The decomposition of sodium chloride by water, which one may predict on the basis of these theoretical considerations, may be demonstrated, slight as it is, by the following experiment.[367]
EXP. A pinch of sodium chloride is brought into a platinum crucible, which is previously heated in a blast lamp to a bright yellow heat (1100°); then 1 c.c. of water is introduced, drop by drop. A steam cushion is formed at once (Leidenfrost's phenomenon). After about half of the water has been evaporated (half a minute), the water is poured into a solution colored with blue litmus; it is changed to red by an excess of hydrochloric acid in the water. The crucible is cooled, and the salt remaining in it is dissolved in a little water and the solution poured into a red litmus solution; the latter turns blue.
The sodium chloride has obviously been partially decomposed, by the water, into its base and its acid; the decomposition is favored by the high temperature and by the fact that the hydrogen chloride [p180] formed can pass through the steam cushion into the water, while the sodium hydroxide is left behind. The removal of a product of the decomposition would favor its progress (see. p. 114).
The conclusions concerning salts of the type of sodium chloride may then be summarized in the statement, that ‹salts formed by the union of a very strong base with an equally strong acid are only very slightly decomposed by water and their solutions show a neutral reaction›.
The decomposition of a salt by water into its component base and acid is called ‹hydrolysis› and the salt is said to be ‹hydrolyzed› in the action.
«Action of Water on the Salt of a Strong Base with a Weak Acid.»—The relations are similar in principle, but quite different in degree and in net result, when the salt of a very strong base, combined with a weak acid, is dissolved in water. Potassium cyanide is a typical salt of this kind, and the study of its hydrolysis will illustrate the behavior of this class of salts. The hydrolysis takes place according to the equations
When the cyanide is dissolved in water, we must obtain, for the same reasons as were developed in the discussion of the hydrolysis of sodium chloride, a ‹little› nonionized potassium hydroxide, from the union of potassium ions with hydroxide ions, formed by the water. Potassium hydroxide being a strong, easily ionizable base, there will be only a ‹slight tendency› towards this union. Hydrocyanic acid, on the other hand, is an exceedingly weak acid. The value of its ionization constant K_{ HCN} = [H^{ +}] × [CN^{ −}] / [HCN] is only 7E−10, as compared with a similar ratio approximating 1 for potassium hydroxide ([K^{ +}] × [HO^{ −}] / [KOH] = 1; see the tables, p. 104 and p. 106 and see pp. 106–7). The hydrogen-ion, formed from the water, must therefore combine with cyanide-ion, ‹to form nonionized hydrocyanic acid›, much more completely than the hydroxide-ion combines with potassium-ion. With the disappearance of the ions of water, in this case notably of its hydrogen ions, more water must ionize to satisfy the ionization constant [p181] for water (p. 176), and the formation of hydrocyanic acid will continue, towards the satisfying of its own constant. It is important to note that, for the reasons given, the hydrogen-ion of water ‹is used up to a far greater extent› than is the hydroxide-ion; ‹the latter therefore accumulates›, and this accumulation results in the formation of smaller and smaller concentrations of the hydrogen-ion, by the water. Since [H^{ +}] × [HO^{ −}] = 1.2E−14 (at 25°; p. 104), as [HO^{ −}] grows larger, [H^{ +}] must grow ‹proportionally smaller›. The ‹suppression of the hydrogen-ion by the accumulation of the hydroxide ion› will, ultimately, make [H^{ +}] so small, that the equilibrium ratio [H^{ +}] × [CN^{ −}] / [HCN] will equal the equilibrium constant. Since the union of the hydrogen-ion with the cyanide-ion, to form little ionized hydrocyanic acid, is the main moving cause for the changes, the latter will then come to a standstill and equilibrium will be established. The net result of the action of water on potassium cyanide may be said to consist in the formation of practically nonionized hydrocyanic acid and the liberation of (chiefly) ionized potassium hydroxide, ‹until all the equilibrium constants› of the system are satisfied. We note that potassium cyanide solution must react strongly alkaline (‹exp.›) and that a free acid (‹e.g.› HCN) may well exist in the presence of a free base (‹e.g.› KOH), provided the acid is present in a nonionized, and therefore chemically inactive, condition (inactive as an ‹acid›).