Note 38 — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

This reaction is taken advantage of in removing sulphurous anhydride from a mixture of gases. Lead dioxide, PbO2, is brown, and when combined with sulphurous anhydride it forms lead sulphate, PbSO4, which is white, so that the reaction is evident both from the change in colour and development of heat. Sulphurous anhydride is slowly decomposed by the action of light, with the separation of sulphur and formation of sulphuric anhydride. This explains the fact that sulphurous anhydride prepared in the dark gives a white precipitate of silver sulphite, Ag2SO3, with silver chlorate, AgClO4, but when prepared in the light, even in diffused light, it gives a dark precipitate. This naturally depends on the fact that the sulphur liberated then forms silver sulphide, which is black.