Why Hot Water Freezes Faster Than Cold Water

Hot water freezes faster than cold water!  Did you know this?  John Lienhard of University of Houston explains:

Water starts freezing only after its temperature has fallen to its freezing point. Then its huge latent heat must be removed at that temperature. Starting out with hot water means only a little more heat has to be removed.

But the flow of heat from a warm place to a cold one is like the flow of electricity. It depends on the resistance. The filament in your light bulb has a very high resistance, so 110 volts drives a current of only about one amp through it.

To see how the heat flow from water in an ice-cube tray is resisted, it helps to break resistance into two parts, internal and external — from inside the water to the metal tray, and from the metal tray to the cooled refrigerator shelf.

Starting out with hot water can affect both. First think about internal resistance: Freezing begins as ice whiskers form on the periphery of the water. They can initially form a layer of insulation and slow the heat flow from the bulk of the water. But if the water is hot, convection currents are set up. They can sweep away those first ice crystals and get all the water down to its freezing temperature more quickly.

If the water is what we call “hard,” with calcium and magnesium bicarbonate dissolved in it, something else happens. Those components reduce the freezing temperature only slightly. But as freezing progresses, their concentration in the remaining water increases, and the freezing temperature really drops. If we preheat the water, we drive these components out of solution and leave them behind in our teakettle.

The incredible thing is that although this phenomenon has been known since Aristotle, we still cannot explain it completely even now.  In 1960’s a Tanzanian School boy named Erasto Mpemba – wrote a paper on this effect.  It is since been called the Mpemba Effect.  His paper was about the effect of preheating on freezing.  He came across that while making ice-cream in his cooking class.