why does hot water freeze faster than cold water?

Question

anonymous · Answer

Cold water is closer to the freezing point of water.

thomaster · Answer

Hot water does not allways freeze faster than cold water. Only under specific circumstances.
It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze before water warmer than 60 degrees C but not before water cooler than 60 degrees C.

anonymous · Answer

have the two of you done any experiment to support your claims?

thomaster · Answer

I did and it turns out that hot water does NOT freeze faster than cold water
hot water DOES freeze faster than lukewarm water.

I filled 3 ice cube trays with 1 cold water (aprox 5 degrees) 2 hot water (about 60 degrees) and 3 boiling water (100 degrees) and checked in which the cubes froze the fastest. 
The fastest one was the tray with cold water. second the boiling water tray and last the 60 degrees hot water tray.

anonymous · Answer

wrong experiment to test why does hot water freeze faster than cold water

thomaster · Answer

its not an actual experiment i just tried it out of boredom xD

thomaster · Answer

But your statement is invalid.. hot water does not freeze faster than cold water.
I dont know the reason why boiling water freezes faster than hot water.

anonymous · Answer

water molecules start crystalizing at 3℃

thomaster · Answer

yes but that does not explain why hot water reaches 3℃ faster than cold water...

anonymous · Answer

try doing that on separate pails of water more than 5 gallons each, with reasonable amount of differences in temperature.

abb0t · Answer

I've heard about this, I think it was called the mpemba effect? But i don't think its referring to hot or boiling water. I think just in terms of warmth. As in one is warmer than the other. If one is already in cold temperature and the other is boiling, of course the one in colder temp might freeze faster.

agreene · Answer

Its all to do with Enthalpy. 
Unfortunately this doesn't get accurately explained in most chemistry courses until you are in graduate level... however in Physics/Mathematics its a common problem set in differental equations...

$$\large dH = TdS +Vdp +\sum_i{\mu_{i}dN_{i}}$$
is the proper formula... as you can see pressure and temp both play a huge role thermodynamically

agreene · Answer

also, i forgot to mention, the specific application here is known as Enthalpy of fusion.

$$\large\mu^{o}_{solid} = \mu^{o}_{solution} \implies \mu^{o}_{solid}=\mu^{o}_{solution}+RT\ln(X_2)$$
through some differential equations i can derive if you're really that interested....

$$\large \ln(x_2)=-\frac{\Delta H^{o}_{fus}}{R}(T^{-1}-T^{-1}_{fus})$$

agreene · Answer

I suppose i should have atleast said the rearrangement.... move all the mu to one side and then realise Gibbs fusion and the apply Gibbs-Helmholtz http://en.wikipedia.org/wiki/Gibbs-Helmholtz_equation

abb0t · Answer

Are you in graduate school @agreene ?

agreene · Answer

Yes, I'm working on my PhD in computational molecular and structural biology. My undergrad degree was a triple major: Envr Sci, CS, Math with minors in Phys and Chem.. Masters in Biology

agreene · Answer

Lol i also just realised i never actually answered the original question. yes hot water freezes faster, and the hotter it is the quicker it freezes because you have more energy in the system--this is assuming you have the same amount of surface area exposed (of the solution)

abb0t · Answer

What?!

agreene · Answer

what are you not understanding?

also i just realised the opposite might be easier to understand....
if you take 3 cubes of ice, at 3 different temps say... -1C, -45C and -255C the rate at which they decompose to liquid would be that he coldest decomposes faster--this is because they are more able to ABSORB the energy in the system.

agreene · Answer

assuming the same heating temp, pressure and cube surface area

agreene · Answer

erg, and technically gravitational pressure and dipole moments

chmvijay · Answer

http://math.ucr.edu/home/baez/physics/General/hot_water.html this may help you

abb0t · Answer

I think you're going far beyond the scope of what is really being asked here and not answering the question directly. I believe she is for high school chemistry.

agreene · Answer

I do that a lot :(

dean.shyy · Answer

Due to a high temperature present, the hot water will take longer to reach the freezing's point when compared to cold water because it is much closer to the freezing point.

chmvijay · Answer

:)

anonymous · Answer

It is called Mpemba effect.