510 PRINCIPLES OF CHEMISTRY 



sulphate be mixed with a solution of aluminium, magnesium, or ferrous 

 sulphate, it gives crystals of a double salt when evaporated. Sulphuric 

 acid itself forms a compound with sodium sulphate, which is exactly 

 like these double salts. It is formed with great ease if sodium sulphate 

 is dissolved in sulphuric acid and the solution be evaporated. On 

 evaporation, crystals of the acid salt separate, Na 2 SO 4 + H 2 SO 4 

 = 2NaHSO 4 . This separates from hot solutions, whilst th'e crystallo- 

 hydrate NaHSO 4 ,H 2 O 10 separates from cold solutions. The crystals, 

 when exposed to damp air, split up into H 2 SO 4 , which deliquesces, and 

 Na 2 SO 4 (Graham, Rose) ; alcohol also extracts sulphuric acid from the 

 acid salt. This shows the feeble force which holds the sulphuric acid 

 to the sodium sulphate. 11 Both acid sodium sulphate and all mixtures 

 of the normal salt and sulphuric acid lose water when heated, and are 

 converted into sodium pyrosulphate, Na 2 S 2 O 7 , at a low red heat. This 

 anhydrous salt, at a bright red heat, parts with the elements of 

 sulphuric anhydride, the normal sodium sulphate remaining behind - 

 Na 2 S 2 O- = Na 2 SO 4 + SO 3 . From this it is seen that the normal salt 

 is able to combine with water, with other sulphates, and with sulphuric 

 anhydride or acid, &c. 116 



Sodium sulphate may, by double decomposition, be converted into 

 a sodium salt of any other acid, by means of heat and taking advantage 

 of the volatility, or by means of solution and taking advantage of the 

 different degree of solubility of the different salts. Thus, for instance, 

 owing to the insolubility of barium sulphate, sodium hydroxide or 

 caustic soda may be prepared from sodium sulphate, if barium hydroxide 

 be added to its solution, Na 2 SO 4 + Ba(HO) 2 = BaSO 4 + 2NaHO. And 

 by taking any salt of barium, BaX 2 , the corresponding salt of sodium 



10 The very large and well-formed crystals of this salt resemble the hydrate 

 H-,SO 4 ,H 2 O, or SO(OH) 4 . In general the replacement of hydrogen by sodium modifies 

 many of the properties of acids less than its replacement by other metals. This most 

 probably depends on the volumes being nearly equal. 



11 In solution (Berthelot) the acid salt in all probability decomposes most in the 

 greatest mass of water. The specific gravity (according to the determinations of 

 Marignac) of solutions at 15/4 = 9992 + 77'92^> + 0'231^ 2 (see Note 7). From these figures, 

 and from the specific gravities of sulphuric acid, it is evident that on mixing solutions of this 

 acid and sodium sulphate expansion will always take place; for instance, H 2 SOi + 25H 2 O 

 with Na2SO4 + 25H 2 O increases from 483 volumes to 486. In addition to which, in weak 

 solutions heat is absorbed, as shown in Chapter X. Note 27. Nevertheless, even more 

 acid salts may be formed. For instance, on cooling a solution of 1 part of sodium 

 sulphate in 7 parts of sulphuric acid, crystals of the composition NaHSO 4 ,H.>SO, 

 are separated (Schultz, 1868). This fuses at about 100 ; the ordinary acid salt, NaHS< ) , . 

 at 149. 



116 In order to demonstrate the weakness of the bond acting in sodium hydrogen 

 sulphate, NaHSO 4 , it is useful to remember that on decreasing the pressure this salt dis- 

 sociates much more easily than at the ordinary pressure ; it loses water and forms the 

 pyrosulphate, Na^O; ; this reaction is utilised in chemical works. 



