Oxygen 253 



that in which the mercury oxide was treated in Experi- 

 ment 44. Test for the evolution of oxygen with a 

 glowing splint. Examine the product left in the tube. 

 It is known as lead oxide or litharge. 



Red lead oxide = yellow lead oxide + oxygen. 



EXPERIMENT 108. Into a small tube made of hard 

 glass a test-tube will not stand the temperature 

 necessary put some black manganese dioxide. Heat 

 this strongly with a powerful Bunsen flame, and test 

 from time to time with a glowing splint. At a red heat 

 oxygen may be recognised. Note any change in the 

 powder after no more oxygen seems to be produced. 



Manganese dioxide == brown oxide of manganese 

 + oxygen. 



171. Many other substances besides oxides give 

 off oxygen on being heated, especially nitrates and 

 chlorates. 



EXPERIMENT 109. Heat some nitre in a small hard 

 tube. Observe the behaviour of the crystals and test 

 for the evolution of oxygen. Continue the heating till 

 no more gas is given off. 



Potassium nitrate = potassium nitrite + oxygen, 

 (nitre or saltpetre) 



EXPERIMENT 110. Heat some potassium chlorate 

 in a test-tube, taking note of the behaviour of the 

 compound and of the amount of heating required to 

 cause oxygen to come off. As before, continue heating 

 till no more change occurs. 

 Potassium chlorate = potassium chloride -f oxygen. 



172. When oxygen is required in quantity for 

 laboratory work with the gas, a mixture of two of the 

 bodies named above is in common use. 



