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Tuesday, February 21, 2012

Salty Roads

So why does salt cause ice to melt? And is there a limit on how cold it can be for this to work?

Salt melts ice because it dissolves in the layer of liquid water that forms on the surface of ice. When you dissolve a substance in water, the freezing point of the resulting solution is lower than that of just water. This is known as freezing point depression. You may know that pure water freezes at 32oF (0oC). If the solution is 10% salt, it freezes at 20oF (-6oC). A 20% solution freezes at 2oF (-16oC). So, if the air is 28oF (-2oC), a container of pure water would freeze solid, but both 10% and 20% salt solutions would remain liquid. At an air temperature of 14oF (-10oC), the pure water and 10% solutions would freeze, but the 20% solution would not. What do you think would happen at -6oF (-21oC)? All three solutions would turn to ice! If you live in an area with very, very cold winter temperatures, the road crews may not even use salt on the roads. If it's too frigid for even pretty salty solutions to melt, there's no point in using salt at all.

Well, no point in using regular table salt, that is. What we normally call "salt" is a chemical called sodium chloride (NaCl). Chemists frequently use the word "salt", in a more general way, to describe substances that form in certain reactions (the reaction of an acid and a base, to be exact). There are other types of "salts" that can melt ice at lower temperatures than sodium chloride. One example is calcium chloride (CaCl2) (not all salts are chlorides). Calcium chloride can melt ice down to a frosty -20oF (-29oC)!

But why do any of these solutions cause a drop in freezing point? When pure water freezes into ice, the water molecules slow down and line themselves up in a very organized pattern. When another substance is dissolved into the water, the dissolved particles keep the pattern from forming. The water molecules have to slow down even more to make ice; this requires a lower temperature. When a salt, such as sodium chloride or calcium chloride, dissolves in water, it breaks up into two parts: sodium chloride produces a sodium and a chlorine, and calcium chloride forms one calcium and two chlorines. The more particles, the more they intefere with freezing. So, CaCl2 (three particles) has a greater effect on the freezing point of water than sodium chloride (two particles), and will melt ice at lower temperatures.

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