How does hot water freeze faster than cold water?

Answer 1

Think about it this way...

If you start out with hot water, it has to cool its freezing temperature before it will freeze. Somewhere along the process it will become the same temperature as "cold" water - whatever temperature you choose to define as "cold". It takes time to reach that "cold" temperature and from the point that it becomes cold until it freezes should be the same as it was for water that started out "cold". All else being equal, cold water will freeze faster than hot water.

Notice that I said "all else being equal". There are situations where water that starts out hot may freeze faster than the cold water. As an example - if you fill two ice trays with water - one with cold water and one with hot - and stick them into the freezer, the hot tray will start melting any frost or ice it is placed on. As it does so, it provides better contact with a cold surface that will act as a heat sink as it cools down. Having formed this better contact, it will begin to cool through both conductive and convective heat transfer. The tray that started out cold but which has poor thermal contact with its surroundings mainly cools by convection. Because conductive heat transfer is usually faster than convective heat transfer, the initially hot tray may catch up to the temperature of the initially cold tray and then continue to cool faster because of the boost from conductive heat transfer. Notice that the conditions have to be right for this to occur - the hot tray has to be warm enough to do the melting and establish the good surface-to-surface contact for the conductive heat transfer; there has to be frost or ice for it to be melting, the cold tray can't be too cold or it will start freezing before the hot tray catches up; likewise, the hot tray can't be too hot or it won't catch up to the cold tray; the freezer can't have too much forced convection (fan blowing in the freezer) but rather be primarily natural convection; the cold tray can't be warm enough to melt the ice or frost like the hot tray.

Answer 2

answ2. This was first bought to the attention of science in recent times by Mpemba, and the Mpemba Effect is named after him.

Optimal temperatures for this astonishing experiment are 35oC for the hot liquid, and 5oC for the cold one.

Still not understood - maybe there is a Nobel in it?

Archimedes noticed that in order to freeze a container of liquid, it was better to have it sun warmed first.