ation: Fig. 42]

~Chemical properties.~ Sulphur dioxide has a marked tendency to combine
with other substances, and is therefore an active substance chemically.
It combines with oxygen gas, but not very easily. It can, however, take
oxygen away from some other substances, and is therefore a good reducing
agent. Its most marked chemical property is its ability to combine with
water to form sulphurous acid (H_{2}SO_{3}).

~Sulphurous acid~ (H_{2}SO_{3}). When sulphur dioxide dissolves in water
it combines chemically with it to form sulphurous acid, an unstable
substance having the formula H_{3}SO_{3}. It is impossible to prepare
this acid in pure form, as it breaks down very easily into water and
sulphur dioxide. The reaction is therefore reversible, and is expressed
by the equation

H_{2}O + SO_{2} <--> H_{2}SO_{3}.

Solutions of the acid in water have a number of interesting properties.

1. _Acid properties._ The solution has all the properties typical of an
acid. When neutralized by bases, sulphurous acid yields a series of
salts called _sulphites_.

2. _Reducing properties._ Solutions of sulphurous acid act as good
reducing agents. This is due to the fact that sulphurous acid has the
power of taking up oxygen from the air, or from substances rich in
oxygen, and is changed by this reaction into sulphuric acid:

H_{2}SO_{3} + O = H_{2}SO_{4},

H_{2}SO_{3} + H_{2}O_{2} = H_{2}S0_{4} + H_{2}O.

3. _Bleaching properties._ Sulphurous acid has strong bleaching
properties, acting upon many colored substances in such a way as to
destroy their color. It is on this account used to bleach paper, straw
goods, and even such foods as canned corn.

4. _Antiseptic properties._ Sulphurous acid has marked antiseptic
properties, and on this account has the power of arresting
fermentation. It is therefore used as a preservative.

~Salts of sulphurous acid,--sulphites.~ The sulphites, like sulphurous
acid, have the power of taking up oxygen very readily, and are good
reducing agents. On account of this tendency, commercial sulphites are
often contaminated with sulphates. A great deal of sodium sulphite is
used in the bleaching industry, and as a reagent for softening paper
pulp.

~Sulphur trioxide~ (SO_{3}). When sulphur dioxide and oxygen are heated
together at a rather high temperature, a small amount of sulphur
trioxide (SO_{3}) is formed, but the reaction is slow and incomplete.
If, however, the heating takes