When you mix solid caustic soda and water, the mixture gets
hot. It can get very hot, and boil. When it boils, it can
and will spit hot caustic soda granules/boiling caustic
solution all over the place.

I demonstrated this to one of the kids over Xmas. One part caustic, one
part water, and stir. I predicted it would melt the yoghourt pot, but it
didn't. Didn't boil, didn't spit. Hand hot I'd say.
Why people bother with acid or alkali drain unblockers is a mystery to
me. Most blockage respond to the smaller diameter garden hose (with a
flow of water if necessary)