68 PRINCIPLES OF CHEMISTRY. 



bonate and calcium chloride form calcium carbonate and sodium 

 chloride. Expressed in chemical equation we say : 



Na,CO s + CaCl 2 = CaCO 3 + 2NaCl. 



Sodium carbonate and calcium chloride are the factors, calcium car- 

 bonate and sodium chloride the products. Adding together the 

 molecular weights of the factors and those of the products we find 

 equal quantities, as follows : 



2Na = 46 Ca = 40 Ca = 40 2Na = 46 



C =12 2C1 = 71 C =12 2C1 = 71 



3O =48 3O =48 



106 + 111=217 100 + 117=217 



Chemical equations not only are 'used for representing chemical 

 changes, but also are the starting-point in all the chemical calcula- 

 tions in which the quantities of substances entering into chemical 

 actions, or the quantities of the product formed, are concerned. 



The above calculation teaches, for instance, that 106 parts by 

 weight of sodium carbonate are acted upon by 1 1 1 parts by weight of 

 calcium chloride, and that 100 parts by weight of calcium carbonate 

 and 117 parts by weight of sodium chloride are formed by this action. 

 These data may, of course, be utilized 1o find how much calcium 

 chloride may be needed for the decomposition of one pound or of any 

 other definite weight of sodium carbonate ; or how much of these two 

 substances may be required to produce one hundred pounds, or any 

 other definite weight, of calcium carbonate. 



While in many cases of chemical decomposition the change which is 

 to take place cannot be foretold, but has to be studied experimentally, 

 there are other chemical changes which can be predicted with certainty 

 (see Chapter 8, page 56). In the latter case especially there is no 

 difficulty in writing out the change in the form of an equation. In 

 doing this it must be borne in mind that equivalent quantities replace one 

 another; that, for instance, two atoms of a univalent element are 

 required to replace one atom of a bivalent element, as, for instance, in 

 the case of the decomposition taking place between potassium iodide 

 and mercuric chloride, when two molecules of the first are required to 

 decompose one molecule of the second compound : 



K I TT W /C1 TT/I K Cl 



K I M S\C1 : ***\I K Cl 

 or 



2KI + HgCl 2 = HgI 2 + 2KC1. 



