Hot water freezing faster then cold water??

Question

I've heart that in some conditions, hot water will freeze faster then cold water, how is this possible?

oh god, ok I'm not understanding this, can this be explained simplier?

Answer 1

On a very cold winter day ( like about 10 - 0 F or colder) take a cup of boiling water and quickly empty it up into the cold air and you will have instant snow. The hot water evaporates faster and that takes heat so in this case it takes less heat to freeze hot water than cold water. After all effect of evaporation is cooling.

The energy required for freezing, evaporation and to heat is:
Cl (fusion) =334 J/g K
Cl(evaporation ) = 2272 J/g K
while to heat a 1 gram of water by 1 deg K ( or C in this case) is only
Cp=4.1813 J/g K

However
If you put hot and cold water in the freezer the cold water will start developing ice crystals faster.

Good thinking Jack! ;-)

________________________
Simpler explanation
________________________
For 1 gram of water (approximately)
to cool from 20 C to 0 C = 4.1813 x 20= 84 J
to evaporate (no change in temperature) = 2270 J
Do you see the change in heat? now
to cool from 100 C to 0 C = 4.1813 x 100= 420 J

Note that I can use the energy required to evaporate 1 gm of water I can cool # grams of water = 2270/420= 5 grams of water. That means if I can force 1 gm of water to evaporate it will bring 5 gm of boiling water to a freezing point.

Since Cl (fusion) =334 J/g K I will need for my 5 gm of cool ( 0 C )water
Q= 334 x 5 = 1670 J

Does that help?

Answer 2

This is true, in some specific, rigorously controlled circumstances, hotter water freezes faster than colder water. This is because the escape of heat of evaporation can cause the hotter water to lose its heat and crystalize immediately. It goes from being a liquid to solid rapidly.

Take a cup of boiling water outside in Antarctica, throw it in the air, and it will hit the ground as ice. Take a second cup of 40 degree F water, do the same, and it will still be water when it hits the ground.

Answer 3

If you took two equal masses of water (say 150grams each), one boiling at 100C and one at around 70C, and threw them in the freezer it could happen. The 100C water will freeze faster but in the end, the mass will be less than the ice that was initially 70C.

That's because each gram of vapor that comes off the water at 100C, takes enough energy from the remaining water, to drop the temperature of 10 grams by 54C. This causes rapid cooling with a slight drop in mass.

Note:
Experiment has shown that boiling hot water will not freeze before water below ~60C, but it will freeze before water above ~60C.

Answer 4

One day I finally decided to try this experiment myself, and the result was just what I expected: cold water freezes faster.

Answer 5

There ain't no real effect like that. It's pretty much a result of poorly done experiments. So if you want to learn how NOT to do a physics experiment, this is a cool example. But if you know how to do them right... you will be disappointed, because the cold water will always freeze before hot water. And both will take a rather well defined time to do so.

:-)

Answer 6

Its called the Mpemba effect.

And highlights just how difficult it is to carry out well controlled experiments. Anyone who argues that the effect is false because the hotter body has to cool to the lower temperature before cooling further has misunderstood the problem.

Answer 7

It's an urban legend.