SODIUM 509 



tin 1 temperature, whilst with respect to decahydrated salt, the solubility 

 rises with the temperature. So that if in the presence of a solution of 

 sodium suljthatc there be only crystals of that heptahydrated salt 

 (p. r>9) Na 2 SO. s ,7H. 2 O which is formed from saturated solutions, then 

 saturation sets in when the solution has the following composition per 

 100 parts of salt : at 19-6, at 10 30-5, at 20 44*7, and at 25 

 52'9 parts of anhydrous salt. Above 27 the heptahydrated salt, like 

 the decahydrated salt at 34, splits up into the monohydrated salt and 

 a saturated solution. Thus sodium sulphate has three curves of solu- 

 bility : one for Na 2 SO 4 ,7H 2 O (from to 26), one for Na 2 SO 4 ,10H 2 O 

 (from to 34), and one for Na 2 SO 4 ,H 2 O (a descending curve begin- 

 ning at 26), because there are three of these crystallo-hydrates, whilst 

 the solubility can only be referred to a definite state of a substance 

 which is present (or separated) in excess. 8 



Thus solutions of sodium sulphate give crystallo-hydrates of three 

 kinds on cooling the saturated solution : the unstable equilibrium of 

 the heptahydrated salt 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 SO 4 ,7H 2 O = 2Na a SO 4 ,10H 2 O + Na 2 SO 4 ,H 2 O. The ordi- 

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

 crystallo-hydrates entirely lose their water, and give the anhydrous salt 

 when dried over sulphuric acid. 9 



Sodium sulphate, Na 2 SO 4 , enters into only 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 that the phenomena of saturation are not able 

 to contribute much towards the understanding of solutions themselves. The solution 

 remains the same, but from the contact of a solid it becomes either saturated or super- 

 saturated, because crystallisation is determined by the attraction to a solid, as the 

 phenomenon of supersaturation clearly demonstrates. 



9 According to the demonstrations of Pickering (1886), the molecular weight in grains 

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

 water, at absorbs (therefore the sign) 1100 heat units, at 10 700, at 15 275, at 

 20 (gives out) + 25, at 25, + 800 calories. For the decahydrated salt Na, 2 SO 4 ,10H.,O, 

 5 -4225, 10 -4000, 15- 3570, 20- 8160, 25 -2775. Hence (just as in Chapter I. Note 56) 

 the heat of the combination Na^SO^lOEUO at 5= +8125,10 = +3250,20= +8200 

 and 25 = + 8050. 



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

 rature. Solutions in hydrochloric acid give a still greater decrease, because the water of 

 crystallisation is here taken in a solid state that is, like ice and on melting absorbs heat. 

 A mixture of 15 parts of Na->SO 4 ,10H.,O and 12 parts of strong hydrochloric acid produces 

 >utiicient cold to freeze water. During the treatment with hydrochloric acid a certain 

 quantity of sodium chloride is formed. 



