Settle the Debate: Boiling Time of Water at Different Temperatures on Stove

[Cyrus]

Prove what? Mpemba? Sure! Let me know when you're visiting these parts.

I was just giving you a hard time, I know its true!

 

[NoTime]

the majority of bubbles do nucleate on the container walls.

In my experience, the bubbles nucleate at the location of the heat source.
Not quite the same thing.
Which makes sense since that's where the energy is to generate the phase change.

In the case of a microwave oven, you can get some peculiar results.

Do you think it all boils instantly into a vapor?

Yep!
At least judging by the number of cups water I've mopped up off the bottom of the microwave after they explosively emptied.

 

[Cyrus]

They did that because the microwaves do not agitate it like an open heat source. They vibrate the polar molecules. When you pick the cup up, you agitate it and it rapidly nucleates.

 

[NoTime]

They did that because the microwaves do not agitate it like an open heat source. They vibrate the polar molecules. When you pick the cup up, you agitate it and it rapidly nucleates.

At least you agree that the entire mass got heated to boiling all at once.

I'm so glad they did it before I picked them up.

 

[Cyrus]

Did you touch them or add anything to it?

 

[Hootenanny]

Do you reckon we've scared the OP off?

 

[WhyIsItSo]

In my experience, the bubbles nucleate at the location of the heat source.
Not quite the same thing.
Which makes sense since that's where the energy is to generate the phase change.

In the case of a microwave oven, you can get some peculiar results.

Yep!
At least judging by the number of cups water I've mopped up off the bottom of the microwave after they explosively emptied.

Both of your comments are related issues.

In the first case (and I recently had cause to research this extensively) we need to be careful with the term "nucleation". It is not specific to boiling, but is also a factor in the process of freezing.

In the case of where the bubbles are forming, yes, you might often observe that they seem to mostly occur at the bottom of the pot. Try it, however, with something that transmits the heat more evenly, and you will see a lot more bubbles forming on the sides.

Now that I see what Cyrus was getting at, I can say he is correct. The nucleation is occurring because the temperature of the water is right about at boiling point, and the air bubbles (and even imperfections in the surface of the pot) make it easier for vaporization to occur. In effect, this is "seeding" the formation of vapor bubbles.

In your microwave, you are probably boiling your water in a glazed cup. This cup has very smooth surfaces, I'm willing to bet, and provides little to assist in the formation of vapor bubbles. Hence, the water can superheat - be hotter than the boiling point without actually boiling. When you add your coffee, spoon, sugar, whatever, you introduce something which seeds nucleation; likely in the form of tiny air bubbles. Since there is far more energy than the minimum required to boil the water, this event violently coverts this excess energy into vaporization; thus the "volcano" effect.

This is the same thing for supercooling; again, it is because there is nothing to seed nucleation. Pure water, undisturbed in a smooth container, will not freeze until something like -45C (not sure of the exact number).

 

[NoTime]

Did you touch them or add anything to it?

The usual scenerio was that I heard a whoomp.
The microwave shuts down and water starts dripping out the door.
Took to setting the time a lot lower after it happened a few times.
I tried watching with the light on to see if I could catch it in the act, but the water just boiled in the cup.

The watched pot never explodes

Try it, however, with something that transmits the heat more evenly, and you will see a lot more bubbles forming on the sides.

I was more referring to the fact that only the part of the pot in contact with the heater bubbles. Sure, if you heat it evenly then it all bubbles. You do need energy to make this happen.

Hence, the water can superheat - be hotter than the boiling point without actually boiling.

I'm sure this is what's happening, when the conditions are just right.
I have seen the bubble up effect from adding stuff.
That's just a minor fizz.
The thing I'm talking about will empty the cup!
You can hear it go off too.

It would also happen with styrofoam cups, not just glazed.

I've never seen the newer turntable models do this, probably too much vibration.

 

[ddavis05]

superheating water

so, is it possible to superheat water in a completely smooth container free of impurities until it simply vaporizes without it ever boiling?

 

[IMP]

If the pan with 100 degree water was at the bottom of Death Valley, and the pan with the 50 degree water was at the top of Mt. Everest (but with equal ambient air temperatures), it might be a close race...