BBC Focus Magazine

[Thinker]

I've been given the eyes of sceptism by my colleague and manager at work when I stated that boiling water will freeze faster than cold water. Now I am 90% certain I am correct in this assumption, but I can't answer their question of why it does that. Hence I come to you good and intelligent people to answer the question so I can pretend I know the answer and look really intelligent.

So, why does boiling water freeze faster than cold water?

[ChrisH]

I have also heard about this.
It is called the "Mpemba Effect"

http://en.wikipedia.org/wiki/Mpemba_effect

I could never understand why it doesn't take as long to freeze once it cools down to the same temperature as the cooler water.
I don't think the Mpemba Effect is fully understood by science.

[Liam Sheppard]

poppycock!

I heard that boiling water freezes clear

[M Paul Lloyd]

Depends how you freeze it. I can attest from first hand experience that throwing a bucket of hot water into the air at minus 40 centigrade results in a lot of small pieces of opaque ice not unlike clumps of frost.
Ice can be really weird stuff.

[The Beige Avenger]

I believe it is somehow connected to the Liedenfrost effect where it is easier to evaporate a liquid on a less hot plate than a scorching hot one due to insulating layers of air surrounding the liquid interfering the heat transfer.

In a similar counter-intuitive manner, hot liquids form ice faster than cool liquids in the appropriate environment.

The wiki page has some good suggestions as to the cause.

[scott fairbrass]

i think its because cold water will freeze a layer at the top insulating it slightly (because of the air bubbles ikn the water) were as hot water will have hardly any air bubble in it and can freeze inside out! Or something along them lines!

[MikeG]

"To the first part of the question--'Does hot water freeze faster than cold water?'--the answer is 'Not usually, but possibly under certain conditions.' It takes 540 calories to vaporize one gram of water, whereas it takes 100 calories to bring one gram of liquid water from 0 degrees Celsius to 100 degrees C. When water is hotter than 80 degrees C, the rate of cooling by rapid vaporization is very high because each evaporating gram draws at least 540 calories from the water left behind. This is a very large amount of heat compared with the one calorie per Celsius degree that is drawn from each gram of water that cools by regular thermal conduction.

"It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze before water warmer than 60 degrees C but not before water cooler than 60 degrees C. This phenomenon is particularly evident when the surface area that cools by rapid evaporation is large compared with the amount of water involved, such as when you wash a car with hot water on a cold winter day. [For reference, look at Conceptual Physics, by Paul G. Hewitt (HarperCollins, 1993).]

"Another situation in which hot water may freeze faster is when a pan of cold water and a pan of hot water of equal mass are placed in a freezer compartment. There is the effect of evaporation mentioned above, and also the thermal contact with the freezer shelf will cool the bottom part of the body of water. If water is cold enough, close to four degrees C (the temperature at which water is densest), then near-freezing water at the bottom will rise to the top. Convection currents will continue until the entire body of water is 0 degrees C, at which point all the water finally freezes. If the water is initially hot, cooled water at the bottom is denser than the hot water at the top, so no convection will occur and the bottom part will start freezing while the top is still warm. This effect, combined with the evaporation effect, may make hot water freeze faster than cold water in some cases. In this case, of course, the freezer will have worked harder during the given amount of time, extracting more heat from hot water."

http://www.scientificamerican.com/artic ... -hot-water

[M Paul Lloyd]

This may have originated with the bucket of water turning to snow in the Arctic trick.
Thing is, when they show you this they always use hot water which they say 'works better' but I'm beginning to suspect that they use hot water to prevent it freezing solid in the bucket which would spoil the whole thing.
However it does rather suggest that hot water freezes more quickly than cold even though it probably doesn't.

[Fleck]

This has got to be a myth. Hot water turns into cold water before it can freeze! Or are we saying water can freeze at a higher temperature? It's nonsense.

[M Paul Lloyd]

Yes I'm inclined to agree Fleck.
So there you go everyone, another myth well and truly busted. Possibly?

[MikeG]

Yes I'm inclined to agree Fleck.
So there you go everyone, another myth well and truly busted. Possibly?

Busted, except for special cases?