Calcium Chloride and Water
Name: Christina N.
In my Elementary Science Teaching Methods class, we did an experiment involving
mixing three different temperatures of water with calcium chloride; the hotter the water before
mixing in the calcium chloride, the hotter the mixture following the addition of the calcium
chloride. I understand that it is an exothermic reaction, but why? It has been eons since I
First, if I could predict the water solubility of ionic salts, I would be on some tropical island
enjoying the royalties such an algorithm would bring in!!! Second, regarding your inquiry: I assume
that the experimental procedure involves the same amount of water at three different temperatures,
and the rise in the temperature was measured. What is not evident from your description is whether
the same amount of calcium chloride was added to each water sample, or was sufficient calcium
chloride to saturate the solution
was added. If it was the latter, calcium chloride is most likely more soluble in hot water than in
cold, so that the more calcium chloride is added to the hot water sample than the cold water
sample. If that is not the experimental setup, then the reaction CaCl2 (solid) ----->
Ca(+2)aq + 2Cl(-1)aq must have a large negative change in heat capacity.
The dH(T hot) = dH(T cold) + dCp(T hot - T cold).
We know already that dH, the heat of solution of calcium chloride is already very negative (
exothermic), if the change in heat capacity: dCp is also negative and large then dH(T hot)
will become more negative, i.e. more exothermic as the temperature increases. The numbers
are available to test this hypothesis, but usually the effect, though significant, is not
large enough to show up in a simple "test tube" apparatus.
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Update: June 2012