric acid & sulphuric acid which causes acid rain. 

3. River water — It is one of the impure forms of natural water since most forms of surface water enters into river water. It contains impurities such as sand partides, organic matter, bacteria, mineral salts which dissolve after soil erosion & dissolved gases. 

4. Lake water — It is another impure form of natural water which also contains impurities and other soluble salts. 

5. Sea water — The most impure form of natural water containing over 3% soluble salts including sodium chloride. It also contains salts of calcium & magnesium. 

(b) Sources of Water below the earth’s surface are — 

1. Well water — Above the impervious rocky layers, of the earth’s surface is well water. which contains soluble impurities. 

2. Spring water — Natural water accumulated above the rocky layers of the earth which forcefully comes out under pressure from the earth’s surface is spring water & contains soluble salts & minerals.