Making Ice Cream and Calcium Chloride
Date: August 2007
I tried making ice cream with calcium chloride instead of
rock salt and it did not work. I ended up with a milkshake
consistency. What is the difference between the two and why did it not work?
My guess is that you used the same amount (either by volume or by mass)
of calcium chloride as that suggested for rock salt. This would result in
the cooling vessel not being as cold as it should be. Rock salt is used to
lower the freezing point of ice. While calcium chloride can also lower the
freezing point of ice, you would need more of it in order to achieve the
same level of cooling as that with rock salt.
The mathematical formula for freezing point depression is dT = ikm, where dT
is the change in freezing temperature, i is called the van Hofft factor (we
will assume this to be 2 for NaCl and 3 for CaCl2), k is the freezing point
depression constant (dependent only on the solvent, water) and m is the mass
of solute (in g) over the molar mass (in g/mol) of the solute over the kilograms
of solvent (the ice).
If we divide this equation with values for CaCl2 by values for NaCl, we find
that the for the same change in temperature (same dT) you would need 1.2x as
much (by mass) CaCl2 as NaCl to get the same kind of cooling. Since the van
Hofft factor changes with high concentration and can start to approach each
other, then we can expect that you could need as much as 1.9x as much CaCl2
to get the same kind of cooling.
Greg (Roberto Gregorius)
Calcium chloride is extremely soluble in water, so it should be even more
effective than rock salt (sodium chloride) at lowering the freezing point of
water. Probably your problem was the very high heat of dissolution of calcium
chloride in water: it generates a lot of heat when it mixes with water. So a
lot of the heat absorbed by melting the ice came from dissolving the calcium
chloride instead of from freezing the ice cream.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012