I dont know why, but alkaline soup expands and contracts Alot with changes in temp. so when you're using a vessel with a small neck that you fill with your nps and put a cap on, NEVER heat the vessel with cap sealed. on purpose or accidentally. if the jar is somewhere that may have a large rise in temp, dont fill it and cap it. it doesn't have to be too warm to touch before it explodes. this has happened to me twice. once sitting in front of a heater, and once in the sunlight. its not fun, and its extremely dangerous.

   i suppose this applies to any vessel that is nearly full with or without a small neck. but air compresses, so if theres enough of it there, i suppose its a little less likely to explode.