Water : Efflorescence

Definite solid chemical compounds of salts with water are called hydrates. White anhydrous copper sulphate at once becomes blue if water is poured on it, and heat is evolved. On cooling a hot solution of copper sulphate in water, deep blue crystals of the hydrate, CuSO₄,5H₂O, called blue vitriol, separate out. If exposed to dry air in a desiccator over sulphuric acid, they fall to a nearly white powder of the monohydrate, CuSO₄,H₂O, which again becomes blue when moistened with water.

Some crystalline hydrates lose water and fall to powder on exposure to the atmosphere. This change, called efflorescence, shows that there must be a pressure of water vapour over the salt, and this is confirmed by passing a crystal of the salt above the mercury in a barometer tube when the mercury falls slightly. The vapour pressure may be measured in this way: it is found to be constant at a given temperature, and to increase with the temperature, in the same way as the vapour pressure of a liquid.

The system has two components, viz., anhydrous salt and water. Since the vapour pressure depends only on the temperature there is only one degree of freedom; hence the Phase Rule, P+F=C + 2, shows that the number of phases is: 2+2-1=3. These are water vapour and too solids. One solid is the original hydrated salt; the second is either the anhydrous salt, if this is produced directly by loss of water, as is the case with Glauber's salt: Na₂SO₄,10H₂O = Na₂SO, + 10H₂O (vap.) or a lower hydrate, as is the case with copper sulphate: SO₄,5H₂O = CuSO₄,3H₂O + 2H₂O.

When the vapour pressure above the hydrated salt at the ordinary temperature is greater than the partial pressure of water in the atmosphere, the salt will lose water on exposure and will effloresce. If, on the other hand, the pressure over the salt is not greatly different from that of atmospheric moisture, the crystals will be stable on exposure. Blue vitriol does not effloresce on exposure, since the vapour pressure over its crystals at 25° is only 7.92 mm., whilst the partial pressure of atmospheric moisture, usually about two-thirds the saturation pressure at the given temperature, is about 15 mm. If the vapour pressure over the hydrate is very small, it may absorb moisture from the air. Ordinary granular calcium chloride used for drying gases, CaCl₂,2H₂O, has a very small vapour pressure, and absorbs moisture from gases, forming CaCl₂,6H₂O.

The name efflorescence first denoted the "creeping" of solutions of salts in open vessels, with the formation of a loose deposit on the sides of the vessel. This action is no doubt caused by the formation of crystals on the walls at the surface of the liquid, which then rises by capillary action between the crystals or between these and the wall of the vessel. More crystals are thus formed above the first layer, and so on, until in some cases the solid reaches to the top of the vessel, when the solution may siphon over and creep down the outside of the vessel. This occurs with salammoniac solutions in Leclanché cells and may be prevented by greasing the upper part of the jar.

Protein crystallography
Chemistry guide
Water
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