The mixture is opalescent and, in reflected light, looks opaque-white; on somewhat prolonged standing a white ‹milk› is produced, but no precipitate. When the mixture is boiled, the same deep white milk is formed, but no coagulated precipitate, the mixture running unchanged through a filter. Hydrogen sulphide converts the mixture into a similar suspension of the black sulphide.
[227] The ratio is affected somewhat by the fineness of division of liquids and solids as a result of surface tension phenomena, as explained below.
[228] An aqueous solution of iodine and potassium iodide shaken with chloroform gives similar results, and the difference in color between the two layers is an advantage for a lecture experiment. But the iodine is partially combined with the iodide, according to KI + I_{ 2} ⇄ KI_{ 3}, or I^{ −} + I_{ 2} ⇄ I_{ 3}^{ −}, and the theoretical relations are not so simple as for bromine in aqueous and chloroform solutions.
[229] To accelerate the action, the mixture is shaken vigorously. After the separation of the layers, the bromine may be recognized in the aqueous layer by its color, or by the addition of potassium iodide (liberation of iodine).
[230] [Br]_{ aq.} and [Br]_{ ch.} are used to express the actual concentrations at any given moment we wish to consider, for instance at the moment equilibrium is reached. The ratio ‹k›_{ 2} / ‹k›_{ 1} for any substance S is found to be equal to the ratio of the ‹solubilities› of the substance in the two solvents. That it must be so can be proved by applying the law of physical equilibrium to the mixed solvents in contact with an excess of the substance, ‹i.e.› to its ‹saturated› solutions (see below).
[231] Similar relations hold for the condition of equilibrium between a liquid and its vapor.
[232] Curie, ‹Bull. Soc. Min.›, «8», 145 (1885); Ostwald, ‹Grundlagen der Anal. Chem.›, p. 22 (1894).
[233] Hulett, ‹Z. Phys. Chem.›, «37», 384 (1901). See also Ostwald, ‹ibid.›, «34», 495 (1900), on the solubilities of finely divided mercuric oxide ("yellow oxide") and of larger crystals ("red oxide").
[234] A 1 / 1000 solution of AuCl_{ 3}, 2 aq., and a 1.2 / 1000 solution of SnCl_{ 2}, 2 aq., may be used conveniently. When equal volumes of the solutions are mixed, the desired precipitate is formed.