154 METALS AND THEIE COMBINATIONS. 



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 1 Bphukled upon pieces of calcium oxide, theTWoHSub- 

 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 



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. 



Experiment 23. Make lime-water according to directions given above. 



Calcium carbonate, Calcii carbonas praecipitatus, CaCO 3 = 

 1OO. Precipitated calcium carbonate is obtained as a white, taste- 

 less, neutral, impalpuble powder by mixing solutions of calcium 

 chloride and sodium carbonate : 



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



Experiment 24. Add to about 10 grammes of marble (calcium carbonate) in 

 small pieces, hydrochloric acid as long as effervescence takes place ; filter the 

 solution of calcium chloride thus obtained and add to it solution of sodium 

 carbonate as long as a precipitate is formed, collect the precipitate on a filter, 

 wash and dry it. 



Calcium sulphate, CaSO 1 = 136 (Dried gypsum, Plaster-of-Paris, 

 Calcined plaster). It has been mentioned above that the mineral 

 gypsum is native calcium sulphate in combination with 2 molecules 



