In my lab report, I am asked to

Question

"Find the solubility of Ca(OH)2 in H2O as the temperature increases. Explain (in terms of energy) the reasons for this trend."

From what I have found, Ca(OH)2's solubility goes down, but I have no idea why! It isn't a gas... it's a solid dissolved in liquid, right?
Thank you for your time.

Answer 1

Here is a site that has a graph of a number of salts versus temperature.
http://galileo.phys.virginia.edu/education/outreach/8thgradesol/TempSolubility.htm

Note that KNO3, NH4Cl, NaCl and some others increase solubility with increasing temperature (although to different extents). These salts are endothermic when dissolved in water. (Next time you're in the lab add a little KNO3 or NH4Cl to H2O in a test tube and shake it. Feel that it gets cold.) So the solution process is endothermic as shown by the following simplistic equation.
solid + heat ==> ions(aq). By Le Chatelier's Principle, adding heat increases solubility and that's what the graph shows.

There are some compounds, notably gases but not all are gases, in which the solution process is exothermic. NH3, HCl, and a few others are in this category. These equations look like this.
gas(or solid) ==> solution + heat
and Le Chatelier's principle tells us that heat will favor the reverse direction. The solution of Ca(OH)2 is exothermic so we would expect the solubility to decrease at higher temperatures. Having said all of that we need to remember that solution, although it sounds simple enough, is a complicated process involving several discrete steps. For example, NaOH and KOH are EXTREMELY exothermic when dissolved but the solubility increases with higher temperatures. Other processes must be at work with those examples.