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“You scream, I scream, we all scream for ice cream.”  Ice cream today is big business, but as with almost all foods we can find a basis for both its construction and its nutritional values in chemistry.  Ice cream itself is considered scientifically to be an emulsion. An emulsion is defined as “as liquid droplets dispersed in another immiscible liquid.” Ice cream is an emulsion because it contains milk fats dispersed in water. In addition to the milk fats and water, ice cream also contains sugars and other flavoring molecules.  Use of the correct amount of these four ingredients is what determines the overall consistency of the ice cream, because each of these molecules affects the freezing point of the overall mixture. The freezing point of a substance is the temperature at which the substance changes from a liquid phase to a solid phase.  Thus freezing is called a phase change.

The freezing point depression of a solution is a colligative property associated with the number of dissolved molecules. Colligative properties are the properties of a solution that are dependent on the concentration of the solute (molecules dissolved in the solution) and the solvent (the main liquid that is supporting the other molecules, normally water).  A molecule’s ability to depress the freezing point is dependent on its molecular weight. The lower the molecular weight, the greater the ability of a certain mass of a molecule to depress the freezing point. Thus monosaccharides (single sugars) such as fructose or glucose produce a much softer ice cream than disaccharides (two sugar molecules linked together) such as sucrose. This is because monosaccharides of the same mass depress the freezing point more than disaccharides, because monosaccharides need a colder temperature to freeze. This limits both the amount and the type of sugar that one can use to make ice cream.

Making ice cream in a lab also requires a means by which to freeze the ice cream as it made. Because the sugars in the ice cream mixture only freeze at temperatures lower than 0^oC, we need to provide an environment around the milk, sugar etc. that is much colder than the freezing temperature of normal ice (0^oC). The way we create this environment is by using salt added to ice to depress the normal freezing point of water.  As salt is applied to ice, a concentrated brine solution forms on the ice, which has a very low freezing point. The freezing point of a 20% solution of salt is -16.6^oC. The reason for this is that the solute (whether they be sugar in the ice cream or salt in the brine) interferes with the water molecules finding each other, so it takes a lower temperature to attain the freezing point. The lowest temperature that can be achieved with sodium chloride brine is –20^oC, at a concentration of 23% salt. In our experiment, the brine is set outside the inner bag of ice cream ingredients. The brine is used to adsorb heat from the freezing ice cream inside the bag, thereby cooling it, thereby allowing the ice cream to freeze. Can you now explain why northern cities use salt on the roads in the winter?