The Halides

Compounds of fluorine, chlorine, bromine, and iodine are thus named. They fall into classes when the elements are arranged according to the periodic system. Taking the chlorides as typical of the halides, we have the following table :-

Besides these compounds, which present considerable regularity, others exist which have less claim to order. Thus, KI₃ is also known; it is unstable, but Csl₃ is relatively stable. Again, CuCl₂ and AuCl₃ exist, also HgCl. In the next group, GaCl₂, InCl, and InCl₂ are also known, as well as T1C1. The following group contains SnCl₂ and PbCl₂; PbCl₄ is very unstable. Besides VC1₅ and VC1₃, VC1₄ and VC1₂ are also known ; and in the next group, CrCl₃, MoCl₃, and MoCl₅, also WC1₅, UC1₃, and UC1₅. MnCl₂ is also known. These compounds are difficult to classify.

The bromides and iodides, as well as the fluorides, corresponding to many of these chlorides in formula, are also known. Where they are of special interest, they will be alluded to in the sequel.

The characteristic of the halides of the elements of the lithium group is that they are all soluble white salts, crystallising in cubes. In dilute solution they are all ionised, and even in strong solution a large percentage of ions are present. Hence they all react as metal ions and as halogen ions. Thus, for instance, with silver nitrate, which is the usual test for ionic chlorine, the following reaction takes place:- Na. Aq + C1-. Aq + Ag. Aq + -NO_g.Aq = Na. Aq + NO₃-.Aq + Ag-CI. Practically insoluble, and therefore practically non-ionised, silver chloride is precipitated, and free ions of sodium and the nitrate group remain in solution. If concentrated solutions are mixed, that portion which is ionised reacts; and as it is removed from solution, the originally non-ionised molecules of sodium chloride are ionised, because the solution becomes more dilute as regards sodium chloride, and they, too, enter into reaction. In a similar way, the alkali metal ions react in presence of a suitable reagent. Another point to be noticed is that these salts are not hydrolysed, that is, do not react with water to give hydroxide and acid to any appreciable extent, and the useful method of preparing them depends on these facts. They may all be obtained by the addition of halogen acids to the hydroxides or carbonates of the metals dissolved in water, thus : K.Aq + -OH.Aq + H.Aq + -Br.Aq = K.Aq + Br-Aq + H₂=0. It will be noticed that the water is not ionised, nor does it hydrolyse the potassium bromide; hence, on evaporation, as concentration increases, the number of ions of potassium and bromine becomes fewer and fewer, and after the water has been removed the pure dry salt is left. With a carbonate the action is similar. The equation is : 2Li.Aq + =CO_r Aq + 2H. Aq + 2-1.Aq = 2Li.Aq + 2-I.Aq + H₂=0 + 0=C=0. In dilute solution the acid H₂CO₃ would be liberated ; it is a very weak acid, i.e. it is comparatively very slightly ionised into 2H.Aq and =CO₃.Aq ; and, moreover, it readily decomposes into H₂O and CO₂; hence it is removed from the sphere of action as it is formed, and on evaporation the salt is left behind, as in the previous example.

Sodium and potassium chlorides occur in nature ; the former in the sea, which contains from 3.8 to 3.9 per cent. Deposits, which have undoubtedly been formed by the drying up of inland seas, are found in many places. At Stassfurth in S. Germany there are large deposits of all the salts present in sea-water, including common salt, chlorides and sulphates of magnesium, potassium, and sodium, and calcium sulphate; these have been deposited in layers in the order of their solubilities, the less soluble salts being deposited first. Bromides and iodides are also present in minute quantity in the residues from the evaporation of sea-water.

Solutions of the halides of the beryllium group of elements can also be made by acting on the hydroxides or carbonates of the metals with the halogen acid. To take barium chloride as an example, Ba. Aq + =CO₃. Aq + 2H. Aq -f 2-Cl.Aq = 2Ba.Aq + 2-Cl.Aq + H₂=0 + 0=C=0. Now barium carbonate is nearly insoluble in water, but the portion which dissolves is ionised; and, as explained above, when the portion which is ionised has reacted, its place is taken by more of the carbonate entering into solution; so that finally all is changed into chloride. With the hydroxides, the same kind of reaction takes place : Ca.Aq + 2-OH.Aq + 2H.Aq + 2-Br.Aq = Ca.Aq+ 2-Br.Aq + 2H₂=0. These salts are also white and soluble in water. There is, however, one exception, namely, calcium fluoride, CaF₂, which occurs native as fluor- or Derbyshire spar. It forms colourless cubical crystals, and is the chief compound of fluorine. It is produced by precipitation : Ca.Aq + 2-Cl.Aq + 2K.Aq + 2-F. Aq = Ca=F₂ + 2K. Aq + 2-Cl.Aq. The calcium fluoride is non-ionised, and comes down in an insoluble form.