110 PRINCIPLES OF CHEMISTRY. 



water used then are the molecular weights of the two salts to each 

 other as is 4 to 7. 



In regard to this method of Raoult it should be stated that it is 

 applicable only to such substances as do not act chemically upon the 

 solvent used, and that the ratio of the lowering of the freezing-point 

 is not the same for all substances, but only for members of the same 

 class of substances. 



8. CHEMICAL EQUATIONS. TYPES OF CHEMICAL CHANGE. 

 REVERSIBLE ACTIONS AND CHEMICAL EQUILIBRIUM. 

 MASS ACTION. ACIDS, BASES, SALTS. RADICAL. CONSTI- 

 TUTIONAL FORMULAS. 



Chemical equations. We have seen that the composition of sub- 

 stances can be expressed by symbols or formulas, which show at a 

 glance the kind of elements and their proportions present in the sub- 

 stances. Similarly, a method of representing what takes place in a 

 chemical change concisely and in a way that can be quickly grasped 

 has been devised. This is done by means of chemical equations. 

 Such an equation is formed by writing the formulas of the substances 

 that react on the left of the sign of equality, and connecting them by 

 the sign +, and the formulas of the products of the reaction on the 

 right of the sign of equality, also connected by the -f sign. For 

 example, hydrochloric acid and silver nitrate mutually decompose 

 each other and give an insoluble white substance, silver chloride and 

 nitric acid. This may be represented thus, HC1 -f AgNO 3 = AgCl 

 -f HNO 3 . The + sign should be read and, and the = sign should be 

 read gives. A chemical equation has nothing in common with an 

 algebraic one, except its form. It cannot be factored, or in any way 

 be handled as an algebraic equation. It is simply a statement of facts 

 learned by experiment. Until we have learned beforehand the com- 

 position of the substances entering into chemical reaction and also of 

 the products formed, and the proportions involved, we have no basis 

 upon which we can legitimately write an equation. The unit of 

 chemical action is the molecule, and the chemical equations are intended 

 to show the action taking place between the molecules. Thus, in the 

 reaction above, one molecule of hydrochloric acid decomposes one 

 molecule of silver nitrate. We often, however, read an equation in a 

 broader and less definite manner, thus, in the above case, we say 

 hydrochloric acid decomposes silver nitrate and gives silver chloride 

 and nitric acid. When more than one molecule is represented, a 

 numeral is placed before the symbol. The symbols 2NaCl and Na 2 Cl 2 



