A. Class Experiments. A Study of Freezing Mixtures.
1. Prepare a small bowl of: a. cracked ice.
b. mixture of one part of salt to one part of ice.
c. mixture of one part of salt to seven parts of ice.
d. mixture of one part of salt to ten parts of ice. Insert a thermometer in each bowl as soon as the ice and salt are mixed and find the lowest temperature obtainable in each case, also the length of time necessary to obtain this temperature.
2. Effect of different freezing mixtures.
Prepare a syrup, using two tablespoons of sugar to one-half cup of water. Pour into four test tubes. Prepare bowls as in (1) and insert one of the test tubes in each. Compare the time required to freeze, and the textures of the frozen syrups. Which freezing mixture will you use to freeze an ice or a sherbet ?
3. Insert in a freezing mixture of one part of salt to seven parts of ice test tubes containing: a. a tablespoon of water.
b. a tablespoon of water and a saltspoon of ground spice.
c. a tablespoon of syrup (one part sugar to four parts of water).
d. a tablespoon of syrup (one part sugar to one part of water).
e. a tablespoon of stiffly beaten white of egg. Notice the time necessary to freeze and the texture of each. Take the temperature of each when frozen. What is the effect of suspended and dissolved sub-stances on the freezing point of water?
B. Prepare Lemon Ice.
Boil two tablespoons of sugar with half a cup of water to make a syrup. Add one tablespoon of lemon juice. Cool, pour into a tin measuring cup or similar container, cover, and surround with ice and salt. (What proportion will you use?) Stir while freezing.
C. Class Work. Prepare Lemon Sherbet.
Prepare a syrup, using the same proportions as in (B) but make enough to serve the whole class. Freeze in a regular freezer.
1. Use two teaspoons of gelatine for every quart of liquid. Soak the gelatine in a little cold water, while the syrup is cooking. Then pour the hot syrup over it. Add lemon in the same proportion given in (B) and strain. Grated lemon peel may be added.
2. When syrup is cool, mix in stiffly beaten white of egg, using one-half to one egg white for each cup of liquid. Add lemon as before.
Without a knowledge of physics it is rather difficult to understand how ice and salt act as a freezing mixture. In order to understand it at all we must know some preliminary facts. In the first place, the subject of energy must be considered. Cold is not a thing in itself, but merely the absence of heat. Heat, light, electricity, magnetism, and motion are all forms of energy and can be transformed one into another. Electricity in our lamps, for example, is changed into light and also gives off heat. In an electric flat-iron, heat is produced without any light at all. The heat of the fuel is, in a locomotive, turned into the motion which carries the train along. It is well-known, too, that some substances, as a piece of hard rubber such as is found in a fountain pen, can easily be electrified on a cold day by rubbing briskly on a woollen surface and that they will then attract bits of paper. These are all cases where one form of energy is turned into another; and it is a law in physics that no energy is ever destroyed, but continues to exist. When a solid like ice is changed into a liquid like water, or a liquid such as water is changed into a gas such as water vapor, heat is necessary for the change. This is said to be used up in performing work and is spoken of as hidden, or latent heat.
The same kind of action is illustrated by boiling water.
The temperature of the water rises until the boiling point is reached, but no further application of heat will raise the temperature above this point. This is because as fast as the heat is supplied it is used up in turning the water into steam; the more the heat supplied, the more steam there is formed, but the temperature of the steam itself is the same as that of the water from which it comes. But when it is said that the heat is used up, it is not meant that it is destroyed; for if the vapor be changed back to water, or the water to ice, the energy again manifests itself and appears as heat.
A second point to be understood is what is shown by the freezing experiments : a liquid that has another substance dissolved in it no longer freezes at the same temperature, but at a lower one. The more substance there is dissolved, the lower the freezing point becomes.
Now what happens when the ice and salt are mixed that makes the two so much colder than before ? The ice is at 32° F. and the salt much warmer, but as soon as they are mixed the temperature falls rapidly. What occurs is this. The ice and salt which are next each other are mixed to form brine. But brine, being really water with salt dissolved in it, should not freeze at 32° but at a much lower temperature. If it cannot be frozen, the ice must melt. But, as has already been said, heat is necessary to bring this about. The only available heat is in the mixture itself or the surrounding objects with which it comes into contact. This heat is used up in doing the work of melting the ice and becomes latent, - that is, disappears and is no longer evident as heat. Some heat also is used in doing the work of dissolving the salt in the water. As a result of these two actions the temperature of the mixture drops.
There are certain substances which conduct heat readily. It is well known how hard it is to hold the end of a metal spoon while the other end is in boiling water. No difficulty is experienced, if a wooden spoon is used. Wood, then, is a poor conductor of heat and metal a good one.
An ordinary ice-cream freezer has a container made of metal. This is so that the heat in the cream can easily be "conducted" to the freezing mixture to be used up in melting the ice and so disappear. On the other hand, the outside tub of the freezer is usually of wood. That is in order to keep the heat of the air from being easily conducted into the freezing mixture, lest this heat be used instead of that in the cream which is to be frozen. The difficulty with the wooden tub is that as it stands unused it is apt to shrink and then leak, and, besides, it is heavy and clumsy. So some ice-cream freezers have a metal outside. They undoubtedly take a little more ice and salt to do the work, but otherwise are quite satisfactory.
Agri. Exp. Sta., Burlington, Vt. "The Principles and Practice of Ice Cream Making."
1. Why does an ice-cream freezer have a dasher?
2. Which is cheaper, ice or salt? How does this point affect the choice of the proportions of ice and salt to be used in a freezing mixture?
3. What proportion of ice and salt would you use for chilling?
4. How is ice cream packed and how is it covered for keeping?
5. Can you make an ice cream or an ice mixture so sweet it will not freeze? Why?
6. Can snow be used to make a freezing mixture instead of ice?
7. Could ice have been frozen as readily in a glass cup as in a tin cup? Explain.
8. Explain why cologne rubbed on the forehead feels cool.
9. There are pressure cookers on the market which boil at a temperature above 212° F. These cookers are of metal with a cover which screws or clamps tightly into place, preventing the escape of the steam until the pressure reaches a certain degree, when an automatic escape valve opens. The steam pressing on the surface of the water prevents the ready formation of more steam. Why does the temperature of the water then rise above that of ordinary boiling water?