Experiencing the heat of solution

It is always fascinating to me when one can experience some scientific concept in everyday life. I particularly like it when one can see things with the naked eye. Recently I realised that a macroscopic manifestation of spin-orbit coupling is ferromagnetic domains and hysteresis. This is because they arise from spin anisotropy which is due to spin-orbit coupling. But I digress.

The other day I was maintaining my pool [a bain of my existence] and I mixed some solid "Hardness increaser" in water. It got really warm! I had noticed this before but not thought about it much. Why does this happen? The chemical is mostly Calcium chloride. It turns out that this has a particularly large "heat of solution" [the enthalpy change associated with dissolving it in water] of -83 kJ/mol. For this reason it is used in "hot packs" and some undergraduate chemistry labs to illustrate heat of solution. [See articles one and two in the Journal of Chemical Education]. In thermal isolation dissolving 100 grams in 1 liter of water should raise the water by 18 degrees C. This is why I experienced it directly.

I am embarrassed that much of the chemistry involved in swimming pool maintenance remains a mystery to me. [e.g. What is the point of increasing the alkalinity and decreasing the pH at the same time?] But hopefully once I read this J. Chem. Ed. paper it will all become crystal clear.