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tekn0lust's avatar

Does hot water freeze faster than cold water?

Asked by tekn0lust (1861points) July 1st, 2008

This has been an age old debate between me and a college buddy. The ice cube question earlier today made me think of asking the collective.

My buddy asserts that if you fill one ice cube tray with 95 degree water and one with 40 degree water then put them in a freezer that the hotter water will form ice cubes quicker.

My buddy is an environmental engineer and has had a lot more chemistry than I have so I’m inclined to believe him, except for the fact that it doesn’t make sense to me.

…and please don’t tell me to go google it. I have I want to know what the collective thinks.

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13 Answers

Jbor's avatar

Yes, it can. Surprisingly :-)
Full explanation here : http://math.ucr.edu/home/baez/physics/General/hot_water.html

soundedfury's avatar

Yes, it can in certain conditions. It’s called the Mpemba effect. There really isn’t any one reason why it happens, though. Part of it has to do with evaporation and convection, but there isn’t a single reason.

Jbor's avatar

With a difference of 55 degrees the cold water should freeze first though. But if the difference is not significant the hot water freezes first. But read the article, I find it difficult to summarize it all :-)

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Lightlyseared's avatar

Yes and given that you probably have access to a freezer, water and a watch you could carry out the experiment your self and prove it.

beast's avatar

No. The cold water is already ready to freeze. The hot water must cool down, then freeze.

Lightlyseared's avatar

sorry beast my experiment would prove you wrong.

richardhenry's avatar

@beast: With more efficient convection due to the heat, the heat dissipates from the warm water faster than the cold water. Ergo, the warmer water reaches freezing point faster.

Lightlyseared's avatar

There are actually 4 theories.

Evaporation—As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze.
Dissolved Gasses—Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. It has been speculated that this changes the properties of the water in some way.
Convection (see above)
Surroundings—A final difference between the cooling of the two containers relates not to the water itself, but to the surrounding environment. The initially warmer water may change the environment around it in some complex fashion, and thus affect the cooling process. For example, if the container is sitting on a layer of frost which conducts heat poorly, the hot water may melt that layer of frost, and thus establish a better cooling system in the long run.

It moust also be pointed out that it does not happen in every set of circumstances ie if the hot water is at 99.9 C and the cold water at 0.1 C then the cold water will freeze first but if you use identical containers and water from the hot and cold kitchen taps it should work for you.

soundedfury's avatar

@lightlyseared – These are 4 theories, they are all factors that play into the same effect. Independently, a single one of these factors will not account for the Mpemba effect.

Lightlyseared's avatar

Thats why we need a unified theory!

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