Ordinary soap is formed by the chemical union of certain fatty substances with soda or potash. In tallow, which is used to make common hard soap, the fatty substances consist of stearin (a solid) and olein (a liquid). When making this soap, the tallow is dissolved in a weak solution of soda; the result being that soap and glycerine are produced. The soap is separated from the water by an addition of common salt, which is capable of performing this office, because soap is insoluble in a solution of salt. The soap is then removed from the "ley" and dried. Soft soap is made by dissolving a liquid fat (for instance, fish oil, which is largely composed of olein) in a solution of potash; the resulting products being potassium oleate, 4 to 5 per cent. of glycerine, and probably some free potash or some free oil. Economy being a matter of great importance in such an industry, it is natural to suppose that there would be nearly always an excess of free potash. As the addition of common salt to a solution of potassium oleate would cause the formation of ordinary hard (soda) soap and potassium chloride, salt cannot be used here for purposes of separation, and consequently the soft soap is obtained by evaporating the solution. Hence, soft soap contains glycerine and all the impurities that were in the pan. Yellow soap differs from ordinary white soap, by the fact of its possessing a considerable percentage of resin, which, by rendering it harder, and by decreasing its solubility in water, makes it more economical to use. Glycerine soap is a mixture of about equal parts of soap and glycerine.

The darkening effect which the continued application of soap has on leather, appears to be chiefly due to the action which the free alkali of the soap has on the tannic acid in the leather. The addition of either soda or potash to a solution of tannic acid turns it brown; the colour becoming deeper from exposure to the air. As soft soap contains more free alkali than ordinary white or yellow soap, its darkening effect on leather will be more marked.

The solution of soap in water causes its decomposition (hydrolysis) into its fatty acids and its alkali. "Its cleansing power is ordinarily explained by this reaction; but it is difficult to see why a solution which has just thrown off most of its fatty acids should be disposed to take up even a glyceride. It is more likely that the cleansing power of soap is due to the inherent property of its solution to emulsionise fats" (Encyclopoedia Britannica).