PHYSICO-CHEMICAL PROCESSES 147 



there is a tendency to exclude from the solution all matter 

 not homogeneous with it, that is, all matter not of the same 

 kind. If a concentrated solution which is impure is allowed 

 to crystallize, the impurities may become enclosed or en- 

 tangled among the forming crystals. This is undesirable 

 and can be prevented by stirring the solution while crystal- 

 lization is taking place. Thus the formation of the very 

 fine crystals, called ''crystal meal," is caused. These fine 

 crystals may be washed free from the "mother liquor" 

 (the liquor from which the impurities are obtained), and may 

 be cleansed of all impurities. 



161. Water of Crystallization. A great many compounds 

 crystallize very easily, and are sold in a crystallized form. 

 In crystallizing they take up more or less water from the 

 solutions and this water forms a definite part of the com- 

 pound. For example, blue vitriol is crystalline copper sul- 

 phate. Its symbol, CuSO 4 .5H 2 O, means that crystalline 

 copper sulphate contains 5 molecules of water. Merchants, 

 in purchasing chemicals, desire them in the crystalline form 

 as this form is considered the purest. Oftentimes compounds 

 are sold on the basis of their dry weight, i.e., the weight of 

 the substance minus the weight of the water. 



The method of figuring the dry weight is as follows : 

 Assume that 34 Ibs. of copper sulphate lose 7 Ibs. on heating. 

 What is the per cent of water of crystallization? 



7 Ibs. = amount lost 



34 Ibs. = whole amount 



g 7 T or .205 of the whole was lost 



As it is customary to express the loss in per cent, the loss is : 

 .205 X 100 = 20.5% 



