MadSci Network: Chemistry

Re: What is the most efficient substance for melting ice?

Date: Mon Dec 21 00:19:30 1998
Posted By: Henry Boyter, Senior Scientist
Area of science: Chemistry
ID: 910407729.Ch

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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