Calcium Hydroxide, Ca(OH)₂

Amorphous quicklime reacts with water with considerable rise of temperature and yields calcium hydroxide, Ca(OH)₂, as a dry powder of rather more than twice its volume. The process is known as " slaking." The molecular heat of solution of calcium oxide in water is 18.12 Cal. The heat of solution of calcium hydroxide is 2.79 Cal., and therefore the heat of formation from calcium oxide and water is 15.33 Cal.

When the oxide has been strongly calcined the rate of reaction with water is very much less. Moissan obtained a sample of fused lime which was practically unaffected by twenty-four hours' immersion in water, although after seventy-two hours complete transformation into hydroxide had taken place. There is also a similar reluctance to react with dilute acids. The velocity of reaction is further affected by the presence of foreign substances. Hydrochloric and nitric acids, ammonium, calcium, and barium chlorides, acetic acid, lactic acid, and alcohol, behave as accelerators, and boric acid, caustic potash and soda, calcium chromate, potassium dichromate, aldehyde, glycerol, cane sugar, and grape sugar, as retarders. It is easy to understand the increase of rate of slaking in presence of an acid, which will tend to bring the lime into solution, and the retardation by an alkali, which tends to diminish the solubility; but the retarding action of sugar and glycerol, for example, which dissolve the lime, is not so readily explained. It might possibly be due to the fact that sugar and glycerol are highly hydrated in solution, thus diminishing considerably the concentration of free water in the liquid, and conceivably the rate of reaction also.

The presence of a large quantity of water also reduces the velocity of reaction, probably by keeping the temperature down.

The hydroxide formed by the slaking of lime is a white amorphous powder of density 2.078. It is also obtained as an amorphous precipitate by the action of an alkali on a soluble calcium salt. Since the hydroxide dissolves in water with the evolution of heat, the solubility diminishes with increase of temperature. By heating to 80° C. a solution saturated in the cold, the hydroxide crystallises out in small transparent hexagonal prisms of density 2.236. The same crystals may also be obtained by evaporating the hydroxide solution under reduced pressure at 28°-30° C.

By calcination the anhydrous calcium oxide may be again obtained.

The dissociation temperatures for different pressures have been determined.

Temperature, °C.	369	389	408	428	448	468	488	507	527	547
Mm. mercury	9.2	17.4	31.5	55	92	149	234	355	526	760

Discordant values for the solubility of lime in water have been found. According to Lamy, the value depends on the temperature of preparation and source of the original calcium oxide, and on the method of slaking. The following values have been obtained for the solubility of a lime prepared by igniting in a platinum dish, in a muffle, a very pure specimen of calcite: The solubility is reduced by the presence of alkali hydroxide to a greater extent than is accounted for by electrolytic dissociation. This has a bearing on the causticising of alkali carbonates by calcium hydroxide, this reaction being more complete in dilute than in concentrated solutions. The presence of alkali chlorides increases the solubility, except in concentrated solutions. In the case of ammonium chloride there is probably a complex salt formed, 2NH₄Cl.Ca(OH)₂ or Ca(NH₃)₂Cl₂.2H₂O.

Calcium hydroxide is also soluble in glycerol and in sugar solution. The crystalline hydrate is less soluble in water and sugar solution than the amorphous form.

The electrical conductivity of calcium hydroxide solutions has been determined.

A transparent cryohydrate is obtained by freezing lime and water. On melting, this deposits elongated hexagonal plates, or small rhombic plates of the semihydrate, 2Ca(OH)₂.H₂O, less soluble than the simple hydroxide and very unstable, passing into the amorphous form when only slightly heated. Seliwanov considered that the amorphous hydroxide is a polymeride of Ca(OH)₂ containing not-fewer than four atoms of calcium in the molecule.

By slaking the oxide with excess of water at 60° C. and then cooling, a hydrate, Ca(OH)₂.H₂O, is obtained. This compound loses water even in the cold and is completely converted into Ca(OH)₂ at 70° C. According to recent work, however, this hydrate does not exist.

By continued addition of water to slaked lime, " milk of lime," a suspension of lime in water, is obtained. Kosmann supposes that a succession of hydrates is thus produced in the order, Ca(OH)₂, HCa(OH)₃, H₂Ca(OH)₄, H₃Ca(OH)₅, H₄Ca(OH)₆, H₅Ca(OH)₇, H₆Ca(OH)₈, and H₇Ca(OH)₉. Kohlschutter and Walther, on studying the rate of sedimentation of calcium hydroxide in water, concluded that, in the formation of a true solution, an intermediate colloidal state is first obtained. Two varieties of colloidal calcium hydroxide have been obtained from a calcined dolomite.

Calcium oxide forms compounds with the alcohols which have been described as ethyl, propyl, amyl, butyl, and glyceryl alcoholates respectively. Compounds with mannitol are also obtained. According to de Forcrand these are addition compounds and not true alcoholates - for example, 4C₂H₅OH.3CaO. The latter is also formed by the action of absolute ethyl alcohol on calcium carbide in a sealed tube at 180° C. Calcium hydroxide forms several crystalline addition compounds with phenol, consisting of one molecule of hydroxide with 2, 4, and 6 molecules of phenol respectively. The first also forms a series of hydrates of composition 2Ca(OH)₂.4C₆H₅OH.(2n. + 1)H₂O, where "n" may have any value from 0 to 4. Thymol gives similar compounds. With nitro-phenols, however, nitro-phenates are formed owing to the greater acidity due to the presence of the nitro-group.