[Jamie]

Jamie asked the Naked Scientists:

Does boiling water freeze faster than cold water?

What do you think?

[lyner]

No.

[Madidus_Scientia]

Yes. Under certain conditions.

Its known as the Mpemba effect. But if you put boiling water in one container and 1 degree c water in another, the almost frozen water will of course freeze faster, but when the margin is a bit more narrowed, the hot water can freeze quicker due to a combination of reasons, like evaporation, which both reduces the volume of the water to freeze and is endothermic, convection currents speeding the transfer of heat, the colder water will be likely to freeze from the top forming an insulative layer of frost, whereas the warmer water will not due to its convection currents.

Here's the wiki link http://en.wikipedia.org/wiki/Mpemba_effect with more contributing factors and explanation.

[Bored chemist]

If you put some water in a strong walled flask and connect it to a suitable vacuum pump the pump will remove the air above the water. After a while the remaining air won't exert much pressure on the water and the water will boil because its vapour pressure exceeds the ambient pressure. As the water boils it needs heat to move molecules from the liquid to the gas phase. It takes this heat from the surroundings and cools down. With a good pump it's possible to get the water so cold it freezes.
Boiling water freezes it; if you just leave water lying about, it doesn't generally freeze so, in the right circumstances, boiling water freezes first.

[lyner]

Yes - but it wouldn't ALL have frozen, would it? The bit that evaporated is not ice.

[Bored chemist]

"Yes - but it wouldn't ALL have frozen, would it? The bit that evaporated is not ice."
Ditto the Mpemba effect.
BTW, don't try that experiment with a vac pump unless you know it's the sort that doesn't mind pumping undiluted water vapour.

[Madidus_Scientia]

Touche.