278 METALS AND THEIR COMBINATIONS. 



line taste and reaction; exposed to the air it gradually absorbs 

 acid among acids, and is used directly or indirectly in many branches 

 of chemical manufacture. 



Calcium hydroxide, Calcium hydrate, Ca(OH) 2 (Slaked lime). 

 When water is sprinkled upon pieces of calcium oxide, the two sub- 

 stances combine chemically, liberating much heat; the pieces swell 

 up, and are converted gradually into a dry, white powder, which is 

 the slaked lime. When this is mixed with water, the so-called milk 

 of lime is formed. 



Freshly slacked lime, made into a thin paste with water and mixed with 3 

 to 4 times as much sand as lime used, forms the ordinary mortar, employed for 

 building purposes. The hardening of mortar is due first to loss of water, fol- 

 lowed by a gradual conversion of calcium hydroxide into carbonate. In the 

 course of years calcium silicate is also formed. 



Lime-water, Liquor calcis (Solution of lime). This is a sat- 

 urated solution of calcium hydroxide in water : 10,000 parts of the 

 latter dissolving about 15 to 17 parts of hydroxide. In making 

 lime-water, 1 part of calcium oxide is slaked and agitated occasionally 

 during half an hour with 30 parts of water. The mixture is then 

 allowed to settle, and the liquid, containing besides calcium hydroxide 

 the salts of the alkali-metals which may have been present in the 

 lime, is decanted and thrown away. To the calcium hydroxide left, 

 and thus purified, 300 parts of water are added and occasionally 

 shaken in a well -stoppered bottle, from which the clear liquid may 

 be poured off for use. 



Lime-water is a colorless, odorless liquid, having a feebly caustic 

 taste and an alkaline reaction. When heated to boiling it becomes 

 turbid by precipitation of calcium hydroxide (or perhaps oxide) which 

 re-dissolves when the liquid is cooled. Carbon dioxide causes a pre- 

 cipitation of calcium carbonate, soluble in an excess of carbonic acid. 



Experiment 27. Make lime-water according to directions given above. 



Calcium carbonate, Calcii carbonas praecipitatus, CaCO 3 = 

 99.35. Precipitated calcium carbonate is obtained as a white, taste- 

 less, neutral, impalpable powder by mixing solutions of calcium 

 chloride and sodium carbonate: 



CaCl 2 + Na^CO, = 2NaCl + CaCO 3 . 



