Question

Why is strontium hydroxide less soluble in hot water than cold water?

Answer 1

Strontium is not often though of as hardness but it and barium are both hardness ions in addition to the more famous calcium and magnesium. If you look at these molecules on a periodic table, you will see they are all alkali earth metals. One of the quirks of this group is that they tend to form compounds that become less soluble as temperature increases, which is different than most other salts.

Ok so I guess I haven't answered the question yet. Why does this happen at all? Its somewhat complex but the simple version of it is that these hardness ions reject heat when they dissolve in a solution. The hotter a fluid gets, the harder it is to reject that heat into the solution which forces the equilibrium back towards the solid form.