Does hot or cold water freeze first?

18 March 2007


If you put some hot water and some cold water in the freezer, which one freezes first?


There's convection currents going on inside the water - if you've got something warm its less dense so it rises upwards and the cooler stuff sinks and this creates a turning circulation. If you put that in the freezer, because the water is stirred up by the heat, the water will start moving and it will carry on moving for quite some time and this will keep mixing the water enabling it after its cooled down to keep loosing heat more quickly than cold water because that's more static and doesn't move so much to start with. Therefore the cold water will be overtaken on the freezing process, possibly by the hot water. The obvious thing would be that the cold water would freeze first, because its got less energy to loose but I have heard that the hot water would freeze sooner but I've not looked in to it in detail.

The convection currents helping the water water freeze first make sense though. The other thing is if you heat up water and boil it you drive off all the gases dissolved in it and those gases might be reducing the freezing point. So if you did the experiment fairly and used distilled water or something it could be an accurate way of testing.

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