104 SMITH'S INTERMEDIATE CHEMISTRY 



the formula ZnS0 4 ,7H 2 O, which indicates at a glance the rela- 

 tionship of the substance to zinc sulphate (ZnSO 4 ). The hydrate 

 is made from zinc sulphate by adding water, and is easily decom- 

 posed into these two substances again. The reaction formula 

 hints at this familiar reaction. Note, however, that the comma 

 (,) does not indicate a mixture of the materials, such as ZnSOi 

 and H 2 0, but a single substance composed of both. The plus 

 (+) sign is used between the formulae of different, uncombined 

 substances in a mixture. 



In accordance with this plan, washing soda, hydrate of sodium 

 carbonate (p. 68), is written Na2CO 3 ,10H 2 O, and bluestone (p. 

 68), hydrate of cupric sulphate, CuS0 4 ,5H 2 O. 



Dissociation. A decomposition which, like that in p. 67, is 

 reversible is called a dissociation. When heat is the agent pro- 

 ducing the change, it is sometimes called a thermal dissociation. 

 Not all decompositions are reversible. Thus potassium chlorate 

 decomposes to give potassium chloride and oxygen, but these 

 products will not combine under any known conditions, directly, 

 to give potassium chlorate. 



Molecular Formulae. In this chapter, for the sake of 

 simplicity, we have so far left in the background the fact that the 

 formula must represent a molecular weight of the substance, as 

 well as its composition. The total weight, for which the symbols 

 in a formula stand, must be equal to the weight of the substance 

 occupying the gram-molecular volume. In other words ? the 

 formula-weight must represent one cube-full (Fig. 36, p. 74) of the 

 substance. This is true, as we have seen, of the formulae H 2 and 

 HC1 (see p. 74). 



In the cases of tin oxide (Sn0 2 ) and ferric oxide (Fe 2 Os), we have 

 substances which cannot be converted into vapor or dissolved, so 

 that their molecular weights are unknown. In such cases, we 

 use the simplest formula that will show the correct proportions. 



