Why does hot water freeze faster than cold water?

Question

It doesn't make sense.

to those who said that it isnt possible: Have you ACTUALLY tried this?

Answer 1

Hot water will drop in temperature at a faster rate, but the cold water will freeze first except under certain circumstances in a phenomenon known as the Mpemba Effect

http://math.ucr.edu/home/baez/physics/General/hot_water.html
for more details on the phenomenon

Answer 2

There are at least five effects that contribute to hot water freezing faster.

1) Evaporation. Some of the hot water evaporates, leaving less water left to freeze.

2) Supercooling. Hot water is more likely to get supercooled below the freezing point. The hotter water near the walls of the hot container keeps the cool water away from nucleation points where the freezing process begins. Supercooled water then freezes faster.

3) Convection. Once the cold water freezes on the surface, it makes an insulating layer against further freezing. But as the hot water cools it sinks setting up currents which stir the liquid as it begins to freeze, cooling it quicker.

4) Dissolved Gases. Dissolved gases lower the freezing point below zero. Heating water drives out the dissolved gases so it freezes at a higher temperature than the cold water.

5) Conduction. When placing a object in a freezer, the hot container will melt ice under it, making a better contact with the cold surface than the insulating layer of ice under the cold container.

Answer 3

LoL. Who says it does?

To lower the temperature of water to the point where it becomes a solid, one must slow down the molecular movement of that water.

Think of the freezing point as 0 and the boiling point as 100. Each degree you lower the water takes energy (the freezer!) and how many degrees you must move that temperature downward determines how much energy the freezer will use. And the freezer can only "move" the heat away from the water so fast.

So, hot water would take more energy and more time to transfer the heat away from the water.

Trust me. Water @ 1 degree C will freeze much, much faster than water at 99 degrees C.

(EDIT: 10 minutes later.)

I have just learned of the "Mpemba Effect" from the answerer after me. Interesting subject. However, the conditions under which the effect occurs are so constraining and specific, for all practical purposes, as to be a rare occurrence.

I'll stand by my original answer/statement.

But... you can teach a old dog new tricks!

Answer 4

It doesn't!!! In order to freeze, the hot water must first be chilled to the colder temperature on the way to freezing. As this cannot be done in negative time, it takes longer for hot water to freeze.

For the same reason: Hot water boils faster than cold water... just in case you were thinking of asking...

Answer 5

That statement is false. The only way hot water freezes faster than cold water is that the hot water is exposed to a lot colder temperatures.

Answer 6

It doesn't, that is a fallacy. Think about it, it has to cool off before it freezes, then it is just cold water, just like if you put it in the freezer cold. Once it is cold, it can start freezing. If you don't believe it, put ot water in one container and an equal amount of cold water in an identical container and watch to see which one freezes first.

Answer 7

Thats true, Ive also heard that cold water boils faster than hot water....weird.

Answer 8

cuase hot water is lighter than cold water and cold water is already use to getting freezed

Answer 9

It doesn't. Cold water does. It's just a myth.

Answer 10

I think it's an old wive's tale! it's called the Mpemba effect, I just looked it up-check it out, I guess under some circumstances it does!