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      Freezing Point Depression

When we add ethylene glycol to automobile radiators as an antifreeze, we are taking advantage of the freezing point lowering of the radiator water. Ethylene glycol evaporates too readily in the summer months, but with other less volatile "year-round" antifreezes, we also make use of the raising of the boiling point of the radiator water to prevent boilover in hot weather.

When we scatter salt on icy sidewalks, Na+ and Cl- ions lower the freezing point of water and cause the ice to melt into a concentrated brine. Similarly, home ice cream makers use rock salt and ice to produce a slush at a lower temperature than can be achieved with pure water and ice.

All of these are applications of the colligative properties of molecules and ions dissolved in water.

Another important application of colligative properties is in determining molecular weights. A freezing point depression measurement can tell us how many moles of a solute are present, and if we already know the number of grams, it is easy to calculate the molecular weight.

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