Interesting question. At a given temperature,
say 32 degrees Fahrenheit, the material that
makes ice melt the fastest is the material that
lowers the freezing point of water the most.
Pure water has a freezing point at 32 degrees
Fahrenheit, below which it behaves as a solid
and above which it behaves as a liquid.
Temperature is an indication of how fast
particles such as molecules, atoms, and ions are
vibration. When liquid water is in contact with
ice at the freezing temperature, liquid water
molecules are vibrating slow enough to be
captured on the surface of the ice. At the same
time, molecules on the surface of the ice are
vibrating fast enough to escape into the liquid,
but not so fast as to outpace the number of
molecules being captured on the surface of the
ice. The rate of capture and escape is the same,
and so the ice will remain as ice and the liquid
water will remain as liquid water. If the
temperature decreases below the freezing point,
the water molecules are vibrating slow enough
that the number of captures is greater than the
number of escapes, and all of the water
To lower the freezing point of water, all you
have to do is add particles! It could be almost
anything that forms a solution (a uniform
mixture) with liquid water. Table salt or sugar
could do the trick. Salts are the most common
molecular compounds used to lower the freezing
point of water. Salts are neutral and bound
positive and negative ions. Table salt, for
example, consists of a sodium ion Na+ bonded to
a chlorine ion Cl-. When mixed with water, salts
dissociate into their constituent ions. A
mixture of water and table salt is really a
mixture of H2O molecules, Na+ ions,
and Cl- ions.
Adding particles like salt ions to liquid
water and ice creates a uniform solution of
liquid water and the ions. However, the
particles don't form a solution in the ice; ice
is pure water. Because the concentration of
water in the solution is lower than the
concentration of water in pure ice, the rate of
water capture on the surface of the ice
decreases, and the rate of escape of water from
the ice of the ice increases. We see this as a
decrease in the freezing point of water.
The more particles you can dissolve in the
liquid water, the more you decrease its freezing
point. Consequently, a mixture of 20% NaCl and
liquid water will have a lower freezing point (2
degrees Fahrenheit) than a mixture of 10% NaCl
and liquid water (20 degrees Fahrenheit). The
20% solution will thus melt ice faster (though
it isn't practical).
If you were to use a salt that dissociates
into a greater number of ions, you would lower
the temperature even further. Calcium chloride,
CaCl2, dissociates into 3 ions, one
Ca2+ ion and two Cl- ions.
The lowest freezing point of a practical
CaCl2:H2O solution is -20
degrees Fahrenheit, compared to 15 degrees
Fahrenheit, the lowest freezing point of a
practical NaCl:H2O solution.
The material that melts ice the fastest is
the material that dissociates into the greatest
number of particles in a solution of that
material and liquid water.
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