How come when you stir a pot of boiling water, the boil stops or slows?

???

Because (simplistically speaking) the only part of the water that is actually boiling are the parts in direct contact with the hottest parts of the pot. When you stir it, you mix the relatively cooler water near the top with the hotter water at the bottom, causing the boiling to slow or stop temporarily.

The effect you note is minimized if a pot is at a truly rolling boil. In a rolling boil, much more of the water is actually boiling.

I’ve been playing around with pots of boiling water and digital thermometers and a quite appreciable fraction of the water does get up to at least 99C even at a simmer as long as you leave it long enough to come up to equilibrium. It’s only the very top 1cm or so that’s exposed to air that cools down.

It only takes a drop of a small fraction of a degree to cause boiling to stop.

When you stir the water, you set up a big toroidal vortex which lifts heat off the bottom, and transports it to the top of the pot. The increased flow rate past any bubbles that do form returns them quickly to equilibrium with the surrounding water. Since the bulk state is liquid, that means the bubbles collapse.

You also cool the water when you put in something to stir it with. A large metal spoon has a good heat capacity and could cause the temp to drop a bit, halting the boil.

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Could you expand upon the mechanism at work here, because I’m having difficulty understanding how this is substantially different from what I said - although, admittedly, my post was more simplified.

The vortex provides rapid organized transport for the heat, which continues for a while after you stop stirring, and actively suppresses bubbles by bringing them more quickly into thermal equilibrium with the bulk fluid.
It’s about the same as you said, just a little more detailed.

Gotcha. You left out the word thermal before the word equilibrium in your previous post, which threw me off slightly.

The sound of water boiling - what’s that?