# Freezing warm water - Mpemba effect

1. Nov 24, 2005

### Astronuc

Staff Emeritus
http://en.wikipedia.org/wiki/Mpemba_effect

This has been brought up in the past, but I did not know the effect had a name.

2. Nov 24, 2005

### mezarashi

Nice. I'd never think I'd see water 'hysteresis' or whatever it is :P

3. Nov 24, 2005

### Gokul43201

Staff Emeritus
4. Nov 27, 2005

### DaTario

Coanda Effect seems to be also a case of african reference.

this effect, if I remember correctly accounts for the flow of liquids, not vertically, but touching the external walls of the cup.

Best Regards,

DaTario

5. Nov 27, 2005

### tariq5

Does the air trapped in cold water delayes its freezing than the hot water?

6. Jan 10, 2006

### ZapperZ

Staff Emeritus
7. Jan 10, 2006

### LURCH

I've tried on many occassions to get hot water to freeze faster than cold, but the cold water allways froze first. Have any of you ever gotten hot water to freeze faster than cold water?

8. Jan 22, 2006

### DaTario

I think it may help if one considers that in the context of a body falling throught an incline, the shape of the incline have deep influences in the time it takes the body to reach the botton of the curve (suppose the path has one) under uniform gravity field.

Tautochrone (I am not sure about the writings here...) is the curve where the time it takes to cover some vertical distance is minimum.
Isóchrone, if I remenber it well, is the curve (an arc of cicloyd) where the time it takes to reach the minimum from the rest is the same no matter the initial height. Probably it is possible to find curves along which, the higher is the initial position, the sooner the body will reach the minimum. The physical analogy between the thermodynamics of the OP and the mechanics of these curves seems to me as an interesting point here.

Best wishes

DaTario

9. Jan 22, 2006

### Gokul43201

Staff Emeritus
I thought that was the brachistochrone !

I also belive that there is not a direct mapping between the thermodynamics of the mpemba effect and the dynamics of a body falling along a curved path.

Lurch : I got Mpemba to work only once (out of about a half-dozen tries). Someone here (can't recall who..one of the 'matt...'s I think) got it to work repeatedly. A noisy compressor on your fridge will likely ruin any chance of your realizing Mpemba.

10. Jan 23, 2006

### pseudovector

When you have the same volume of hot and cold water, then the mass of the hot water must be smaller than that of the cold water due to differences in the mass densities. So, although the temperature of the hot water is greater then that of the cold water, there less water to freeze, so overall, it might take less time to freeze hot water.

11. Jan 23, 2006

### brewnog

Nope, works when actually using the same amount of water. Really pseudovector, you're making us look like amateurs!

I got it to work once, it was just a case of getting the mass of water right with respect to the container dimensions, and trying a few different samples over the weekend. Only worked a couple of times out of 10 or so.

12. Jan 23, 2006

### pseudovector

There's nothing amateur about overlooking this possibility. I personally know numerous highschool physics teachers and some university lecturers who made the same mistake.
When you conducted that experiment, did you cover the top of the glasses?

13. Jan 23, 2006

### brewnog

Some I did, some I didn't. Some I left air gaps above the water, some I didn't. Some I surrounded with 'stuff' (frozen peas, I think!), some I left with air surrounding. I was curious as to whether the effect was due to convection, evaporation, or something else. Didn't see anything conclusive, but it's interesting when you see something which really counters intuition and logical thought.

14. Jan 26, 2006

### Stobbe

come on guys- think about Newton’s law of cooling, the temperature differential establishes the heat transfer rate, so if you have a bigger temperature differential (water temp vs. ambient temp) you will have a faster heat transfer gradient. Here's the fun part. Since the hot water has a larger temperature change to make before freezing it sets up a nice tortoise and the hare race.

Back to the mechanics analogy, its like racing two boxcars down two hills. One is a short track but shallow slope. The other a very long track but a steep slope. Anyway, I wish someone would do a serious experiment on this. My guess is there is some critical temperature value and differential set in which the cold can freeze first on one side and the hot on the other.

15. Jan 27, 2006

### brewnog

16. Jan 27, 2006

### Stobbe

lol, no I did not read the link. But now that I have read it, I feel rather clever yet much less original.

"...This shows that the initially hot water goes faster, but of course it also has farther to go..."

Although I have my MS is in Mechanical Engineering, I wholly despise thermodynamics and have quite a remedial knowledge of it, and therefore it's not surprising that the solution is more complicated than my simple explanation.

Anyway, thanks,

and if anyone is looking for a job and is skilled in optics, they should apply to Northrop Grumman in Chicago IL, we need some more physics nerds :)

17. Jan 27, 2006

### jackiefrost

I always argued against this effect using a bit of "hare and tortoise" style reasoning as applied to temperature gradients and heat transfer mechanisms. I always seem glued to the notion that at any instant on their journey toward phase change, L1 (the hotter body of water) must always be at some previous L2 state. But, for any delta t interval, L1 does transfer more heat energy to its surroundings' molecules than L2 so maybe there's some choice set of initial conditions or prevailing circumstances (density of surrounding air, water/air interface surface area, molecular characteristics of container, etc) where they could combine to cause a miraculous finish to the race. However, I'm still skeptical and think that the validity of this phenomena (if there is any) would probably be found in a plethora of deliberate as well as unintentional experimental design choices rather than any inherent fundamental thermodynamic principles. I, like Zeno, really can't see L1 at any instant jumping ahead of L2 - or even catching it for that matter.

At least that's my very biased and probably totally ignorant perspective for the moment :-)

Later...

Last edited: Jan 27, 2006
18. Apr 1, 2006

### ZapperZ

Staff Emeritus
19. Apr 1, 2006

### quantumcarl

Does this phenomenon relate in anyway to cold water rinsing suds off of dishes more efficently than hot water?:uhh:

20. Apr 9, 2006