Combined Water

When their concentrated aqueous solutions are cooled or allowed to evaporate many substances separate out with combined water or so-called water of crystallisation. This latter term is intended merely to imply that the actual crystalline form and not the crystalline nature of the deposit is dependent on the combined water present, because generally the anhydrous substances are also crystalline. The simplest type of a substance with water of crystallisation is to be seen in the case of the crystalline compound chlorine hydrate, Cl₂.8H₂O, but the best known examples are amongst the salts, especially the "vitriols," CuSO₄.5H₂O; ZnSO₄.7H₂O; FeSO₄.7H₂O, and the alums, of which potassium alum, K₂SO₄.Al₂(SO₄)₃.24H₂O may be regarded as typical. Generally speaking, the presence of water of crystallisation is more common amongst the salts (simple and double) of the weaker acids. The number of molecules of water which thus combine with a molecule of a substance varies with the substance and even with the conditions such as the temperature of crystallisation; e.g. above 32.4° C., sodium sulphate crystallises from aqueous solution in the anhydrous condition, whereas below this temperature the crystals have the composition Na₂SO₄.10H₂O. Each salt containing water of crystallisation exerts a characteristic vapour pressure, which increases with rise of temperature. Such hydrated compounds can therefore be deprived of their water, in part or entirely, by raising the temperature or by placing them in an atmosphere containing less water-vapour than corresponds with the vapour pressure of the hydrate. By enclosing a hydrated salt, preferably mixed with a little of the dehydrated salt, in a desiccator over a drying agent, not only does dehydration take place, but it may occur in stages revealing the existence of intermediate hydrates; thus blue vitriol undergoes dehydration by the stages, CuSO₄.5H₂O; CuSO₄.3H₂O; CuSO₄. H₂O; CuSO₄, each stage being marked by a reduction in the vapour pressure at any one temperature.

If the vapour pressure of the water of crystallisation in a substance exceeds the pressure of the atmospheric moisture, spontaneous dehydration may occur so that the substance becomes coated with a powdery layer of a less hydrated form. Glauber's salt, Na₂SO₄.10H₂O, and "washing soda," Na₂CO₃.10H₂O, are typical of this class, to which the term "efflorescent" is applied. On the other hand, a hygroscopic substance is one which absorbs moisture from the atmosphere; if the aqueous vapour tension of the atmosphere is much greater than that of the damp substance, the absorption may continue until finally a solution is obtained, the process continuing until the vapour pressure of the solution attains that of the vapour in the atmosphere. Potassium carbonate, sodium nitrate, calcium chloride, and zinc chloride are common examples of such deliquescent substances.

Both sunlight and ultra-violet light accelerate the escape of water of crystallisation from salts in certain cases.

The transition temperature at which a hydrated salt liberates all or part of its water and passes into another less hydrated form is almost as definite as an ordinary melting-point, and can therefore be used as a standard for the calibration of thermometers; thus the transition temperature for Glauber's salt, Na₂SO₄.10H₂O, occurs at 32.383° C., whilst that of sodium bromide, NaBr.2H₂O, is at 50.674° C.

From the relative difficulty with which the last molecule of water of crystallisation is expelled from certain salts, e.g. CuSO₄. H₂O and MgSO₄.H₂O, the dehydration of these salts requiring a temperature above 200° C., whereas the other water molecules in the ordinary pentahydrate and heptahydrate respectively are eliminated at a temperature a little above 100° C., it has been suggested that this last molecule is in some way more intimately associated than the others with the salt molecule and the name water of constitution has been applied to it. By some, it is considered that this molecule may be especially associated with the acid radicle indicating, for example, in the above cases of monohydrated sulphates, that the salts are derived, not from sulphuric acid H₂SO₄, but from an orthosulphuric acid, H₄SO₅. On the other hand, it is equally possible that the molecule of water of constitution may be associated with the basic radicle, in which case the monohydrated salt would be regarded as a basic salt of the type Cu(OH)SO₄H, which would thus be copper hemisulphate hemihydrol. There hardly appear to be sufficient grounds, however, for regarding water of constitution as distinct from ordinary water of crystallisation; in both cases it is probable that the molecules of water arc attached to the salt molecules by additional valencies at the oxygen atoms, and, as explained above, the removal of each successive molecule of water of crystallisation will be a matter of increasing difficulty.

Interesting ideas as to the nature of water of crystallisation have been promulgated by A. Werner, who regards six molecules of water of crystallisation as the normal quantity, and suggests that the water is combined with the basic radicle forming a complex radicle, e.g. [Ca(OH₂)₆]Cl₂ and [Co(OH₂)₆]Cl₂. Certainly a group of six molecules of water of crystallisation recurs frequently in hydrated, salts.

In hydrated salts containing seven molecules of water, the sulphates supplying numerous examples, the seventh molecule is supposed to be attached to the acid radicle. This view receives confirmation in the frequency with which such heptahydrated sulphates form derived double sulphates containing only 6H₂O, the additional salt being a sulphate which, when alone, crystallises in the anhydrous condition. Thus ferrous sulphate, FeSO₄.7H₂O, and zinc sulphate, ZnSO₄.7H₂O, yield such derivatives as (NH₄)₂SO₄.FeSO₄.6H₂O, and K₂SO₄.ZnSO₄.6H₂O respectively. With salts containing more than seven molecules of water, the number is frequently twelve, and this is attributed to the water being combined in dihydrol molecules; and in accordance with this view potassium alum would be written [Al(H₄O₂)₆](SO₄)₂K.

The application of Bragg's X-ray spectrum analysis to hydrated crystals is throwing further light upon the manner of attachment of combined water, and, in many cases, it would appear that no distinction can be made between water of crystallisation and water of constitution.

For a study of the optical behaviour of combined water, the reader is referred to the subjoined references.

There is a possibility of small quantities of water being present in salts in another form than that of water of crystallisation. T. W. Richards, in his attempts to prepare salts in an extremely pure condition for the determination of atomic weights, has observed indications of the presence of water in a state of solid solution in crystals.