Answer. Burning of fossil fuels produces oxides of sulphur and nitrogen in air. Rain water converts SO2 into H2SO4 and NOX to HNO2 and HNO3. Normal rain water is weakly acidic because it consists of dissolved CO2 of the air. Its pH is about 5.6 to 6. But
rain water on dissolving air pollutants (acids) becomes more acidic and its pH reduces to 4. Thus, acid rain is formed on dissolving acidic air pollutants such as sulphur dioxide and nitrogen dioxide by rain water.