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 32^oF (0^oC). If the solution is 10% salt, it freezes at 20^oF (-6^oC). A 20% solution freezes at 2^oF (-16^oC). So, if the air is 28^oF (-2^oC), a container of pure water would freeze solid, but both 10% and 20% salt solutions would remain liquid. At an air temperature of 14^oF (-10^oC), the pure water and 10% solutions would freeze, but the 20% solution would not. What do you think would happen at -6^oF (-21^oC)? 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 -20^oF (-29^oC)!

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.

Remove De Ice

Ice is great if you want to cool a drink, or as a surface to skate on, but it can be really dangerous when it covers streets, sidewalks, and other places where people have to drive or walk. What common substance is spread on roads and walkways? Salt! This is the same old table salt that you might sprinkle over your French fries, although the particles are usually larger. What effect does salt have on ice? Here's an experiment you can try that will reveal its effect!

What you'll need:

Glass

Water

Ice cube

Sewing thread

Salt in shaker

Cut a piece of thread about 12 inches (30 cm) long. Fill the glass about 3/4 full of water and put the ice cube into the water. Lay the thread over the top of the ice cube. Hold the ends of the thread up, and pour salt onto the ice cube, covering the area with the thread. Wait about one minute, then take hold of both ends of the thread. Lift them up. What do you notice?

The ice cube comes out of the water with the thread! The salt melts the ice, which then wets the thread. As the salt continues to dissolve, it moves away from the ice cube, and the ice re-freezes. However, by now the salt solution has covered the string, fastening it to the cube.

Shrinking and Growing

In the last experiment, the volume of the water increased as it turned to ice, and the volume of the oil decreased, or stayed the same, as it froze. Why do these two liquids act so differently?

Most liquids contract (get smaller), at least a little, when they freeze. Molecules in a liquid move in all directions very quickly. They slow down as they get colder, and get really lazy around the freezing temperature. When the molecules are about to turn into a solid, they begin to crowd together and stick to one another. This arrangement of molecules takes up less space than the liquid form, and the volume of the solid that results after freezing is less than the volume of the original liquid.

Water (H₂O) acts differently because of its molecular structure. A water molecule has its oxygen atom in the center, with hydrogens on either side. However, the molecule is not straight, but rather, is bent at an angle of 105^o. So the water molecules cannot pack tightly like most other molecules can. They end up with a structure that looks like a honeycomb, in a six-sided, or hexagonal arrangement. This structure actually takes up more space than the original water, and so solid water has a larger volume than liquid water.

Here's a video you can watch that shows the motion of water molecules in ice (solid), water (liquid), and steam (gas).