(2) H2SO4 + 2H = H2SO3 + H2O,
(3) H2SO3 = H2O + SO2.
By combining these equations the following one is obtained:
Cu + 2H2SO4 = CuSO4 + SO2 + 2H2O.
4. Action on salts. We have repeatedly seen that an acid of high boiling point heated with the salt of some acid of lower boiling point will drive out the low boiling acid. The boiling point of sulphuric acid (338°) is higher than that of almost any common acid; hence it is used largely in the preparation of other acids.
5. Action on water. Concentrated sulphuric acid has a very great affinity for water, and is therefore an effective dehydrating agent. Gases which have no chemical action upon sulphuric acid can be freed from water vapor by bubbling them through the strong acid. When the acid is diluted with water much heat is set free, and care must be taken to keep the liquid thoroughly stirred during the mixing, and to pour the acid into the water,—never the reverse.
Not only can sulphuric acid absorb water, but it will often withdraw the elements hydrogen and oxygen from a compound containing them, decomposing the compound, and combining with the water so formed. For this reason most organic substances, such as sugar, wood, cotton, and woolen fiber, and even flesh, all of which contain much oxygen and hydrogen in addition to carbon, are charred or burned by the action of the concentrated acid.
Salts of sulphuric acid,—sulphates. The sulphates form a very important class of salts, and many of them have commercial uses. Copperas (iron sulphate), blue vitriol (copper sulphate), and Epsom salt (magnesium sulphate) serve as examples. Many sulphates are important minerals, prominent among these being gypsum (calcium sulphate) and barytes (barium sulphate).
Thiosulphuric acid (H2S2O3); Thiosulphates. Many other acids of sulphur containing oxygen are known, but none of them are of great importance. Most of them cannot be prepared in a pure state, and are known only through their salts. The most important of these is thiosulphuric acid.
When sodium sulphite is boiled with sulphur the two substances combine, forming a salt which has the composition represented in the formula Na2S2O3: