Boiling liquids

[bob_builder]

When boiling say, water in a saucepan, why does the 'boiling activity' (bubbling) seem to increase momentarily when the heat is removed?

[mrbatfink]

Cant say i have noticed it. But it could be due to the heat transfer time from the saucepan to the water.

[bob_builder]

I don't mean it carries on boiling in the same way - I mean there is a tangible increase in bubbling activity once the heat is removed

[Space]

I don't notice any change when I turn the heat down.
Unless I start moving the saucepan, but that's another matter.

[Bob A Job]

If you are using gas, then the hottest part of the flame can often be just above where the pan will lay on the hob,
Therefore as you 'lift' the pan off, it gets to the hottest part and boiling increases slightly. if the gas is cut off, the flame height will reduce and at that point the same thing will occur.

I think.... although ive never observed this phenomonomonmononemthingy

[bob_builder]

Thanks Bob, I see your theory. Although, it was my belief that the hottest part of a gas flame was the tip of the inner flame i.e. the deep azure blue cone within the whole flame.

This would confirm your theory if it moves nearer to the pan, but I don't think it does.

Personally I think there is something more fundamental - the activity increases because there is LESS heat albeit for a moment.

Bob ('s not your uncle)!

[Bob A Job]

As i recall, the tip of the flame is normally set higher than the pan bottom so the flame spreads on contact and creates an even heat... although i may be wrong.

try a full heat flame without a pan and look at it on the level.

[bob_builder]

In the interest of science........

I put a pyrex dish on the gas to view how the flame behaved, one possible explanation is that there is a cold spot in the centre of the gas ring. As the gas is switched off the ring contracts toward the centre of the pan. Just a thought.

Thanks Bob