DECOMPOSITION OF COMPOUNDS. 53 



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. DECOMPOSITION OF COMPOUNDS. GROUPS OF COMPOUNDS. 



Action of heat upon compounds. All phenomena taking place 

 in nature are, without exception, due to motion. Chemistry considers 

 the motion of atoms, without which no chemical change takes place. 

 The causes for chemical changes are either physical actions (heat, 

 light, electricity), or the decomposing influence of one substance upon 

 another caused by the atoms rearranging themselves into new bodies, 

 so as to better satisfy their affinities. 



The decomposing action of heat upon compounds has been men- 

 tioned before in connection with the decomposition of red oxide of 

 mercury into mercury and oxygen. Similarly to this process, many 

 other compound substances are decomposed by heat either into ele- 

 ments, or, more frequently, into simpler forms of combination. This 

 means that the molecule of a .substance containing, for instance, 10 

 atoms, is split up into 2, 3, or more molecules, each one containing a 

 portion of the 10 atoms. 



For instance : A piece of marble, which is calcium carbonate, or 

 CaCO 3 , is decomposed by heat into calcium, oxide, CaO, and carbon 

 dioxide, CO 2 . 



That heat has such decomposing influence upon compounds is readily 

 accounted for, if we bear in mind that increase in heat means increased molec- 

 ular vibration, which most likely weakens the stability of the molecule, and 

 diminishes the attractions of its component atoms. On the other hand, heat 

 will in many cases facilitate chemical combination between two substances, 

 because the increased molecular vibration brings the molecules closer within 

 the sphere of each other's attraction, thereby facilitating chemical union. For 

 instance : Mercury and oxygen do not act upon each other at ordinary tem- 

 perature, but when heated to a temperature somewhat below the boiling-point 

 of mercury, they combine slowly, forming oxide of mercury. This compound, 

 however, as shown before, readily decomposes into mercury and oxygen when 

 heated to a low red heat. 



The quantity of heat required for decomposition differs widely 

 according to the nature of the substance. Some substances can be 

 produced only at a temperature below the freezing-point of water, a 

 higher temperature causing their decomposition ; other substances may 

 be decomposed at temperatures between the freezing- and boiling- 



