ICE, WATER, AND STEAM 127 



water even from the air and becomes wet, as all of us have 

 had occasion to notice when using a salt shaker in damp 

 weather. Salt not only absorbs gaseous water from the air 

 but also absorbs water from a piece of ice, thereby changing 

 the ice to the liquid form. This explains why salt is used to 

 melt ice on sidewalks and in other places. 



When ice is melted, heat is required to carry on the process. 

 When salt melts ice, the source of heat is the ice and the sur- 

 rounding air or other objects. If salt and ice are well mixed, 

 the ice will be melted rapidly by the salt, but the temperature 

 of the mixture will not remain at C. It will fall to 1 7 ( '. 

 or even slightly lower. The heat needed is taken from the 

 mixture itself, and the presence of the salt prevents the water 

 from freezing at the lower temperature, as it otherwise would 

 do. Such a freezing mixture of salt and ice is often used to 

 produce a temperature slightly below C., as in freezing 

 ice cream. 



