Lime-Water

Lime-water is the name given to the solution in water of slaked lime, called in chemical language hydrate of lime, calcium hydrate, and calcium hydroxide, Ca(OH)₂. To prepare it, quicklime, which has been made by burning (as it is commonly called) a pure marble, or, preferably, Iceland spar, is slaked with distilled water. The calcium hydrate formed is placed in a wide-mouth stoppered bottle, and covered with several times its bulk of distilled water. The object of this treatment is to dissolve soda and some other soluble impurities, the major part of which will be removed when the watery liquid in the bottle is decanted from the undissolved excess of calcium hydrate which should then be again covered with distilled water which has been recently boiled. The stopper should be well ground and smeared with vaseline. The bottle should be shaken at intervals in order that the water may take up as much calcium hydrate as it can dissolve. After all, this amount is very small, not exceeding, at 15° C, 0.172 part by weight per hundred measures of lime-water. Thus a gallon of lime-water, saturated at about 60° F., could not contain more than 120 grains of calcium hydrate, corresponding to 90 grains of pure lime or calcium oxide, CaO. In ordinary practice such a perfectly saturated solution is not attainable, while the most carefully prepared and strongest solution is sure to become weakened each time the stopper of the containing vessel is withdrawn by the removal of some of the lime in solution in the form of carbonate of lime.

The clearest lime-water, from this cause and from its action on glass, always appears turbid after a time.

Although so dilute a solution, lime-water gives the most marked reactions of an alkali: it turns red litmus paper blue, embrowns yellow turmeric paper, and imparts a crimson hue to colourless phenolphthalein paper. It acts energetically upon many organic and some inorganic pigments, owing to its alkaline or basic properties. The ease with which the lime in lime-water unites with carbonic acid, forming carbonate of lime (= calcium carbonate), and the bearing of this action, and of other properties of caustic lime upon the materials and processes of painting are discussed in Chapters II. and XXIII.