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. Rainwater: The purest form of natural water almost free from impurities is rainwater. Rainwater may dissolve oxygen, nitrogen & carbon dioxide gas forming weak acids. In industrial regions, nitrogen dioxide and sulphur dioxide evolved may dissolve in rainwater 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 enter into the river water. It contains impurities such as sand particles, 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. Seawater:The most impure form of natural water containing over 3% soluble salts including sodium chloride. It also contains salts of calcium & magnesium.