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Does hot water freeze faster than cold water?


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1 minute ago, The Generation Game said:

It doesn't make sense but there are scientific papers confirming it happening under certain specific conditions. 

There are papers saying they can't replicate the effects.

To be fair the exact parameters of the experiments aren't defined.

I could envisage the hot water forming ice crystals on the surface faster. But the whole body of water won't decrease it's temperature to a set point faster if it starts off hotter.

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6 minutes ago, Great Guy said:

I've read about this before. I think one theory is that hot water melts the ice in  the freezer and there is subsequently a better connection between the freezer and the water.

If you put your hand on ice it gets a lot colder than just keeping your hand in equally cold air.

I’ve also notice that my freezer will go into some kind of turbo mode when something obviously hot is put into it that, I assume, warms the air inside up enough. Sticking and already frozen bag of peas in there for example doesn’t trigger that.

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17 minutes ago, Poseidon said:

There are papers saying they can't replicate the effects.

To be fair the exact parameters of the experiments aren't defined.

I could envisage the hot water forming ice crystals on the surface faster. But the whole body of water won't decrease it's temperature to a set point faster if it starts off hotter.

I must admit that I have heard about the effect and never pressed the matter further. I didn't realise, from the Wikipedia article, that it was so disputed. 

Perhaps Wikipedia didn't exist when I first heard about it?

Note that the figure in that article shows a timescale of around 6hrs. 

image.thumb.png.c93e4cdb7c06acb8137259350593a9fa.png

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The rate of heat transfer is proportional to the difference in temperatures. Therefore something hot placed in a freezer will lose its heat at a faster rate than something cold. It also loses heat from the top by evaporation which in turn cools the body of the water cooling it faster.

The boiling water could possibly  freeze faster than cooler water but it  would depend on the exact circumstances.

It's called the Mpemba effect and has been debated for many years.

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1 hour ago, snaga said:

I think Dave Bloke should test this for us tonight, should be cold enough. Take a bowl of room temperature and boiled water outside at 1am, report back which freezes first.

And what the difference is in the volume of ice remaining in the beakers. I would suspect that where this “effect” is observed a significant volume evaporates off almost immediately, allowing the remaining liquid to freeze faster because of its higher surface area to volume ratio.

 

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