CHEMICAL EQUILIBRIUM 241 



gaseous. Thus, the compound (HC1) having been reduced in 

 concentration to the point of being removed entirely, there would 

 be no direct action to undo the work of the reverse action. The 

 whole chlorine would, therefore, soon have passed through the 

 form HC1. Hence, by another mechanical arrangement, an action 

 which ordinarily could progress to only 20 per cent would be 

 turned into a complete one : 



2C1 2 + 2H 2 ^ 2 + 4HC1 (+ CaCO 3 -> CaCl 2 + H 2 + CO 2 ). 



Reversibility Usually Avoided. In every-day chemical 

 work, since our object is usually to prepare some one substance, 

 chemists either avoid chemical changes which are notably rever- 

 sible, or adjust the conditions, as is done in the foregoing illus- 

 trations, so that the reverse of the action which they desire is 

 prevented. In consequence of this, when carrying out the direc- 

 tions for making familiar preparations, the fact that such actions 

 are reversible at all very readily escapes our notice. Arranging 

 the conditions so that the separation of a solid body by precipita- 

 tion, or the liberation of a gas, takes place, are the two commonest 

 ways of rendering a reversible action complete. Excellent exam- 

 ples of both of these are furnished by the chemical change used 

 in producing hydrogen chloride by the interaction of salt and 

 sulphuric acid, the full discussion of which (p. 127) should now be 

 studied attentively in the light of these explanations. 



Double decompositions between electrolytes in solution may 

 also be carried to completion by arranging that one of the products 

 of the reaction shall be a non-ionized, or practically non-ionized, 

 substance. Neutralization of an acid by a base is a case in point 

 (see p. 193). 



The Influence of Temperature on the Speed of any Reac- 

 tion. The activity of chemical change, and therefore the 

 speed of all chemical changes, is increased by raising the temper- 

 ature and diminished by lowering it (cf. p. 27). Different actions 



