Chemical Change and Energy 347 



In Experiment 36, where you stoppered a test tube 

 containing a little water and then held the tube over a 

 flame until the cork flew out, you were causing an 

 explosion. As the water changed to steam, the steam 

 was an expanding gas. It was at first confined to the 

 test tube by the cork. Then there was an explosion; 

 the gas freed itself by blowing out the cork. 



Steam boilers have safety valves to prevent explosions. 

 These are valves so arranged that when the steam ex- 

 pands and presses hard enough to endanger the boiler, 

 the steam will open the valves and escape instead of 

 bursting the boiler to get free. 



Explosives. Dynamite, gunpowder, and most explo- 

 sives are mixtures of solids or liquids that will combine 

 easily and will form gases that expand greatly as a result 

 of the combination. One of the essentials in explosives is 

 some compound of oxygen (such as the manganese 

 dioxid or potassium chlorate you used to make oxygen 

 in Experiment 93) which will easily set its oxygen free. 

 This oxygen combines very swiftly with something else 

 in the explosive, releasing heat and forming a gas that 

 takes much more room. In its effort to free itself, this 

 expanding gas will blast rocks out of the way, shoot 

 cannon balls, or do any similar work. 



But if gunpowder does not have to push anything of 

 much importance out of its way to expand, there is no 

 explosion. That is why a firecracker merely fizzes when 

 you break it in two and light the powder. The card- 

 board no longer confines the expanding gas ; so there 

 is nothing to burst or to push violently out of the way. 



Useful explosions are generally caused by a chemical 



