HEAT OF VAPORIZATION 85 



steam must be in the steam. This heat is given out 

 again, or liberated, when the steam condenses and forms 

 water. The heat of condensation is equal to the heat of 

 vaporization. This can be proved by forcing steam into 

 cold water. The water will rapidly increase in tempera- 

 ture. This is also illustrated by the steam-heating 

 system. The steam comes into the radiators at a tem- 

 perature of about 1 00 C., it condenses, and the water 

 flows out at about the same temperature, but the room 

 containing the radiator becomes warm. The heat comes 

 from the steam condensing to form water. 



61. Boiling Temperature. When a liquid in an open 

 vessel is heated, it will be found that there is a certain 

 temperature above which it cannot be raised, no matter 

 how fast the heat is applied. At this temperature bubbles 

 of steam or vapor form at the bottom of the vessel and 

 rise to the surface, increasing in size as they rise. The 

 temperature at which this occurs is called the boiling 

 temperature or boiling point. Boiling temperature may 

 also be denned as the temperature at which the pressure 

 of the steam in the steam bubbles in the liquid is equal 

 to the pressure of the air or gas on the surface of the 

 liquid. In an open vessel the pressure of the air on the 

 liquid determines at what temperature the bubbles of 

 steam will form in the liquid. So the pressure of the air 

 determines at what temperature water will boil. At 

 "standard pressure," which is about average air pressure 

 at sea level, the boiling point of pure water is 100 C. 

 If salt or sugar is dissolved in the water, it will not boil 

 until it is heated above 100 C., depending upon the 

 amount of salt or sugar put into the water. Try it. 



Water on the top of a mountain will boil at a lower 

 temperature than at sea level. Why? It will take 



