Why does hot water freezes faster than cold water?

Answer 1

While this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1-3], it was not introduced to the modern scientific community until 1969, by a Tanzania high school student named Mpemba. Both the early scientific history of this effect, and the story of Mpemba's rediscovery of it, are interesting in their own right -- Mpemba's story in particular provides a dramatic parable against making snap judgements about what is impossible. This is described separately below.
It seems likely that there is no one mechanism that explains the Mpemba effect for all circumstances, but that different mechanisms are important under different conditions.

Evaporation -- As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze.
2. Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. It has been speculated that this changes the properties of the water in some way, perhaps making it easier to develop convection currents (and thus making it easier to cool), or decreasing the amount of heat required to freeze a unit mass of water, or changing the boiling point. There are some experiments which favor this explanation but no supporting theoretical calculations.
3. As the water cools it will eventually develop convection currents and a non-uniform temperature distribution. At most temperatures, density decreases with increasing temperature, and so the surface of the water will be warmer than the bottom -- this has been called a "hot top." Now if the water loses heat primarily through the surface, then water with a "hot top" will lose heat faster than we would expect based on its average temperature
The fact that hot water freezes faster than cold has been known for many centuries. The earliest reference to this phenomenon dates back to Aristotle in 300 B.C. The phenomenon was later discussed in the medieval era, as European physicists struggled to come up with a theory of heat. But by the 20th century the phenomenon was only known as common folklore, until it was reintroduced to the scientific community in 1969 by Mpemba, a Tanzanian high school student. Since then, numerous experiments have confirmed the existence of the "Mpemba effect," but have not settled on any single explanation.
VR

Answer 2

This is a totally illogical question. Obviously hot water has to cool before it can freeze. So it can't freeze faster than cold water because it has to become cold water in order to freeze.

Answer 3

It depends on what kind of gases that dissolved in the water. The amount of energy to freeze the water is totally different issue with how fast hot/cold water to freeze.

Answer 4

It doesn't. Try for yourself. Put same amount of hot water and cold water in two containers in freezer. See what freezes first.

Answer 5

HOT WATER FREEZES FASTER THAN COLD WATER JUST BECAUSE THE RATE OF FREEZING OF HOT WATER IS FASTER THAN THE THE RATE OF FREEZING OF COLD WATER,IN OTHER WORDS WE CAN SAY THAT IT IS THE PHYSICAL NATURE OF HOT WATER.

Answer 6

Same can be the phenomena that a person comes home from walk and turn on the AC ,he is waiting for room to be cool not knowing that it has been since last 5 mins. After 20 mins his friend come from football match ,sweating alot.As he enters the room he feels cold atmosphere immediately than the 1st person and chills.

Answer 7

It doesn't. The specific heat of water is the same at 100 as it is at 50 as it is at 0. The specific heat is the amount of energy required to change the temperature of the water by a certain amount.

Answer 8

because evaporation cooling helps in reducing the temperature
faster

Answer 9

It doesnt! This is an old tale and is not true!