Nitrogen dioxide and sulphur dioxide are water-soluble gases. They mix with water vapour present in the air and form nitric acid and sulphuric acid. These fall down to the earth along with rain. This rain is known as acid rain. Acid rain causes damage to trees, plants, buildings, etc.

Hydrocarbons are a class of organic compounds consisting entirely of hydrogen and carbon. Hydrocarbons are generally colourless and hydrophobic with only weak odours. Hydrocarbons are the principal constituents of petroleum and natural gas. They are used as fuels and lubricants as well as raw materials for the production of plastics, fibres, rubbers, solvents, explosives and industrial chemicals. Methane, ethane, propane, butane, etc., are examples of some common hydrocarbons.