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.
Thus solutions of sodium sulphate may give crystallo-hydrates of three kinds on cooling the saturated solution: the unstable heptahydrated 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 heptahydrated salt decomposes into them, probably according to the equation 3Na2SO4,7H20 = 2Na2SO4,10H2O + Na2SO4,H2O.
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, Na2SO4, only enters into a few reactions of combination with other salts, and chiefly with salts of the same acid, forming double sulphates.
Thus, for example, if a solution of sodium 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 when sodium sulphate is dissolved in sulphuric acid and the solution evaporated. On evaporation, crystals of the acid salt separate, Na2SO4 + H2SO4 = 2NaHSO4.
This separates from hot solutions, whilst the crystallo-hydrate, NaHSO4,H2O,10 separates from cold solutions. The crystals when exposed to damp air decompose into H2SO4, which deliquesces, and Na2SO4 (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, Na2S2O7, at a low red heat.12
This anhydrous salt, at a bright red heat, parts with the elements of sulphuric anhydride, the normal sodium sulphate remaining behind—Na2S2O7 = Na2SO4 + SO3.
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.
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, Na2SO4 + Ba(HO)2 = BaSO4 + 2NaHO.
And by taking any salt of barium, BaX2, the corresponding salt of sodium may be obtained, Na2SO4 + BaX2 = BaSO4 + 2NaX. Barium sulphate thus formed, being a very sparingly-soluble salt, is obtained as a precipitate, whilst the sodium hydroxide, or salt, NaX, is obtained in solution, because all salts of sodium are soluble. Berthollet's doctrine permits all such cases to be foreseen.