3. By the action of an acid on a sulphite. Sulphites are salts of sulphurous acid (H2SO3). 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
Na2SO3 + 2HCl = 2NaCl + H2SO3,
H2SO3 = H2O + SO2.
Explanation of the reaction. In this case we have two reversible reactions depending on each other. In the first reaction,
(1) Na2SO3 + 2HCl <--> 2NaCl + H2SO3,
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 H2SO3 is constantly diminishing, owing to the fact that it decomposes, as represented in the equation
(2) H2SO3 <--> H2O + SO2,
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.