THE SULPHATES 73 



XVIII. THE SULPHATES 



The sulphates of other bases can be prepared Hke sulphate 

 of soda by neutralising sulphuric acid with the basic oxide or 

 its hydrate. Many of these sulphates occur in nature, or are of 

 importance in commerce. Procure and examine the following :— 



Sulphate of potash .... KgO.SOg. 



Sulphate of soda (Glauber's salt) . Na^.SOg; loHp. 



Sulphate of magnesia (Epsom salt) . MgO.SOg; 7H2O. 



Sulphate of lime (gypsum, selenite) . CaO.SOgj 2H2O. 



Sulphate of baryta (heavy spar) . BaO.SO^. 

 Sulphate of alumina and potash 



(potash alum) . . AI2O3.3SO3; K2O.SO3; 24H,0. 



Note that these salts are crystalline. Expose a weighed 

 crystal of sulphate of soda to the air ; observe that it effloresces 

 and loses its crystalline form and decreases in weight. Heat a 

 crystal in a test-tube ; note that it liquefies, water vapour escapes, 

 and the salt finally dries up as a white amorphous {i.e, form- 

 less) mass. The crystalline salt is therefore a hydrate of sul- 

 phate of soda. Many other salts combine with water forming 

 crystalline compounds. This water of crystallisation is generally 

 loosely combined, sometimes escaping at the ordinary tempera- 

 ture in dry air, and usually at 100° C, but occasionally needing 

 a much higher temperature for complete dehydration. 



Determine the water of crystallisation of gypsum, or its crys- 

 talline form, selenite. Heat a weighed quantity (say .5 gram) of 

 the finely powdered material in a weighed porcelain capsule in the 

 water oven at a temperature of 100° C. until no further appreci- 

 able loss of weight occurs. (Before each weighing the capsule 

 must be cooled in the dessicator.) Having obtained the exact 

 weight, raise the temperature to a low red heat until the weight 

 is again constant. The residue is anhydrous sulphate of lime. 

 Calculate the percentage composition of the original gypsum, 



