Q.What is the pH of rain water?

H2SO4 is the major acid present in acid rain.

Acid rain is caused mainly by SO2 and NOx released from burning fossil fuels. SO2 is oxidised in the atmosphere (helped by moisture and catalytic dust particles) to SO3, which dissolves in rainwater to form H2SO4. While HNO3 also contributes (from NOx), H2SO4 is typically present in the …

Normal rainwater has a pH of about 5.6, slightly acidic, because atmospheric CO2 dissolves in raindrops and forms carbonic acid (H2CO3).

As rain falls through the atmosphere, it dissolves ambient carbon dioxide:

CO2 + H2O <=> H2CO3

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Normal rain water has a pH of about 5.6 due to dissolved atmospheric CO2 forming carbonic acid.

Rain water is never perfectly neutral (pH 7) even without pollution, because as raindrops fall through the atmosphere they dissolve some of the CO2 present in air:

CO2 + H2O ⇌ H2CO3 (carbonic acid)

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