Inorganic compound, heavy, colourless, poisonous gas (SO 2 ).
It has a pungent, irritating odour (the smell of a just-struck match). It occurs in volcanic gases and dissolved in the waters of some warm springs. Huge quantities are made industrially for use as a bleach, as a reducing agent, and as sulfites, which are food preservatives. It is a precursor of the trioxide (SO 3 ), used to make sulfuric acid . Sulfur dioxide is formed when sulfur-containing fuels are burned; in the atmosphere it can combine with water vapour to form sulfuric acid , a major component of acid rain .