Hydrates

Hydrates are compounds of water with elements or with other compounds. The term hydroxide is one which is sometimes used as a synonym of hydrate, and indeed it may be said that we have no certain means of distinguishing the one from the other. The distinction between the two is that in the hydrate the water is supposed to be present as water, and without any rearrangement of the molecules, while in the hydroxide the water is considered to have lost its identity, its constituent atoms having entered into new combinations. As a typical example of a hydrate we may instance crystallised sulphate of copper, $\backslash text\{CuSO\}\_4\; \backslash cdot\; 5\backslash text\{H\}\_2\backslash text\{O\}$, which contains the water so loosely combined that it is driven off by prolonged heating, and the white anhydrous sulphate, $\backslash text\{CuSO\}\_4$, is produced. Here the water is apparently present as water, and necessary to the crystalline form, and is therefore called water of crystallisation. When nitric anhydride, $\backslash text\{N\}\_2\backslash text\{O\}\_5$, unites with water it forms nitric acid, $\backslash text\{N\}\_2\backslash text\{O\}\_5\; \backslash cdot\; \backslash text\{H\}\_2\backslash text\{O\}$ or $\backslash text\{HNO\}\_3$, but this is not regarded as a hydrate, because the nitric acid cannot lose the water without also losing its characteristic properties. The whole question is full of difficulties, and is at present quite theoretical; different chemists using the terms above mentioned in different senses.

Hydraulic Cranes. See CRANES.