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Name: Danna
Status: other
Grade: other
Location: NY
Country: N/A
Date: August 2007

Question:
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?



Replies:
Danna,

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.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming



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