94 GENERAL SCIENCE 



Metals, as a rule, expand when they change from a 

 solid to the liquid form and contract when they become 

 a solid. Cast iron freezes at 1200 C., silver at 954 C., 

 lead at 330 C., mercury at - 39.5 C. That is, these 

 metals become solids at these temperatures. At what 

 temperature do these same substances melt? 



68. Effects of Salts on Freezing. In freezing ice 

 cream salt is mixed with the ice. The salt melts the ice. 

 It requires heat to melt the ice and to dissolve the salt. 

 The heat necessary to do this comes from the cream, and 

 so its temperature is soon lowered to the freezing point. 

 The application of ice and salt is continued until the 

 cream is all frozen. When water has as much common 

 salt dissolved in it as is possible, the solution will not 

 freeze until its temperature is 22 C. Why does ocean 

 water not freeze as easily as river water? 



If equal weights of ammonium nitrate (NH 4 N0 3 ) and 

 water at 15 C. are mixed, the temperature will decrease 

 to 10 C. If three parts of calcium chloride (CaCy, 

 in crystal form, are mixed with two parts of snow, a 

 temperature as low as 55 C. will result. This mixture 

 will freeze mercury. These two salts are not used in 

 making ice cream, because they are too expensive. 



PROBLEMS 



1. How much heat is required to change the temperature of 

 500 grams of ice at -20 C. to o C? (Specific heat of ice is .5). 



2. How much heat is required to change 500 grams of ice at 

 o C. into water at o C? (Heat of fusion of ice is 80 calories). 



3. How much heat is required to change 500 grams of water 

 at o C. into steam at 100 C? (Heat of vaporization of water 

 is 536 calories). 



4. How much heat is required to change 100 grams of ice at 

 -25 C. into steam at 100 C? 



5. If an iceberg in the form. of a cube is 40 feet above the water, 

 how far does it extend down into the water? 



