SODIUM 519 



which, on being considerably heated, very easily absorb carbonic 

 anhydride. 38 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 

 distinctions from it. Thus, for example, if a solution of sodium 

 carbonate be added to a normal magnesium salt a turbidity (precipi- 

 tate) is formed of magnesium carbonate, MgCO 3 . No such precipitate 

 is formed by the acid salt, because magnesium carbonate is soluble in 

 the presence of an excess of carbonic anhydride. 



Sodium carbonate is used for the preparation of caustic soda 23 

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

 with 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, 24 

 and boiled in a cast-iron, wrought-iron, or silver boiler (sodium 

 hydroxide does not act on these metals), and lime is added, little by 

 little, during the boiling. This latter is soluble in water, although 

 but very slightly. The clear solution becomes turbid on the addition 

 of the lime because a precipitate is formed ; this precipitate consists 

 of calcium carbonate, almost insoluble in water, whilst caustic soda 

 is formed and remains in solution. The decomposition is effected 

 according to the equation : Na. 2 CO 3 + Ca(HO). 2 =CaCO 3 + 2NaHO. On 

 cooling the solution the calcium carbonate easily settles as a precipitate, 



22 Crystalline sodium carbonate (broken into lumps) also absorbs carbonic anhydride, 

 but the water contained in the crystals is then disengaged: Na 2 CO3,10H 2 O + CO2 = 

 Na.jCO 3 , HoCO,-, + 9H..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 rapid generation of considerable quantities of 

 .carbonic anhydride in the domestic preparation of water charged with carbonic acid, for 

 the manufacture of digestive lozenges like those made at Essentuki, Vichy (pastilles 

 digestives de Vichy), &c. 



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

 guished from the word sodium, meaning the metal. 



24 With a small 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- 

 fluence of the mass of water is evident. According to Gerberts, strong solutions of 

 sodium carbonate are, however, decomposed by lime, which is very interesting if confirmed 

 by further investigation. 



