EXPERIMENT 79: MIXTURES FOR FREEZING.—Prepare 2 cupfuls of cracked ice. Place 1 cupful of the cracked ice in each of two bowls. To one bowl of ice add 1/3 cupful of rock salt, and mix thoroughly. Insert thermometers into both bowls and note temperature. What effect does the salt have upon the temperature of the ice?
Allow the ice mixture to stand a few minutes, then observe the difference in the condition of the ice in the two bowls. Besides lowering the temperature, what does the salt do to the ice?
FREEZING WITH ICE AND SALT.—When a solid substance is changed to a liquid, heat is absorbed from surrounding materials. When solid ice liquefies, heat is absorbed from surrounding materials. Salt makes ice liquefy at a lower temperature, thus absorbing more heat from its surroundings. Since foods must be cooled, i.e. heat drawn from them, in order to freeze them, a mixture of ice and salt rather than ice is used in freezing.
If ice and salt surround a tin can containing substances to be frozen, from what is the heat absorbed when the ice is changed to liquid form? Explain why it is that a mixture of ice and salt, rather than ice alone, is used to freeze a dessert.
CONDUCTORS OF HEAT.—In Experiment 2 and in, The Principle of Fireless Cookery, it is shown that some materials are better conductors of heat than others. Which is a better conductor of heat, wood or metal? Explain why it is that most freezers consist of an inner can of metal and an outer bucket of wood. A few freezers have an outside metal bucket. Such freezing devices have been found more satisfactory when heavy paper is tied around the outer metal bucket.
COOLING BY EVAPORATION.—If a few drops of alcohol, ether, or gasoline are poured in the palm of the hand and allowed to evaporate, the hand feels cold. During evaporation, the liquid takes heat from the hand. When any liquid evaporates, heat is absorbed from surrounding materials. Water may be cooled by placing it in a porous jar and hanging it in a breeze.
When there is no ice, this principle of cooling by the rapid evaporation of a liquid may be applied to the cooling of butter and other foods. Wrap butter in an oiled paper and place it in a flower crock or any porous jar. Place the crock in a draft; put a bowl of water beside it. Wrap a wet cloth about the crock and place one end of it in the bowl of water. The continuous evaporation of the moisture keeps the food cool.
PREPARING AND PACKING THE FREEZER.—Scald the can, the cover, and the dasher of the freezer; cool it before the mixture that is to be frozen is placed in it. Adjust the can carefully in the bucket; put in the dasher; pour in the mixture, cover; adjust the crank. Crush the ice for freezing by placing it in a strong bag and pounding it with a wooden mallet. Mix the ice with rock salt in the proportion given below. Then pour the ice and salt mixture around the can of the freezer. The ice and salt mixture should be higher around the can than the level of the mixture inside.
For freezing ice creams and most ices use three parts of cracked ice to one of rock salt. If ice of coarse grain is desired, use a greater quantity of salt. The less salt in
proportion to ice used, the finer the grain; the process of freezing, however, takes place very slowly when little salt is used.