Why is it that hot water cools faster than cold water in a freezer?
1 Answer
The Mpemba Effect, named for Tanzanian Erasto Mpemba who, in 1963 discovered that hot ice cream mix freezes faster than ice cream mix prepared cold, is a special reaction where hot water reaches freezing faster than colder water but this happens only under certain conditions. Mpemba went on to perform the experiment of cooling two similar containers with equal volumes of water, one starting at 95 degrees F and the other starting at 212 F, the hotter water cools faster.
Modern studies of the Mpemba effect actually show the hot water supercooling to between -6 and -18 degrees.
The specific cause of the reaction is still a matter of debate but is probably a combination of the following factors:
A) Hot water is less dense than cold water. Water at 35 degrees C is closer to maximum density doing very little work to expend energy whereas water at 100 degrees C is at minimum density and doing as much work as it possibly can to expend energy.
B) Hot water at rest lets off steam and vapor more readily than cold water so throughout the course of the experiment there is always less water weight for the hot than for the cold.
C) The hot water is at a higher rate of entropy so the formation of ice can occur all through it. Hot water seems to freeze from the outside in while cool or cold water freezes from the surface down; that can also have an wffect on the freezing rate as surface frost helps to retain heat within the water.
According to Mpemba, himself who selected Nikola Bregovic as winner of a competition to explain the cause of the Mpemba effect, convection and supercooling are the reasons for the effect.
