THE CHEMISTRY OF COMBUSTION 87 



(c) The Product Formed When Hydrogen Burns in Air. Prepare 

 another burning splinter of wood. Take the dry bottle containing 

 the mixture of air and hydrogen, and keeping it mouth downward, 

 ignite the mixture. After the combustion is over carefully examine 

 the inside of the bottle. Do you find any moisture on the walls? 

 Where did this come from? What then is the product arising from 

 the burning of hydrogen in air? Why was it necessary to use a 

 dry bottle for this part of the experiment? 



97. Discussion of the Exercise. When hydrogen burns, it 

 unites with oxygen, liberating much heat and producing water, 

 in the form of vapor, as a product. This vapor condenses to 

 water when it is cooled. Hydrogen is the lightest known 

 substance. It is much lighter than air, and hence the bottles 

 containing hydrogen are kept inverted so that the hydrogen 

 will not run out. Mixtures of hydrogen and air containing 

 more than 5 per cent, by volume of hydrogen and less than 

 72 per cent, are explosive. That is to say, a mixture of 95 

 cu. ft. of air and 5 cu. ft. of hydrogen will explode; so will 

 other mixtures containing relatively less air and more hydro- 

 gen until a mixture of 28 cu. ft. of air and 72 cu. ft. of hydrogen 

 is obtained, beyond which the mixture is no longer explosive. 

 This means that, within the limits named, the concentration 

 of hydrogen and oxygen is sufficient to maintain rapid com- 

 bustion. In the experiment (&) the mixture was probably 

 about 50 per cent, of each, 



98. Hydrocarbons. 



Exercise 27. Burning Compounds of Hydrogen and Carbon 



(a) Light the Bunsen burner and hold over the flame an inverted, 

 dry, cold tumbler. What substance appears on the inside of the 

 glass? Pour a tablespoonful of limewater into the tumbler, cover it 

 and shake it. Notice the change in the limewater. What two sub- 

 stances were formed by burning the gas? 



(6) Repeat (a) but use a candle flame instead of the gas flame. 

 Study the products arising from the burning of the candle. 



(c) Repeat (a) using a kerosene lamp flame, studying the products 

 of the combustion. 



What products of combustion may we expect when fuels contain- 

 ing hydrogen and carbon are burned? 



