Freezing warm water - Mpemba effect

[Astronuc]

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.

The Mpemba effect is a name for the observation that hot water freezes faster than cold water.

The effect is named for its rediscoverer, the Tanzanian high-school student Erasto B. Mpemba. He first noticed the effect in 1963 after his account of the freezing of hot ice cream mix in cookery classes, and went on to publish experimental results with Dr. Denis G. Osborne in 1969. At first sight, the effect is contrary to thermodynamics in general.

 

[mezarashi]

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

 

[Gokul43201]

https://www.physicsforums.com/showpost.php?p=518499&postcount=37
https://www.physicsforums.com/showthread.php?t=59180&highlight=mpemba
https://www.physicsforums.com/showthread.php?p=425986&highlight=mpemba#post425986

It's perhaps the only fairly well-known physical phenomenon/theorem/concept that is named after an African.

 

[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

 

[tariq5]

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

 

[ZapperZ]

A more recent and careful analysis of this effect appeared on ArXiv with the past couple of weeks.

http://arxiv.org/abs/physics/0512262

Zz.

 

[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?

 

[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

 

[Gokul43201]

Tautochrone (I am not sure about the writings here...) is the curve where the time it takes to cover some vertical distance is minimum.

I thought that was the brachistochrone !

I also believe 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.

 

[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.

 

[brewnog]

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.

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.

 

[pseudovector]

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

 

[brewnog]

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

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.

 

[Stobbe]

gradient ;)

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.

 

[brewnog]

Stobbe, did you read the link?!

 

[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 :)

 

[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...

 

[ZapperZ]

Even MORE new articles on this. Physics World this month has a good coverage of the ongoing curiosity into this effect.

http://physicsweb.org/articles/world/19/4/4/1

Zz.

 

[quantumcarl]

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

 

[jackiefrost]

Even MORE new articles on this. Physics World this month has a good coverage of the ongoing curiosity into this effect.

Zz.

Thanks for the article link.

jf

 

[ZapperZ]

This thing just won't go away. There's MORE "hypothesis" about this effect.

http://arxiv.org/abs/physics/0604224

Zz.