The Romance of Modern Chemistry

FROM SOLUTIONS TO CRYSTALS Crystallisation is a common laboratory operation, and is an efficient means of purifying salts and other sub- stances. This depends on the fact that when an impure material is dissolved in water and crystallisation is allowed to take place, the separated crystals are com- paratively free from impurities. These are found to have accumulated in the liquid which is alongside the crystals—the “mother liquor,” as it is called. If the crystals are redissolved, and the process of crystallisa- tion repeated, a still purer product is obtained. Some- times it is necessary to carry out a recrystallisation repeatedly in order to get absolutely pure material, and cases are on record in which the operation has been performed twenty to thirty times. The reader will perceive, therefore, that patience is an essential part of the chemist’s equipment. If a strong salt solution always behaved as it ought, and as we might reasonably expect it to behave, then when cooled to the temperature at which it is saturated, it would begin to deposit crystals. But just as there are not a few individuals who have a great reluctance to get out of bed when they ought to be up, so there are some salts which exhibit a curious hesitancy to leave the dissolved condition ; their solutions deposit no crystals even when cooled far below the saturation-point. In these circumstances we have what is known as a “ super- saturated ” solution. We do not require to go very far afield to find a salt which exhibits this curious inertia. Sodium thio- sulphate, better known, perhaps, as the “ hypo ” of the photographer, is a very good case in point. A strong solution of this substance may be made by nearly filling a flask with the crystals of the salt, adding a little 315