Adding a solute to a liquid lowers its freezing point, allowing ice cream to be made and roads to be de-iced. Salt, sand, and antifreeze are commonly used solutes. The addition of foreign molecules causes fluctuations in temperature that accelerate melting. The percentage of antifreeze used should be adjusted for different climates.
A freezing point depression is when the normal freezing point of a liquid is lowered beyond normal by the addition of a solute. Every liquid has a normal freezing point; for example, pure water freezes at 32 degrees Fahrenheit (F) (zero degrees Celsius or C). The salt in seawater brings seawater to a lower freezing point than pure water; seawater can still freeze, however, the temperatures at which it does are lower than water from a pure water source. This fact is what allows ice cream to be made in ice cream shops, when the metal container in the center is surrounded by salty ice. The melting of the ice by the rock salt allows the ice cream mixture to lose heat to the surrounding ice water, thereby freezing the ice cream.
Scientists have noticed that the amount of solute added to a liquid is directly proportional to the resulting depression of the freezing point. Not only do molecules in a solution freeze at a lower temperature, but cooling to freezing point proceeds at a more moderate rate than in pure liquids. The freezing point depression ability of a solution is the basis for the use of salt or sand on icy roads in winter. Molecules of salt or sand, scattered on the roadways, mix with the ice and cause it to melt, which is particularly useful for black ice so dangerous for vehicles to travel on. The strong diffusion of salt can lead to a freezing point depression down to zero degrees F (-18 degrees C).
When salt is sprinkled on ice, there is a chemical exchange between the molecules. The ice surface molecules begin to escape into the melt water and the water surface molecules get trapped in the water surface molecules. This shift between the molecules causes fluctuations in the temperatures of each that gradually heat and melt the remaining ice as the released molecules melt and accelerate. The added salt mixture introduces foreign body molecules that do not compact easily into a solid and accelerate liquefaction. For this reason, any foreign substance – alcohol, sugar or salt – introduced would have the same effect, although salt is widely available and less expensive to use.
A freezing point depression is the intent to add antifreeze to automotive vehicle radiators. The addition of ethylene glycol, a main ingredient in antifreeze, creates a solution with water. People in different cold climates may use different percentages of antifreeze in the water to reach different lows, however, when there is more antifreeze in the water than there is less, the opposite effect occurs and freezing points rise rather than fall. You may want to read the sides of your antifreeze bottle to determine the best percentages to use in particular climates.
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