This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
When a soluble substance is dissolved in a liquid the freezing point is lower than that of the pure solvent. A gram-molecular weight (mole) of a non-ionized substance in a liter of water lowers the freezing point 1.86°C. The lowering of the freezing point can be continued as long as the substance is soluble. If no more than 3 moles of a non-ionized substance are soluble in a liter of water, the freezing point cannot be depressed below - 5.58°C. Substances that are not very soluble do not affect the freezing point particularly, because such a small portion of a mole of the substance is dissolved. Thus substances in suspension, such as fat in milk or cream, do not affect the freezing point of an ice-cream mix in the same way that sugar does, for the fat is not soluble.
Ionized substances and the freezing point. Sugar does not ionize or dissociate in the solution to an extent great enough to influence the freezing point. Salt belongs to the groups of electrolytes which dissociate into ions in the solution. The amount of ionization depends upon the concentration of the solution: the less concentrated the solution the greater the dissociation. In a very dilute solution, complete dissociation into ions may occur. If 58 grams of salt or sodium chloride in 1 liter of water are completely ionized into sodium and chlorine ions, there will be 1 mole of sodium ions and 1 of chlorine ions. Thus the two lower the freezing point (2 X 1.86) or - 3.72°C. The molecular weight of sucrose is 342 grams, that of levulose is 180, and that of dextrose is 180 grams. None of the sugars ionize to an extent that affects the freezing point. Compare the freezing points, if 3/4 cup, or 150'grams, each, of sucrose, levulose, or dextrose is added to a quart of cream.
Calcium chloride ionizes into 3 ions. Therefore, a mole of calcium chloride depresses the freezing point more than a mole of sodium chloride.
The calcium chloride is also more soluble than sodium chloride, so that a greater number of moles can be dissolved in a liter of water.
The Freezing Point of Ice Cream and Ices
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 3/4 cup of sugar to a quart of cream freezes between - 1° and - 2°C, or about 29°F. Ice cream usually contains a smaller percentage of sugar than ices or sherbets.