I took water off of the heat, it stopped boiling, then boiled really quickly for a second. Anyone know why?

Question

I was boiling some plant material on a hot plate, then took the flask off. The flask was stopped with cork, but had a tube running to a condensation flask across the bench. It stopped boiling after a second, but then boiled really quickly for about half a second before calming down. It was really strange, because it's not like it was superheated, as not only had it been boiling a second before but it was also filled with so much organics from the plant that it was a deep brown, so it was about as far from distilled as one can get.

Any help is appreciated :)

Answer 1

If the tube was sufficiently narrow and the boiling rate sufficiently high, then the vapor in the flask could be substantially above atmospheric pressure, which would mean that the liquid would be substantially above its normal boiling point at 1 atm.

Once you remove the flask from the hot plate, it stops boiling but remains above its normal boiling point. The excess pressure would bleed off almost immediately, dropping the pressure back to 1 atm - at which point the liquid in the flask would be superheated. As a result, you'd get a very rapid, spontaneous boil until the temperature of the liquid dropped back below its 1 atm boiling point.

If you had the ability to measure the pressure inside the flask, or perhaps the flow rate of the vapor through the tube, I suspect that you would see that the brief delay between taking the flask off of the hot plate and the onset of the rapid boiling would correspond to the time it takes for the pressure in the flask to drop back down to 1 atm (and the flow rate to drop to nearly zero).