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~ (H_{2}S_{2}O_{3}); ~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 Na_{2}S_{2}O_{3}: Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}. The substance is called sodium thiosulphate, and is a salt of the easily decomposed acid H_{2}S_{2}O_{3}, called thiosulphuric acid. This reaction is quite similar to the action of oxygen upon sulphites: Na_{2}SO_{3} + O = Na_{2}SO_{4}. More commonly the salt is called sodium hyposulphite, or merely "hypo." It is a white solid and is extensively used in photography, in the bleaching industry, and as a disinfectant. ~Monobasic and dibasic acids.~ Such acids as hydrochloric and nitric acids, which have only one replaceable hydrogen atom in the molecule, or in other words yield one hydrogen ion in solution, are called monobasic acids. Acids yielding two hydrogen ions in solution are called dibasic acids. Similarly, we may have tribasic and tetrabasic acids. The three acids of sulphur are dibasic acids. It is therefore possible for each of them to form both normal and acid salts. The acid salts can be made in two ways: the acid may be treated with only half enough base to neutralize it,-- NaOH + H_{2}SO_{4} = NaHSO_{4} + H_{2}O; or a normal salt may be treated with the free acid,-- Na_{2}SO_{4} + H_{2}SO_{4} = 2NaHSO_{4}. Acid sulphites and sulphides may be made in the same ways.