. Elementary physics and chemistry: first stage . Fig. 85.âAn eight-sided crystal of alum. (From a photograph by Mr. H. E. Hadley.) How Crystals can be made.â Warm water when saturated with any soluble substance, as you learnt in a previous lesson, often contains more of the solid dissolved than an equal quantity of a cold saturated solution. The con- sequence of this is, that if you allow a warm saturated solution to get cold, the water can no longer keep all the substance in solution, and it separates out in the solid state, which, under these cir- cumstances, always takes a crystalline char
. Elementary physics and chemistry: first stage . Fig. 85.âAn eight-sided crystal of alum. (From a photograph by Mr. H. E. Hadley.) How Crystals can be made.â Warm water when saturated with any soluble substance, as you learnt in a previous lesson, often contains more of the solid dissolved than an equal quantity of a cold saturated solution. The con- sequence of this is, that if you allow a warm saturated solution to get cold, the water can no longer keep all the substance in solution, and it separates out in the solid state, which, under these cir- cumstances, always takes a crystalline character. The crystals of alum, formed in this way, generally have eight sides, or the shape of the crystal is the same as the solid called the octahedron, shown in Fig. 85. But in some cir- cumstances the crystals only have six sides, or are cubes. Some Crystals contain V/ater.âBy heating a crystal of alum or iDlue vitriol in a clean dry test-tube, it is easy to show that they both contain water. This water is given off in the form of steam, which condenses into drops of water on the cold upper part of the tube. There are many other crystals besides those of alum and blue vitriol which also contain water. This water, which is contained in some crystals, is known as â water of crystallisation. It is necessary for these crystals to have this water in them to form the regular shape of which you have learnt. If the water of crystallisation is got rid of they become powdery. Some coloured crystals not only lose their shape but also their colour when the water of crystallisation is driven out. Other crystals again, if simply exposed to the air, lose this water and become powdery. Such crystals are said to be efflorescent, and crystals of soda are a good example (Figs. 86 and 87). Other substances do just the opposite thing and take up more water from the air, becoming very moist. These are called deliquescent. The white residue obtained by evaporating the solution formed when marbl
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