|MadSci Network: Chemistry|
First, none of these substance actually melt ice. What some do is mix with (dissolve in) the water (ice/snow) and lowers the freezing point of the water, so the water can not freeze. This is the reason that if it snows and the temperature is very low, cities don't even put down salt, because even when mixed with the snow, the water will stay frozen. The others you list (sand,kitty litter) provide traction for your tires and may keep the ice from refreezing by mixing with it, but they don't lower the freezing point or do any melting. Note: I am not counting melting due to the fact that what you add is warmer than the ice or snow. When chemicals such as salts lower the freezing point of the water, it signals a change in the colligative properties of the water. Colligative properties (look up in textbook or library) are those that depend on the number of particles (ions or molecules) in the solution. They do not depend on what the particles are. So if a salt dissolves as three ions (CaCl2), the freezing point will be lowered more than with two ions (NaCl). NaCl will lower it more than sugar (one particle, no ions). So theoretically your list would be CaCl2>NaCl=rock salt>rubbing alcohol. As before sand and kitty litter do not cause melting because they do not dissolve. The alcohol is a special case because it breaks down into ions only partially in water. Note that I said theoretically. It also depends on how well the materials mix with the ice/snow and how easily they can dissolve. So for NaCl (rock salt) and NaCl (powder), the powder will do better per pound added to the ice/snow.
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