SODIUM 525 



Sodium carbonate, like sodium sulphate, loses all its water on 

 being heated, and when anhydrous fuses at a bright-red heat (1098). 

 A small quantity of sodium carbonate placed in the loop of a platinum 

 wire volatilises in the heat of a gas flame, and therefore in the furnaces 

 of glass works part of the soda is always transformed into the condition 

 of vapour. Sodium carbonate resembles sodium sulphate in its relation 

 to water. 18 Here also the greatest solubility is at the temperature of 

 37 , both salts, on crystallising at the ordinary temperature, combine 

 with ten molecules of water, and such crystals of soda, like crystals of 

 Glauber's salt, fuse at 34 Sodium carbonate also forms a super- 

 saturated solution, and, according to the conditions, gives various com- 

 binations with water of crystallisation (mentioned on page 108), <fec. 



A.t a red heat superheated steam liberates carbonic anhydride from 

 sodium carbonate and forms caustic soda, Na 2 CO 3 + H 2 O = 2NaHO 

 + CO 2 . Here the carbonic anhydride is replaced by water ; this depends 

 on the feebly acid character of carbonic anhydride. By direct heating, 

 sodium carbonate is only slightly decomposed into sodium oxide and 

 carbonic anhydride ; thus, when sodium carbonate is fused, about 

 1 per cent, of carbonic anhydride is disengaged. 19 The carbonates of 

 many other metals for instance, of calcium, copper, magnesium, iron, 

 &c. on being heated lose all their carbonic anhydride. This shows 



cold water, and then shake up with a strong solution of ammonia, pour off the residue, 

 and heat. The impurities will then remain in the mother liquors, &c. 



Some numerical data may be given for sodium carbonate. The specific gravity of the 

 anhydrous salt is 2'48, that of the decahydrated salt 1*46. Two varieties are known of 

 (he heptahydrated salt (Lb'wel, Marignac, Bammelsberg), which are formed together by 

 allowing a saturated solution to cool under a layer of alcohol ; the one is less stable (like 

 (he corresponding sulphate) and at has a solubility of 82 parts (of anhydrous salt) in 100 

 water ; the other is more stable, and its solubility 20 parts (of anhydrous salt) per 1QO of 

 water. The solubility of the decahydrated salt in 100 water = at 0, TO ; at 20, 21-7 ; .at 

 80, 87-2 parts (of anhydrous salt). At 80 the solubility is only 46'1, at 90 45'7. at J00, 

 45-4 parts (of anhydrous salt). That is, it falls as the temperature rises, like Na 2 SO 4 . 

 The specific gravity (Note 7) of the solutions of sodium carbonate, according to the data 

 of Gerlach and Kohlrausch, at 15/4 is expressed by the formula, s= 9,992 + 104'6p 

 + 0'165p 8 . Weak solutions occupy a volume not only less than the sum of the volumes 

 of the anhydrous salt and the water, but even less than the water contained in them. For 

 instance, 1,000 grams of a 1 p.o. solution occupy (at 15) a volume of 990-4 c.c. (sp. gr. 

 T0097), but contain 990 grams of water, occupying at 15 a volume of 990*8 c.c. A 

 similar case, which is comparatively rare occurs also with sodium hydroxide, in thoae 

 dilute solutions for which the factor A is greater than 100 if the sp. gr. of water at 4 = 

 100,000, and if the sp. gr. of the solution be expressed by the formula S=S * Ap + Bp*, 

 where S is the specific gravity of the water. For 6 p.c. the sp. gr. 15/4 = r0520 ; for 10 

 p.c. T1057; for 15 p.c. 1'1608. The changes in the sp. gr. with the- temperature are 

 here almost Che same as with solutions of sodium chloride with an equal value of p 



18 The resemblance is so great that, notwithstanding the difference in the molecular 

 composition of Na^SC^ and NaaCOg, they ought to be classed under the type (NaO) 2 E, 

 where R= 8O 2 or CO. Many other sodium salts also contain 10 mol. H 3 O. 



19 According to the observations of Pickering. According to Rose, when solutions of 

 eodium carbonate are boiled a-oertain amount of carbonic anhydride is disengaged. 



