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 compound. For example, blue vitriol is crystalline copper sulphate. Its symbol, CuSO4.5H1O, 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 lbs. of copper sulphate lose 7 lbs. on heating. What is the per cent of water of crystallization?

7 lbs. = amount lost

34 lbs. = whole amount

7 / 34 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%