Chemical Change and Energy 323 



SECTION 48. Chemical change caused by heat. 



Why do you have to strike a match to make it burn ? 



How does pulling the trigger make a gun go off? 



What makes cooked foods taste different from raw ones? 



Has it struck you as strange that we do not all burn 

 up, since burning is a combining with oxygen, and we 

 are walking around in oxygen all the time? The only 

 reason we do not burn up is that it usually requires heat 

 to start a chemical change. You already know this in a 

 practical way. You know that you have to rub the 

 head of a match and get it hot before it will begin to 

 burn ; that gunpowder does not go off unless you heat 

 it by the sudden blow of the gun hammer which you 

 release when you pull the trigger; that you have to 

 concentrate the sun's rays with a magnifying glass to 

 make it set a piece of paper on fire ; and that to change 

 raw food into food that tastes pleasant you have to heat 

 it. If heat did not start chemical change, you could 

 never cook food, partly because the fire would not burn, 

 and partly because the food would not change its taste 

 even if heated by electricity or concentrated sunlight. 



Here is an experiment to show that gas will not burn 

 unless it gets hot enough : 



Experiment 97. Hold a wire screen 2 or 3 inches above 

 the mouth of a Bunsen burner. Turn on the gas and light 

 a match, holding the lighted match above the screen. Why, 

 do you suppose, does the gas below the screen not burn? 

 Hold a lighted match to the gas below the screen. Does it 

 burn now? 



The reason the screen kept the gas below it from 

 catching fire although the gas above it was burning was 



