THE CARBONATES 109 



But by far the most abundant is the carbonate of calcium. This occurs in 

 chalk, limestone, marble, calc-spar, etc., substances which, though geologi- 

 cally different, are chemically identical, and consist of the nearly pure car- 

 bonate. From these minerals lime is obtained by " burning" in kilns, i.e. 

 by heating the carbonate by means of burning fuel, either placed beneath it 

 or mixed with it to dissociate it into lime, which remains in the kiln, and 

 carbonic acid gas, which escapes into the air, {Dissociation is reversible 

 decomposition. Thus the decomposition of chalk is dissociation because 

 the lime and carbonic acid gas recombine at a lower temperature. The 

 decomposition of red oxide of mercury is another instance of dissociation, 

 the chemical change being reversible.) 



Mortar and Cements. — Mortar is prepared by slaking quicklime, adding 

 enough water to make into a paste, and mixing with three or four times its 

 weight of sharp sand. It "sets" in a few days owing to evaporation 

 of water, and gradually increases in hardness owing to absorption of 

 atmospheric carbon dioxide and the formation of chalk. Portland and 

 Roman cement are prepared by grinding mixtures of chalk and clay (or 

 river mud) with water, drying the mixture, "burning" in kilns, and 

 powdering. Unlike mortar, these cements, after mixing with water, have 

 the power of setting under water, and are therefore called "hydraulic 

 mortars." The setting is due to the power of absorbing water possessed 

 by both quicklime and anhydrous silicate of aluminium and the production 

 of solid- hydrates. The dissolved carbonic acid in the water is also 

 gradually absorbed. 



If the foregoing determination shows that carbonate of 

 calcium is a compound containing lime and carbonic acid gas 

 in single combining proportions, it may be supposed that the 

 carbonate of sodium or potassium has a similar composition. 

 With a view to determine this, try whether these carbonates 

 are decomposed by heat, by heating strongly in test-tubes. If, 

 unlike the carbonates of other metals, those of the alkali metals 

 prove to be incapable of dissociation by heat, the method of 

 titration with an acid, using litmus as an indicator, can be 

 employed ; for soluble carbonates are, unlike most salts, 

 strongly alkaline to litmus. 



Weigh out two portions of pure carbonate of sodium, say 

 I gram each, into two flasks, dissolve in distilled water, render 



