CBSE Class 9 Answered

Acid rain is rain or any other form of precipitation that is unusually acidic. It has harmful effects on the environment and on structures. Acid rain is mostly caused by emissions due to human activity of sulfur and nitrogen compounds which react in the atmosphere to produce acids

The more accurate term is "acid precipitation."  Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are basic. "Clean" or unpolluted rain is slightly acidic, its pH being about 5.6, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid.

H₂O (l) + CO₂ (g) → H₂CO₃ (aq)

Carbonic acid then can ionize in water forming low concentrations of hydronium ions:

2H₂O (l) + H₂CO₃ (aq) ⇌ CO₃^2- (aq) + 2H₃O⁺(aq)