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Re: Water Boil Water

On Sun, 15 Mar 1998, Steven Su wrote:


I thought it would. I did not try it out yet. But my HOD was actually
trying out
the experiment in his home when his grandmother came in to the kitchen, saw
what he was doing, gave him a thorough scolding, and told him that the
bottle of
water will never boil in this way.

My question is: Is she correct? If she is, what's the theory behind it?

Hi Steven!

Hey, I was just thinking about this a month ago. I don't recall why.
However, I came to the same conclusion.

If we place a cup of water in a microwave and heat it up, it invariably
superheats. If a tiny bit of sand (etc.) is dumped into the heated water,
FOOOSH! Tiny air bubbles in the grains provide water/gas interfaces which
serve as "nucleation points," and the overtemperature suddenly cures
itself with massive boiling.

But on the stove burner, this is not the rule. Must be because the
surface of the pot is way above the boiling temperature, which "seeds" the
boiling even without water/gas nucleation points. It's easy to get
superheating with a microwave oven because the water is heated more than
the container.

So.... If we place a glass jar in boiling water, the glass surface can
approach boiling temperature but never exceed it. Any water in the glass
jar might be heated to boiling, but it will never actually "roil"
(although it might effervesce a tiny bit at the start, as dissolved gas
starts coming out of solution.)

I can think of one way this would be violated and boiling would still
work. If the glass jar is pushed solidly against the hot metal pot, a
steam bubble could develop between the pot and the glass. The steam
temperature could go way higher than boiling temperature, the glass would
be heated, and the water in the jar might actually boil.


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