ic anhydride.

~Sulphur dioxide~ (SO_{2}). 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 = SO_{2}.

It is also formed when substances containing sulphur are burned:

ZnS + 3O = ZnO + SO_{2}.

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 + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.

3. _By the action of an acid on a sulphite._ Sulphites are salts of
sulphurous acid (H_{2}SO_{3}). When a sulphite is treated with an acid,
sulphurous acid is set free, and being very unstable, decomposes into
water and sulphur dioxide. These reactions are expressed in the
equations

Na_{2}SO_{3} + 2HCl = 2NaCl + H_{2}SO_{3},

H_{2}SO_{3} = H_{2}O + SO_{2}.

~Explanation of the reaction.~ In this case we have two reversible
reactions depending on each other. In the first reaction,

(1) Na_{2}SO_{3} + 2HCl <--> 2NaCl + H_{2}SO_{3},

we should expect an equilibrium to result, for none of the four
substances in the equation are insoluble or volatile when water is
present to hold them in solution. But the quantity of the H_{2}SO_{3} is
constantly diminishing, owing to the fact that it decomposes, as
represented in the equation

(2) H_{2}SO_{3} <--> H_{2}O + SO_{2},

and the sulphur dioxide, being a gas, escapes. No equilibrium can
therefore result, since the quantity of the sulphurous acid is
constantly being diminished because of the escape of sulphur dioxide.

~Physical properties.~ Sulphur dioxide is a colorless gas, which at
ordinary temperatures is 2.2 times as heavy as air. It has a peculiar,
irritating odor. The gas is very soluble in water, one volume of water
dissolving eighty of the gas under standard conditions. It is easily
condensed to a colorless liquid, and can be purchased in this condition
stored in strong bottles, such as the one represented in Fig. 42.

[Illustr