142 ELEMENTARY SCIENCE 



does not cause an increase in the temperature of the sub- 

 stance. The heat applied to it is transformed into that 

 potential energy which a substance acquires when it changes 

 from a solid to a liquid, and which it gives up again in the 

 kinetic form of heat when it changes back to a solid. This 

 principle was illustrated in the freezing of ice-cream. 

 The temperature of the freezing-mixture does not increase 

 although the temperature of the substance frozen decreases 

 greatly; the heat withdrawn from the cream is used in 

 the change of ice and salt to liquid; it is "stored" in this 

 liquid; it has become potential rather than kinetic. You 

 have also learned that the temperature of boiling water 

 does not increase no matter how much heat is applied to 

 it, for the heat is used in the transformation of molecules 

 from the state of liquid to the state of gas. 



We can demonstrate this principle even more simply 

 and clearly by placing in different vessels equal amounts of 

 ice and water, and applying to them equal amounts of 

 heat. In the case of the ice, the temperature of the water 

 which results from its melting will not rise until all the 

 ice is melted; all the heat applied is being consumed in 

 overcoming the solidity of the ice; it ceases to be heat, 

 having been expended in the work necessary for changing 

 a solid to a liquid. But in the case of the water, its tem- 

 perature will begin to rise from the moment that the heat 

 is applied. By the time that the ice in the other vessel 

 is all melted, the water, to which no more than an equal 

 amount of heat is being applied, will have increased in 

 temperature 80 C. 



This gives a simple means for computing the amount 

 of heat consumed in the changing of ice to water; the 

 amount of heat involved in changing any solid to a liquid 



