



CHAPTER XV 

 EVAPORATION 



89. Conditions Affecting Evaporation. Evaporation is the 

 name given to that change of state in which a liquid assumes 

 the gaseous form. Liquids will evaporate at any temperature 

 but, since heat is always absorbed in the process, the warmer 

 the substance is made the faster will evaporation take place. 

 When a substance evaporates without the special application 

 of heat we assume that the heat necessary is derived from sur- 

 rounding objects, and this proves to be the case, for evapo- 

 rating liquids are always somewhat cooler than their surround- 

 ings. The heat thus absorbed becomes latent; that is, it is 

 used up in the change of state and does not raise the tempera- 

 ture of the liquid. Other things being equal, the greater the 

 surface exposed, the more rapid will be the evaporation. In 

 sugar making, the syrup to be evaporated is placed in large 

 shallow pans to facilitate the process. Evaporation is also 

 promoted by removing the vapor as fast as it is produced and 

 thus clearing the way for other molecules of the evaporating 

 liquid to fly off. There is, as might be expected, a consider- 

 able difference in the, rate at which different liquids evaporate, 

 but all are alike in requiring heat for the process. Liquids 

 which evaporate rapidly of course take up heat rapidly and 

 feel colder than those which evaporate more slowly. If a 

 liquid disappears very quickly when exposed to the air, it is 

 said to be volatile. Sublimation, already mentioned (52), 

 may be considered as a form of evaporation which takes place 

 in some solids at a temperature lower than their melting or 



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