(!(Conc’n H^{+})^{2} x Conc’n S^{—­})/Conc’n H_{2}S = Constant!, and a marked increase in the concentration of the H^{+} ions, such as would result from the addition of even a small amount of the highly ionized hydrochloric acid, displaces the point of equilibrium and some of the S^{—­} ions unite with H^{+} ions to form undissociated H_{2}S.  This is of much importance in studying the reactions in which hydrogen sulphide is employed, as in qualitative analysis.  By a parallel course of reasoning it will be seen that the addition of a salt of a weak acid or base to solutions of that acid or base make it, in effect, still weaker because they decrease its percentage ionization.

To understand the changes which occur when solids are dissolved where chemical action is involved, it should be remembered that no substance is completely insoluble in water, and that those products of a chemical change which are least dissociated will first form.  Consider, for example, the action of hydrochloric acid upon magnesium hydroxide.  The minute quantity of dissolved hydroxide dissociates thus:  Mg(Oh)_{2} <—­> Mg^{++} + 2Oh^{-}.  When the acid is introduced, the H^{+} ions of the acid unite with the Oh^{-} ions to form undissociated water.  The concentration of the Oh^{-} ions is thus diminished, more Mg(Oh)_{2} dissociates, the solution is no longer saturated with the undissociated compound, and more of the solid dissolves.  This process repeats itself with great rapidity until, if sufficient acid is present, the solid passes completely into solution.

Exactly the same sort of process takes place if calcium oxalate, for example, is dissolved in hydrochloric acid.  The C_{2}O_{4}^{—­} ions unite with the H^{+} ions to form undissociated oxalic acid, the acid being less dissociated than normally in the presence of the H^{+} ions from the hydrochloric acid (see statements regarding hydrogen sulphide above).  As the undissociated oxalic acid forms, the concentration of the C_{2}O_{4}^{—­} ions lessens and more CaC_{2}O_{4} dissolves, as described for the Mg(Oh)_{2} above.  Numerous instances of the applications of these principles are given in the Notes.

Water itself is slightly dissociated, and although the resulting H^{+} and Oh^{-} ions are present only in minute concentrations (1 mol. of dissociated water in 10^{7} liters), yet under some conditions they may give rise to important consequences.  The term !hydrolysis! is applied to the changes which result from the reaction of these ions.  Any salt which is derived from a weak base or a weak acid (or both) is subject to hydrolytic action.  Potassium cyanide, for example, when dissolved in water gives an alkaline solution because some of the H^{+} ions from the water unite with CN^{-} ions to form (HCN), which is a very weak acid, and is but very slightly dissociated.  Potassium hydroxide, which might form from the Oh^{-} ions, is so largely dissociated