29-31] 



13 



SECTION II 



THE CONSTITUENTS OF AIR AND WATER 

 AIR 



29. Air consists mainly of a mixture of nitrogen and 

 oxygen gases : it also contains small quantities of carbon 

 dioxide, water-vapour, ammonia and nitric acid. 



Preparation and Properties of Oxygen 



30. Place a few crystals of potassium chlorate in a per- 

 fectly dry test-tube and heat in the Bunsen flame. The 



Fig. 15 



substance melts and gives off bubbles of oxygen gas. 

 When this takes place, introduce a glowing splinter of wood 

 into the mouth of the test-tube. The splinter will burst 

 into flame. This behaviour of the glowing splinter is a test 

 for the oxygen which is given off from the heated potassium 

 chlorate. The following equation represents the chemical 

 action that takes place : KC10 3 =KCl + 3 . 



31. In order to investigate the properties of oxygen gas 

 a much larger quantity of the gas is required than can be 

 prepared by the above method. 



