I did this again this morning and, although their was still foaming, I felt no heat in the reacting vessel. Perhaps I was wrong in inferring a chemical reaction last time -- I'm sure if you poured ordinary water into bleach you'd still get some foaming.
Lesson is, science is built on repeatable observations, and I couldn't repeat my own. Who knows why the mixture grew warm last time; maybe it was nothing but overstimulated imagination.
I guess I'll have to try it one last time, just to make sure.
