Consider the following

  1. Carbon monoxide
  2. Nitrogen oxide
  3. Ozone
  4. Sulphur dioxide

Excess of which of the above in the environment is/are cause(s) of acid rain?

(a) 1, 2 and 3

(b) 2 and 4 only

(c) 4 only

(d) 1, 3 and 4


The correct answer is (b) 2 and 4 only.

Nitrogen oxide (NOx) and sulphur dioxide (SO2) are the two oxides responsible for causing acid rain. Here’s an explanation of each option:

  • Carbon monoxide does not cause acid rain. It is a poisonous gas produced from the incomplete burning of carbon-containing fuels like gasoline and wood.
  • Nitrogen oxides (NOx) like nitrogen dioxide (NO2) react with water, oxygen and other chemicals to form nitric acid, one of the main causes of acid rain. The major source of NOx emissions is the burning of fossil fuels.
  • Ozone is not a direct cause of acid rain. It is formed when NOx and volatile organic compounds react in sunlight. Ozone causes respiratory problems but does not directly contribute to acid rain.
  • Sulphur dioxide (SO2) reacts with water, oxygen and other chemicals to form sulphuric acid, another major cause of acid rain. The burning of fossil fuels releases SO2 into the atmosphere.

Therefore, the oxides responsible for acid rain are nitrogen oxides (NOx) and sulphur dioxide (SO2).

Learn more

What is acid rain?

  • Acid rain refers to precipitation (rain, snow, sleet, hail, fog) that is more acidic than normal.
  • It is caused when sulphur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere and react with water, oxygen and other chemicals.
  • The main sources of SO2 and NOx emissions are burning of fossil fuels in power plants, vehicles, refineries and other industries.
  • Acid rain can have harmful effects on ecosystems, buildings, human health and visibility.

Effects of acid rain:

  • Makes lakes and streams acidic, harmful for aquatic life like fish and insects.
  • Leaches nutrients like calcium and magnesium from soil, harmful for trees and plants.
  • Erodes structures and buildings made of stone or metal.
  • Causes respiratory issues in humans when SO2 and NOx gases are inhaled.

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