SODIUM 527 



solution of the latter gives crystals of the acid salt if carbonic 

 anhydride be passed through it. The acid salt may be yet more 

 conveniently formed from effloresced crystals of sodium carbonate, 

 which, on being considerably heated, very easily absorb carbonic anhy- 

 dride. 22 The acid salt crystallises well, but not, however, in such large 

 crystals as the normal salt ; it has a brackish and not an alkaline taste 

 like that of the normal salt ; its reaction is feebly alkaline, nearly neutral. 

 At 70 its solution begins to lose carbonic anhydride, and on boiling the 

 evolution becomes very abundant. From the preceding remarks it is 

 clear that in most reactions this salt, especially when heated, acts 

 similarly to the normal salt, but has, naturally, some distinction from 

 it. Thus, for example, if a solution of sodium carbonate be added to a 

 normal magnesium salt, a turbidity (precipitate) is formed of magnesium 

 carbonate, MgCO 3 . No such precipitate is formed by the acid salt, 

 i>ecause magnesium carbonate is soluble In the presence of an excess of 

 carbonic anhydride. 



Sodium carbonate is used for the preparation of caustic soda 

 that is, the hydrate of sodium oxide, or the alkali which corresponds 

 to sodium. For this purpose the action of lime on a solution of 

 sodium carbonate is generally made use of. The process is as follows : 

 a weak, generally 10 per cent., solution of sodium carbonate is taken,* 4 



the latter. The solubility of the bi- or acid salt varies with considerable regularity , 

 100 parts of water dissolves at 15 9 parts of the salt, at 30 11 parts. 



The ammonium, and more especially the calcium, salt, is much more soluble in water. 

 The ammonia process (see p. 524) is founded upon this. Ammonium bicarbonate (acid 

 carbonate) at has a solubility of 12 parts in 100 water, at 80 of 27 parts. The solu- 

 bility therefore increases very rapidly with the temperature. And its saturated solution 

 is more stable than a solution of sodium bicarbonate. In fact, saturated solutions of 

 these salts have a gaseous tension like that of a mixture of carbonic anhydride and water- 

 namely, at 15 and at 50, for the sodium salt 120 and 750 millimetres, for the ammonium 

 salt 120 and 563 millimetres. These data are of great importance in understanding the 

 phenomena connected with the ammonia process. They indicate that with an increased 

 pressure the formation of the sodium salt ought to increase if there be an excess of ammo- 

 nium salt. 



82 Crystalline sodium carbonate (broken into lumps) also absorbs carbonic anhydride^ 

 but the water contained in the crystals is then disengaged: Na^COgjlOELjO + COj 

 = Na-jCOs, HijCOg + 9H 2 O, and dissolves part of the carbonate ; therefore part of the sodium 

 carbonate passes into solution together with all the impurities. When it is required to 

 avoid the formation of this solution, a mixture of ignited and crystalline sodium carbonate 

 is taken. Sodium bicarbonate is prepared chiefly for medicinal use, and is then often 

 termed carbonate of soda, also, for instance, in the so-called soda powders, for preparing 

 certain artificial mineral waters, for the manufacture of digestive kfzenges like those 

 made at Essentuki, Vichy, &c. 



83 In chemistry, sodium oxide is termed ' soda,' which word must be carefully distin- 

 guished from the word sodium, meaning the metal. 



M With a email quantity of water, the reaction either does not take place, or even 

 proceeds in the reverse way that is, sodium and potassium hydroxides remove carbonic 

 anhydride from calcium carbonate (Liebig, Watson, Mitscherlich, and others). The in- 



