HEAT NECESSARY TO DISSOLVE A SUBSTANCE 49 



Small cracks and crevices are enlarged, massive rocks are pried 

 up out of position, huge slabs are split off, and particles large 

 and small are forced from the parent rock. The greater part 

 of the debris and rubbish brought down from the mountain 

 slopes by the spring rains owes its origin to the fact that water 

 expands when it freezes. 



Heat necessary to dissolve a substance. It requires heat 

 to dissolve a substance, just as it requires heat to change ice 

 to water. If a handful of common salt is placed in a small 

 cup of water and stirred with a thermometer, the temperature 

 of the mixture falls several degrees. This is just what one 

 would expect, because the heat needed to liquefy the salt must 

 come from somewhere, and naturally it comes from the water, 

 thereby lowering the temperature of the water. We know 

 very well that potatoes cease boiling for an instant if a pinch 

 of salt is thrown into the water; this is because the temperature 

 of the water has been lowered by the amount of heat necessary 

 to dissolve the salt. 



Let some snow or chopped ice be placed in a vessel and 

 mixed with one third its weight of coarse salt; if then a small 

 tube of cold water is placed in this mixture, the water in the test 

 tube will soon freeze solid. As soon as the snow and salt 

 are mixed the snow melts and the salt dissolves. The heat 

 necessary for this comes in part from the air and in part from 

 the water in the test tube. The water in the tube loses so 

 much heat that it is changed to ice. But the salt mixture does 

 not freeze, because its freezing point is far below that of pure 

 water. The use of salt and ice in ice-cream freezers is a practi- 

 cal application of this principle. The heat necessary for melt- 

 ing the mixture of salt and ice is taken from the cream, which 

 becomes so cold that it freezes. 



