[Illustration: FIG. 21.--The destruction caused by freezing water.]

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
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 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 soon freeze
solid. As soon as the snow and salt are mixed they melt. The heat
necessary for this comes in part from the air and in part from the
water in the test tube, and the water in the tube becomes in
consequence cold enough to freeze. 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 practical application
of this principle. The heat necessary for melting the mixture of salt
and ice is taken from the cream which thus becomes cold enough to
freeze.




CHAPTER IV