142 THE WATERS OF THE EARTH 



It is found that when salt or any other solid is in solution 

 in water, more heat is required to boil the solution and a 

 lower temperature to freeze it than are required by pure 

 water. A saturated salt solution freezes only at -22C. 

 (-7F.) although pure water freezes at C. The freezing 

 point of a salt solution may, therefore, be anywhere from 

 slightly below C. to - 22 C., dependent upon the strength 

 of the solution. Salt placed directly upon ice will cause the 

 ice to melt and form a solution if the temperature is above 

 -22 C. This explains why salt may be used successfully 

 to melt ice on porch steps, sidewalks, and car-track switches. 



When ice is placed in salt water it takes from its surround- 

 ings the heat necessary to change it from the solid to the 

 liquid state and continues to do this until the freezing point 

 of the solution is reached. It thus happens that the tem- 

 perature of such a solution may become much lower than the 

 freezing point of water and yet the solution remain unfrozen. 

 Most substances placed in such a solution become quickly 

 frozen. A solution of this kind is used -in freezing ice-cream. 

 About three parts of snow or ice to one part of salt are the 

 best proportions to use. 



Substances in Suspension and in Solution in Water. 

 Experiment 64. Into a glass of clear water stir a half teaspoonful 

 of sand and fine dust. Cover the glass and set it aside. After 

 an hour or so examine the glass and see if any of the sand and 

 dust has settled to the bottom. If so, stir it up again. What 

 happens? 



It was found in Experiment 64 that water is able to hold 

 solids in suspension and that the finer the solid particles 

 the longer they stay suspended. It was also found that 

 when the water was in motion (stirred) it held more and 

 iarger particles. 



