Experimental cookery

Milk, which contains sugar and other dissolved substances, has a lower freezing point than water. When sugar is dissolved in water and some fruit juice added to make a sherbet or ice the freezing point of the sherbet mixture is about — 4° to — 5°C., depending upon the proportion of sugar added. Ice cream containing about % cup of sugar to a quart of cream freezes between — 1° and — 2°C., or about 29°F. Ice cream usually con- tains a smaller percentage of sugar than ices or sherbets. Freezing Mixtures When a crystalline substance is added to a liquid its freezing point is lowered. However, as a solution freezes it separates into crystals of the solvent and crystals of the dissolved substance. 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 solu- tion 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 con- centration 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 86 FREEZING 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 sur- roundings 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 surround- ^ -10 o u V D- E H-20 -30 A ^ \ A" N \y •/ 10 30 40 20