Rain is the primary source of all water. A part of the rain water sinks into the ground to form ground water; part of it evaporates back into the atmosphere, and some runs off to form streams and rivers which flow ultimately into the sea. Some of the water in the soil is taken up by the plants and is evaporated in turn by the leaves, the process called "water cycle".
Characteristics of Rain Water:
Purest water in nature
Bright and sparkling
Chemically soft water containing only traces of dissolved solids (0.0005 per cent)
Being soft it has a corrosive action on lead pipes.
Free from pathogenic agents.
Purest rain water tends to become impure as it comes in contact with the atmosphere by picking up suspended impurities such as dust, soot, microorganisms and gases such as carbon dioxide, nitrogen, oxygen and ammonia. Gaseous sulphur and nitrogen oxides emitted from power plants also serve as source of contamination. By reacting with atmospheric water these gases form dilute solutions of sulphuric and nitric acids. The precipitation of these acids, referred to as acid rain, has serious impacts on the quality of surface water and has an impact on plants.