What is acid rain? Discuss its harmful effects. 

When fossil fuels are burnt, a large amount of CO₂ enters the atmosphere. The gaseous CO, dissolves in water droplets to produce carbonic acid (weakly acidic).

Rainwater normally is not acidic but the weak carbonic acid can give water a pH value of about 5.6. If no other acid comes in contact with water, the pH of the water should remain 5.6 and 7.0. But actually acidity of rain water has increased significantly in many regions of the words. This is due to a large quantity of SO₂ and oxides of nitrogen present in the air. These gases react to produce HNO₃ and H₂SO₄.

Nitric acid and sulphuric acid are brought down from the atmosphere during rain/snow and can lower the pH of the water to 4 or even less that that.
Harmful effects of acid rain:
(i) Acid rain damages iron and steel structures.
(ii) Acid rain causes extensive damage to buildings and sculptural materials made of marble, limestone, slate, mortar etc.

Taj mahal is threatened by acid vapour from Mathura Refinery:
(iii) Acid rain increases the acidity of water in the lakes which are lethal for the fishes. Thus some of the lakes have become fishless and rain water has greatly affected the fish population. 
(iv) Acid rain damages leave of trees and plants and retards the growth of forests.