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. FREEZING MIXTURES 93 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 dis- solved 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 con- tact. 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 94 FOOD STUDY spoon while the other end is in boiling water. No diffi- culty 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 con- ducted 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. REFERENCES Agri. Exp. Sta., Burlington, Vt. "The Principles and Practice of Ice Cream Making." QUESTIONS 1. Why does an ice-cream freezer have a dasher?

AI-modernized reading of the original text

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?

Food study