Monday, January 29, 2007


Oooh - this is a nice video. The idea is that when you heat or cool something, it doesn't change state automatically. There needs to be a bit of dirt or something for the new state to start on. So here they've got a bottle of clean water and it's cooled quite a bit below room temperature but hasn't frozen because it was clean. Then they shake it, which introduces an irregularity so it freezes.

You can get the same with boiling water, especially if it's in a microwave in a very clean cup. You can heat the water to 120C or so without it boiling. But then, if you let it touch your lips, the lips provide an uneven surface, so it all boils then and you end up with a faceful of steam, which is really not nice. Don't try that at home.

And I referred to that in an essay I wrote last week on the Reformation.


Anonymous said...

As it becomes ice, why doesn't it expand and deform the bottle?

John said...

As I recall, a fair bit of expansion takes place on the cooling rather than the freezing, so it's already expanded.

Fast transitions like that also generally don't involve expansion or contraction - the molecules don't have time to rearrange fully... I think.

Anonymous said...

So how does one supercool something?

In the past i have put unopened, chilled bottles of water into the freezer compartment of fridges and then had the tops pop off.

The bottles in the video were free from condensation, how come?
They did appear to be placed over the vents which may have been the a/c vents, so the air could have been really chilly and the passing breeze may have kept away the condensation, but then how come the jiggling of the car didn't start the reaction?

Can we be sure it wasn't a bottle full of that cunning gel stuff that sets hard and fast, as in the video, when a bimetalic thingy is clicked, but rather gives out heat?

John said...

It'd need to have had a fair bit of air in the bottle to start with, otherwise the expansion would, as you point out, have burst it.

Free from condensation - I assume that it hadn't been above 0 for a while. You need water vapour in the air for condensation.

I don't think the engine was running.

Yes, it's possible it's some kind of fast phase transition in gel or something (which is still usually an example of supercooling), but I don't think it should be too difficult to do with water.

You supercool something by getting it a fair way below its freezing point (-6C is the value they quote for water) while making sure there's no irregularities - needs to be still, etc.

The further you get below the freezing point, the harder it is for things not to condense. That's why pretty much all water will freeze in a freezer at -18C.

Anonymous said...

So water won't boil in a microwave (I've seen this), but will boil in a hot pan because the hotter pan around the cooler water generates currents which brings on the rolling boil?
...and there seem to be somethings on or in the base of nearly all the pans I've looked at, from where the bubbles of boilling seem to originate.

John said...

The issue in the pan is partly the dents / localised hot spots on the bottom (I'd guess due to imperfections in the pan) which as you point out serve as nuclei for the boiling.

Are they put there deliberately on modern pans? Don't know.