For acid rain, the pH of rain water should be less than
- A.7.0
- B.6.6
- C.5.6
- D.8.0
Correct answer
Explanation
The correct answer is C, 5.6. Rain is called acid rain when its pH falls below 5.6.
Even clean rain is slightly acidic, because carbon dioxide in the air dissolves in it and forms weak carbonic acid, which brings its pH down to about 5.6. When sulphur dioxide and nitrogen oxides from power plants, vehicles and industry mix with water vapour, they form sulphuric and nitric acids and push the pH lower still. Such rain damages buildings (the Taj Mahal marble), soils, lakes, fish and forests.
A is wrong because pH 7.0 is neutral, and normal rain is already below it.
B is wrong because rain at pH 6.6 is less acidic than ordinary rain, so it is not acid rain.
D is wrong because pH 8.0 is basic, not acidic.
Exam tip: normal rain pH about 5.6 (due to CO₂); acid rain below 5.6, caused mainly by SO₂ and NOx.
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