A colourless liquid or pungent gas, SO2, formed by sulphur burning in air; r.d. 1.43 (liquid); m.p. –72.7°C; b.p. –10°C. It can be made by heating iron sulphide (pyrites) in air. The compound is a reducing agent and is used in bleaching and as a fumigant and food preservative. Large quantities are also used in the contact process for manufacturing sulphuric acid. It dissolves in water to give a mixture of sulphuric and sulphurous acids. See also acid rain.