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Tuesday, April 15, 2014

The Mpemba Effect

Mpemba effect (source: Daily Mail)

A homely article in the Daily Mail includes a tip for making ice cubes faster: use hot water.

Warm water cools faster than cold, a counterintuitive fact known as the Mpemba Effect. There are a number of suggested explanations but the latest (covered a few months ago in the DM) focuses on the weak bonds between water molecules, which is why water forms long chains. The latest theory says the bonds are stretched when the liquid is warmed, and snap back as it cools.

It seems that this new theory is not complete.

I have an amateur tweak: boiling not only stretches the long molecule chains, but breaks them, and it takes some time for them to re-form. During this period it may be easier for the separated molecules (or short molecule chains) to reassemble into the crystalline structure of ice, than for long liquid chains to be converted into the solid form.

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4 comments:

Paddington said...

One argument concerns air bubbles.

A K Haart said...

Two identical vessels full of water, one warm one cold.

When the warm water cools, then at some point it will reach the same temperature as the cold water, so it must take longer to cool.

On the other hand, as the identical vessels were full, the one containing warm water contained a smaller mass of water because water expands when warmed.

So the warm water vessel could conceivably cool more quickly because of the reduced mass of water.

Sackerson said...

AKH: that is, I think, one of the explanations considered in one of the links. And, of course, I'm not a scientist.

But I recall seeing an explanation of evaporation that said molecules of a liquid at room temperature, bashing into each other, are in a variety of heat states (I'm not putting this elegantly) so that at some are energised enough to boil off whereas most don't. As the liquid is heated, a greater proportion "boil".

So perhaps as water cools, the converse is true. Some may be starting to form ice-like structures even before the body of water as a whole (or on average)reaches zero C.

Any sense in that?

Changqing Sun said...

Kindly share update on Mpemba effect (with newest refs):
http://www.wikiwand.com/en/Mpemba_effect#/Mechanism
Best!