This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Suppose 10 grams of salt are added to 90 grams of water to give a 10 per cent salt solution. The salt dissolves and the temperature of the solution is lowered. The freezing point of salt solutions may be shown in the diagram by the line AB. The temperature is represented vertically and the percentage of salt in the solution is represented horizontally.
The freezing point of water is 0°C. This is represented on the line AB of Fig. 13, at A. The freezing point of a 10 per cent salt solution is about - 5°C. This is shown at point X, where the freezing-point line AB cuts the line indicating a 10 per cent salt solution. If the salt solution is cooled to - 5°C. some of the water freezes. This leaves a greater concentration of salt than 10 per cent in the rest of the solution. Thus the freezing point of the rest of the solution is lowered. If cooling of the solution is continued, ice will continue to form, and the remaining salt solution will become more concentrated and its freezing point lower. The lowering of the freezing point can be continued until the line AB cuts the line BC. The point B represents the limit of the solubility of the salt at a temperature of - 22°C. Below this temperature the salt and water separate, both crystallizing. This is called the cryohydric point. Both the ice and salt may be cooled to lower temperatures than - 22°C. after they are in solid form.
If salt is mixed with water a saturated solution is obtained when the solution contains 35.6 grams of salt per 100 grams of solution at 0°C. If this solution is cooled below 0°C. the solubility of the salt is not so great at the lower temperatures. This is shown on the line CB. A little below 0°C. salt combines with 2 molecules of water. Thus the slope of the curve for solubility of salt changes below 0°C, and the salt is found in solution as the dihydrate. If a saturated salt solution is cooled below 0°C. the excess salt beyond saturation point is precipitated from solution. This will continue until the cryohydric point is reached.
Salt is often added to ice for a freezing mixture. When salt is added to water and ice at 0°C. they are not in equilibrium, even if the surroundings are at 0°C, for the addition of salt lowers the freezing point of the solution. Since they are not in equilibrium the ice melts and the salt dissolves in the water. There is always a film of water on the surface of the ice. As the salt dissolves in this small amount of water it absorbs heat. This absorbed heat is taken from the brine or from the surroundings. If the heat is taken from the brine the temperature of the brine is lowered.
Fig. 13. - The freezing point of solutions containing different percentages of sodium chloride and the solubility of sodium chloride below 0°C.
When salt is added to ice, one of three things may happen, depending upon the proportion of ice and salt, and the surrounding temperature or insulation. All the salt may be dissolved. When this happens the melting of the ice lessens the concentration of the salt solution and the temperature is not lowered beyond the point obtained when the solution contains the highest concentration of salt. All the ice may melt. When the ice is melted the temperature cannot be lowered to a greater extent, for there is no more ice to melt to absorb the heat. The cryohydric point may be reached. At this point the solution becomes solid and separates into ice and salt.
Ice-cream freezers. An ice-and-salt mixture is used to lower the temperature of other substances. An ice-cream freezer is a utensil made to freeze a substance placed in a center metal container. Outside this metal container there is a space in which the cooling medium is placed. The outer wall of the ice-cream freezer is often of wood, which is a poor conductor of heat. Since it conducts heat slowly it partially insulates the ice and salt from the surrounding air. The freezing mixture absorbs heat. In doing this it lowers the temperature of the brine and removes heat from the contents of the center can. In order to freeze the contents of the can the temperature of the brine formed must be lower than the freezing point of the mixture to be frozen.
To freeze the contents of the ice-cream can, considerable heat must be removed from it. As a result, a corresponding amount of ice must melt to absorb the heat. Therefore, the contents of the can do not begin to freeze until considerable brine has formed. Conduction of heat is more rapid by water than by air. Before the brine forms and replaces the air spaces around the ice, conduction of heat from the can is slow. After all the salt is dissolved or all the ice is melted, the temperature of the brine cannot be lowered, unless the temperature of the surrounding air is lower than the temperature of the brine. In ice-cream making the temperature of the surroundings is usually much higher than the temperature of the brine. When the quantity of ice left in the freezer is small the temperature of the surrounding air may heat the brine more than the melting of the small amount of ice cools it. Before this stage is reached the freezer needs fresh ice and salt packed around it.
The lowest temperature obtainable for a brine from a salt and ice mixture is about - 22°C. (Walker reports - 21° and Bigelow - 22.4°C.) This temperature is called the cryohydric point. If calcium chloride is used with ice the lowest temperature obtainable is - 55 °C.
At the cryohydric point 29 parts of salt are soluble in 71 parts of water. No more salt can be dissolved at this temperature. This proportion of salt to ice is about 1 to 3 and is often used in freezing mixtures.
Ice in the refrigerator is probably about 0°C, but ice from out of doors on a cold winter day may have a temperature far below 0°C. When ice at a very low temperature is used for freezing, a brine will form more quickly and freezing start sooner if water is poured over the ice and salt.