54 PRINCIPLES OF CHEMISTRY. 



The formula more correctly written would be : 

 2HC1 + 2Na = 2NaCl -f 2H. 



3. HC1 -f AgNO 3 = AgCl + HNO S . 



Hydrochloric Nitrate of Chloride of Nitric acid, 



acid. silver. silver. 



This form of decomposition, known as double decomposition, 

 is one of the most common kinds of chemical changes met 

 with in chemical operations. 



4. H 2 S + 30 = H 2 + SO 2 . 



Hydrosulphuric Oxygen. Water. Sulphur 



acid. dioxide. 



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 the 

 hydrochloric acid by sodium must be explained by saying, that 

 sodium has a greater affinity for chlorine than hydrogen, as the 

 latter is expelled by the sodium. 



!No general rule can, however, be given for the amount 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, oxide of iron 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 oxide of iron the re- 

 verse action takes place, water and free iron being formed : 



FeO -f 2H = Fe -f H 2 0. 



This reaction would indicate that the affinity between oxygen 

 and hydrogen is greater than that between oxygen and iron. 

 Many similar instances are know r n and will be spoken of later. 



Chemical reactions. This expression is used for any chemical 

 change^brought about with the intention of studying the nature 

 of a substance. The expression reagent is applied to those sub- 

 stances used for bringing about such changes. 



