MAKING OF FORMULA AND EQUATIONS 103 



Again, iron and hydrochloric acid (HC1) give hydrogen and ferrous 

 chloride (FeCl 2 ) : 



Fe + 2HC1 -> H 2 + FeCl 2 . 



In the last two equations the variety of chemical changes called 

 displacement (p. 51), where one elementary substance displaces 

 another from a compound, is well illustrated. 



The equation for the formation of water by union of hydrogen 

 and oxygen, 



2H 2 + O 2 -> 2H 2 O, 



has already (p. 59) been discussed. The reduction of an oxide, 

 such as magnetic oxide of iron or cupric oxide, by hydrogen (p. 57), 

 gives the metal and water: 



Fe 3 4 + 4H 2 <=* 3Fe + 4H 2 0. 

 CuO + H 2 -> Cu + H 2 O. 



Upon examining these equations for reductions, we perceive that 

 they are illustrations of displacement also. 



Reversible Actions (p. 69), like the decomposition of water by 

 heating, and the recombination of the elements on cooling (p. 67), 

 are shown by using two arrows : 



2H 2 <= 2H 2 + O 2 . 



The equation may be read from either end. The decomposition 

 and formation of hydrates (p. 68) are also reversible actions. In 

 the case of zinc sulphate, the equation is 



ZnS0 4 + 7H 2 O <=> ZnSO 4 ,7H 2 O. 



Reaction Formulas. In the foregoing formula for the 

 hydrate of zinc sulphate, it will be seen that we do not add 

 together all the atoms of oxygen, and write ZnHi 4 SOn. The 

 latter would show the composition of the substance correctly, 

 but it would show nothing more. Now chemists find it convenient, 

 frequently, to alter the formula so that it shall indicate also some 

 important chemical property or reaction of the substance. Hence 



