Sulphur Dioxide

Sulphur dioxide is a colourless gas, about 2.5 times as heavy as air, with a suffocating smell, faint sweetish odour.


Sulphur dioxide occurs in volcanic gases and thus traces of sulphur dioxide are present in the atmosphere. Other sources of sulphur dioxide is the combustion of the iron pyrites which are contained in coal. Sulphur dioxide also results from various metallurgical and chemical processes.

Preparation of Sulphur Dioxide

Sulphur dioxide is prepared by burning sulphur in oxygen or air.

		S   +   O2   ==>   SO2  

Sulphur dioxide is usually made in the laboratory by heating concentrated sulphuric acid with copper turnings.

	Cu   +   2 H2SO4   ==>   CuSO4   +   SO2   +   2 H2O    

Sulphur dioxide is also reduced when heated with mercury, silver or charcoal, C. Reduction with Sulphur, S, is very slow.

Sulphur dioxide is released by the action of acids on sulphites or acid sulphites (e.g. by dropping concentrated sulphuric acid into a concentrated solution of sodium hydrogen sulphite).

	NaHSO3  +  H2SO4  ==>  NaHSO4  +  SO2  +  H2O

Properties of Sulphur Dioxide

Sulphur dioxide is a colourless liquid or pungent gas, which is the product of the combustion of sulphur on air. Its melting point is -72.7 degC, its boiling point is -10 degC and its relative density is 1.43.

Sulphur Dioxide is an acidic oxide which reacts with water to give sulphurous acid.

		SO2   +   H2O   ==>   H2SO3     

Sulphur dioxide is a good reducing and oxidising agent.

The structural formula of sulphur dioxide is SO2, and it is a covalent, planer molecule which is v-shaped with an angle of 120 degrees.

Uses of Sulphur Dioxide

Sulphur dioxide is a reducing agent and is used for bleaching and as a fumigant and food preservative.

Large quantities of sulphur dioxide are used in the contact process for the manufacture of sulphuric acid.

Sulphur dioxide is used in bleaching wool or straw, and as a disinfectant.

Liquid sulphur dioxide has been used in purifying petroleum products.

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