56 PRINCIPLES OF CHEMISTRY. 



All the decompositions mentioned above are caused by the affinity 

 which the atoms of one substance have for atoms of another substance. 

 For instance : The decomposition of hydrochloric acid by sodium 

 may be explained by saying that sodium has a greater affinity for 

 chlorine than for hydrogen, as the latter is expelled by the sodium. 



No general rule can, however, be given for the intensity of affinity 

 with which the atoms of different elements attract each other, because 

 this attraction differs under different conditions. For instance: 

 Water passed in the form of steam over red-hot iron is decomposed, 

 iron oxide and free hydrogen being formed : 



Fe + H 2 O = FeO + 2H. 



This decomposition would indicate that the attraction between iron 

 and oxygen is greater than between hydrogen and oxygen. But in 

 passing free hydrogen over heated iron oxide the reverse action 

 takes place, water and free iron being formed : 



FeO + 2H = Fe + H 2 O. 



This reaction would indicate that the affinity between oxygen and 

 hydrogen is greater than between oxygen and iron. 



As a general rule it may be stated that the quantity of a product 

 formed by chemical action of two substances upon one another is 

 influenced by the relative proportions of the reacting substances. In 

 the above instance iron decomposes water when the iron is in large 

 excess, while a liberal supply of hydrogen causes the reverse action. 

 As a second instance may be mentioned the decomposition of sodiuin 

 nitrate by sulphuric acid, with the formation of sodium acid sulphate 

 and free nitric acid. On the other hand, sodium acid sulphate is 

 decomposed by a large excess of nitric acid into sodium nitrate and 

 free sulphuric acid. 



A consideration of this mass-action, as it is now termed, has led to 

 the establishment of the law, that* Chemical action is proportional to 

 the active mass of each substance taking part in the change, j 



While the power of affinity possessed by atoms or compounds does 

 not furnish us with data sufficient to predict all chemical changes, we 



y lay down a general rule which governs the decomposition of 



rtain compounds and which may be stated thus / When two (or 

 more) substances are brought together in solution, which substances by 

 any rearrangement of the atoms may form a product insoluble in the 

 quid present, this product will form and separate as a precipitate. 



As instances of this kind of decomposition may be mentioned the 

 formation of all the hundreds of insoluble metallic salts, which are 



