CHAPTER XII 

 HEAT OF VAPORIZATION 



60. When water in a vessel is placed on a stove, it can 

 be made to escape by evaporation, and the faster heat is 

 applied the faster it evaporates. The temperature of 

 boiling water in an open vessel cannot be raised above 

 the boiling point. The escaping steam carries the heat 

 away as fast as the water receives it. When more heat 

 is applied, the steam escapes faster and so carries off 

 the extra amount of heat. The temperature of the 

 steam is the same as that of the boiling water, but a 

 gram of steam has more heat in it than a gram of water 

 has. Since the temperature of the boiling water cannot 

 be increased, and since the temperature of the steam 

 coming from the boiling water is the same as the tempera- 

 ture of the water, a large amount of heat is required to 

 change the water from a liquid into a gas or steam. The 

 number of calories of heat required to change one gram of a 

 boiling liquid into steam is called the heat of vaporization 

 of that liquid. Since it takes about 5! times as long com- 

 pletely to evaporate an open vessel of water as to raise 

 it from o C. to 100 C., we conclude that the heat of 

 vaporization of water at 100 C. is about 533 calories. 

 Accurate experiments show that the heat of vaporiza- 

 tion of water is 536 calories. Other liquids have a dif- 

 ferent number of calories for their heat of vaporization. 

 From Chapter X we learned that energy cannot be 

 created or destroyed but can be transformed. The 

 large amount of heat required to change water into 



