Mpemba effect: Why hot water can freeze faster than cold

(PhysOrg.com) -- Scientists have known for generations that hot water can sometimes freeze faster than cold, an effect known as the Mpemba effect, but until now have not understood why. Several theories have been proposed, ...

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Freezing

Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting.

All known liquids, except liquid helium, freeze when the temperature is lowered enough. Liquid helium remains liquid at atmospheric pressure even at absolute zero, and can be solidified only under pressure. For most substances, the melting and freezing points are the same temperature; however, certain substances possess differing solid–liquid transition temperatures. For example, agar displays a hysteresis in its melting and freezing temperatures. It melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F).

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