Now, the very slight ionization of mercuric cyanide enables us to realize, in the following experiment, the case where an exceedingly weak acid without the formation of any precipitate involving physical constants, may displace a much stronger acid from its salts. Hydrocyanic acid is one of the weakest acids (table, p. 104), the constant for the ratio [H^{+}] × [CN^{−}] / [HNC] being 0.7E−9. It is so weak an acid that the addition of a dilute solution to methyl orange will not redden the indicator, but will have only a barely perceptible effect on it (‹exp.›). Mercuric chloride solutions also are almost neutral to methyl orange (‹exp.›) (very slight decomposition of the salt by water makes the solution very slightly acid, not enough to produce more than an orange color with methyl orange). Now, mercuric chloride, while it is not very easily ionized, is, we found, very much more readily ionized than is mercuric cyanide. The consequence is that when we add hydrocyanic acid to a mercuric chloride solution, the equilibrium between mercuric chloride and its ions and between hydrocyanic acid and its ions will be decidedly displaced, ‹the mercuric-ion combining with the cyanide-ion to form the scarcely ionizable mercuric cyanide›. As a result, more and more of the molecular mercuric chloride and hydrocyanic acid will be ionized; and since the other ions, the chloride and the hydrogen ions, form a readily ionizable electrolyte, hydrochloric acid, ‹these ions› (H^{+} and Cl^{−}) ‹will accumulate in the solution› and we shall have sufficient ‹ionized hydrochloric acid› liberated to make the solution decidedly acid. EXP. When the two solutions described above are mixed, a strongly acid solution, colored a bright pink, results. In the following equations the dark arrows indicate the direction in which the action goes when the solutions are mixed: HgCl_{2} ⥂ Hg^{2+} + 2 Cl^{−} 2 HCN ⥂ 2 CN^{−} + 2 H^{+} 2 CN^{−} + Hg^{2+} ⥂ Hg(CN)_{2} 2 Cl^{−} + 2 H^{+} ⇄ 2 HCl FOOTNOTES: [166] ‹Concentrations are usually measured in moles or gram-molecular weights per liter, and a gram-molecular or molar weight of a compound is its molecular weight expressed in grams.› Hence the number of grams of a given substance in a liter divided by its molecular weight represents its concentration. [167] Nernst, ‹Theoretical Chemistry›, 423, 433; Ostwald, ‹Lehrbuch›, II_{2}, 104, etc., 296; Walker, ‹Introduction to Physical Chemistry› (1909), 259, etc. [168] Not every collision of a molecule of ‹A› with one of ‹B› is supposed to result in a chemical interaction, but the number of collisions with such a result is considered to be directly proportional to the total number of collisions. Van 't Hoff, ‹Lectures on Physical Chemistry›, I, 104.