HEAT NECESSARY TO DISSOLVE A SUBSTANCE 43 



frosts occur, the destructive action of water works constant 

 changes in the appearance of the land ; small cracks and 

 crevices are enlarged, mas- 

 sive 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. 



34. Heat Necessary to 

 Dissolve a Substance. It 

 requires heat to dissolve 

 any 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 sev- 

 eral degrees. This is just what one would expect, because 

 the heat needed to liquefy the salt must come from some- 

 where, and naturally it comes from the water, thereby lower- 

 ing the temperature of the water. We know very well that 

 potatoes cease boiling if a pinch of salt is put in 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 freeze immediately. As soon as the snow and salt 



FIG. 21. The destruction caused by freezing 

 water. 



