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We care about molality because freezing point depression is a colligative property, a property that depends on how many solute particles are in the solvent, not the kind of solute particles. Molality, m, is defined as moles (mol) solute per kilograms (kg) solvent, as shown in Equation 1:Įquation 1:

We will take a look at each of these factors. But, by how much? One degree? Ten degrees? The answer to that question depends on three things: the molality of the solution, the Van 't Hoff factor of the solute, and the molal freezing-point-depression constant of the solvent. In this science project, you will use water as the solvent and sodium chloride (table salt) and sucrose (granulated sugar) as the solutes.Īdding a solute, like salt, to a solvent, like water, lowers the freezing point of the solvent. The solvent does the dissolving, and the solute is the thing that is dissolved. To make a solution, you dissolve a solute in a solvent. Pure water freezes at 0° Celsius (C), but water mixed with salt freezes at less than 0° C.įreezing point depression is not unique to water and salt it happens with all solutions. The decrease in freezing point that happens when salt isĪdded to water is called freezing point depression. In both cases, salt is used to lower the temperature at which water freezes, a temperature called the freezing point. This is because the salt lowers the temperature at which water freezes, so ice can melt even when the temperature is below the normal freezing point of water. If you live in a cold climate, you may also have seen trucks spreading salt or sand on the streets to prevent roads from getting icy after a snowfall.

This extra-cold ice-salt mixture freezes the cream in the ice cream machine. The salt allows the ice-salt mixture to get colder than pure water ice. If you have ever made ice cream with an old-fashioned hand-crank machine, you probably packed a mixture of ice and rock salt around the container holding the cream.

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