SODIUM 517 



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of sodium sulphate there are only crystals of that heptahydrated salt 

 (Chapter I., Note 54), Na 2 SO 4 ,7H 2 O, which is formed from saturated 

 solutions, then saturation sets in when the solution has the follow- 

 ing composition per 100 parts of salt : at 19-6, at 10 30-5, at 20' 

 447, and at 25 52'9 parts of anhydrous salt. Above 27 the 

 heptahydrated salt, like the decahydrated salt at 34, splits up 

 into the monohyd rated salt and a saturated solution. Thus sodium 

 sulphate has three curves of solubility : one forNa 2 SO 4 ,7H 2 (fromO 5 

 to 26), one for Na 2 S0 4 ,10H 2 (from to 34), and one for 

 Na 2 S0 4 ,H 2 (a descending curve beginning at 26), because there are 

 three of these crystallo-hydrates, and the solubility of a substance 

 only depends upon the particular condition of that portion of it which 

 has separated from the solution or is present in excess. 8 



Thus solutions of sodium sulphate may give crystallo-hydrates of 

 three kinds on cooling the saturated solution : the unstable hepta- 

 hydrated salt is obtained at temperatures below 26, the decahydrated 

 salt forms under ordinary conditions at temperatures below 34, and 

 the monohydrated salt at temperatures above 34. Both the latter 

 crystallo-hydrates present a stable state of equilibrium, and the hepta- 

 hydrated salt decomposes into them, probably according to the equa- 

 tion 3Na 2 S0 4 ,7H 2 O = 2Na 2 S0 4 ,10H 2 O + Na 2 S0 4 ,H 2 0. The ordinary 

 decahydrated salt is called Glauber's salt. All forms of these crystallo- 

 hydrates lose their water entirely, and give the anhydrous salt when 

 dried over sulphuric acid. 9 



Sodium sulphate, Na 2 SO 4 , only enters into^ a few reactions of com- 

 bination with other salts, and chiefly with salts of the same acid > 



forming double sulphates. Thus, for example, if a solution of sodium 







8 From this example it is evident the solution remains unaltered until from the 

 contact of a solid it becomes either saturated or supersaturated, crystallisation being 

 determined by the attraction to a solid, as the phenomenon of supersaturation clearly 

 demonstrates. This partially explains certain apparently contradictory determinations 

 of solubility. The best investigated example of such complex relations is cited in 

 Chapter XIV., Note 50 (for CaCL). 



9 According to Pickering's experiments (1886), the molecular weight in grams (that 

 is, 142 grams) of anhydrous sodium sulphate, on being dissolved in a large mass of water, 

 at absorbs (hence the - sign) - 1,100 heat units, at 10- 700, at 15-275, at 20 

 gives out + 25, at 25 + 300 calories. For the decahydrated salt, Na 2 SO 4 ,10H 2 O, 

 6-4,225, 10-4,000, 15-8,570, 20-3,160, 25-2,775. Hence (just as in Chapter "l., 

 Note 56) the heat of the combination Na. 2 S0 4) 10H 2 at 5 =+3,125, 10 =+3,250, 

 20= +3,200, and 25= +3,050. 



It is evident that the decahydrated salt dissolving in water gives a decrease of tempera- 

 ture. Solutions in hydrochloric acid give a. still greater decrease, because they contain 

 the water of crystallisation in a solid state that is, like ice and this on melting absorbs 

 heat. A mixture of 15 parts of Na 2 S0 4 ,10H 2 O and 12 parts of 'strong hydrochloric acid' 

 produces sufficient cold to freeze water. During the treatment with hydrochloric acid 

 a certain quantity of sodium chloride is formed. 



