OXIDES OF SULPHUR
Sulphur forms two well-known compounds with oxygen: sulphur dioxide (SO2), sometimes called sulphurous anhydride; and sulphur trioxide (SO3), frequently called sulphuric anhydride.
Sulphur dioxide (SO2). Sulphur dioxide occurs in nature in the gases issuing from volcanoes, and in solution in the water of many springs. It is likely to be found wherever sulphur compounds are undergoing oxidation.
Preparation. Three general ways may be mentioned for the preparation of sulphur dioxide:
1. By the combustion of sulphur. Sulphur dioxide is readily formed by the combustion of sulphur in oxygen or the air:
S + 2O = SO2.
It is also formed when substances containing sulphur are burned:
ZnS + 3O = ZnO + SO2.
2. By the reduction of sulphuric acid. When concentrated sulphuric acid is heated with certain metals, such as copper, part of the acid is changed into copper sulphate, and part is reduced to sulphurous acid. The latter then decomposes into sulphur dioxide and water, the complete equation being
Cu + 2H2SO4 = CuSO4 + SO2 + 2H2O.