Correct Answer - Option 1 : 5.6
Concept:
In clean air, rain water picks up some acidic oxides like CO2 and SO2 obtained from volcanic eruption. These substances make the rain slightly acidic with the pH of 5.6 to 6.
The technical definition of pH is a measure of the activity of the hydrogen ion (H+). It is essentially a measure of acidity. The pH scale ranges from 0 to 14. In the scale, the reading `7' denotes the neutral point. A substance can be rated acidic or basic depending on its pH value. It will be rated as acidic if it has a pH of less than 7 and basic if it is greater.
Normal rain water has a pH of 5.6 (slightly acidic). This is because it is exposed to the carbon dioxide in the atmosphere. The carbon dioxide gets dissolved in the rain water and forms carbonic acid.
Rain water with pH value below 5.6 is considered as acid rain. There are both natural and non-natural sources of materials that cause pH of rain water to change. The pH of rain water centres around this value then it is fit for drinking. Also, the pH of rain water differs from place to place. This is due to heavy pollution in one area and clean air in another.
Rainfall made so acidic by atmospheric pollution that it causes environmental harm, chiefly to forests and lakes. The main cause is the industrial burning of coal and other fossil fuels, the waste gases from which contain sulphur and nitrogen oxides which combine with atmospheric water to form acids.
But, when the rain falls through polluted air, it consumes oxides of sulphur (SO
x) and oxides of nitrogen (NO
x) mists of HCl etc. They make the rain more acidic called acid rain with the pH range 3.5 to 5.6.
