CHEMICAL EQUATIONS, TYPES OF CHEMICAL CHANGE, ETC. Ill 



represent the same number of atoms and the same proportions by 

 wciuht of the elements, but they are quite different, for NaCl is the 

 actual size of the molecule, and 2NaCl stands for two molecules, while 

 Xa.,CU represents a molecule double the size of that of sodium chloride, 

 Na( -1, as actually known. Often in writing equations, for convenience 

 we represent elements in the atomic state, while in reality they exist 

 in the molecular state. The equations are true, however, as far as 

 proportions are concerned. For example, we represent the union of 

 hydrogen and oxygen to form water thus, H 2 + O = H 2 O, but to be 

 in keeping with the fact that molecules of hydrogen and oxygen are 

 really involved, we should write 2H 2 -|- O 2 = 2H 2 O. 



Every correct chemical equation is correct mathematically also 

 i. e.y the sum of the atoms as well as that of the molecular weights of 

 the factors equals the sum of the atoms and that of the molecular 

 weights of the products respectively. For instance : Sodium car- 

 bonate and calcium chloride form calcium carbonate and sodium 

 chloride. Expressed in chemical equation we say : 



NaaCOs + 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 : 



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 105.31 parts by 

 weight of sodium carbonate are acted upon by 110.16 parts by weight 

 of calcium chloride, and that 99.35 parts by weight of calcium car- 

 bonate and 116.12 parts by weight of sodium chloride are formed by 

 tins action. These data may, of course, be utilized to 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 



