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Does Higher Temperature Water Freeze Faster?

Posted: Wed Jun 29, 2011 3:33 pm
by adhiluhur
I've read an article about Mpemba Effect that warmer water freeze faster. It was first observed by a 13 years old student from Tasmania, Erasto Mpemba.
When he presented this phenomenon, he was ridiculed by his classmates and teacher. But later on Dr. Denis G. Osborne visited his school, experimented on the issue and proved that it was true.
In 1996, Mpemba together with Osborne published their result.

Oddly enough, I experienced the same thing and was ridiculed as well by my teacher and classmates in high school :(

At first I thought warmer water freeze faster because of evaporation, in which the equation of Q=mc(T2-T1) made sense.
However, it must not be true because Mpemba collaborated with Dr. Osborne to experiment and even published their result.

So, given two bodies of water, which are identical in all parameters (mass, shape, surroundings, etc.) except that one is initially at a higher uniform temperature than the other, the hotter water will freeze first. What do you think?

Re: Does Higher Temperature Water Freeze Faster?

Posted: Wed Jun 29, 2011 8:41 pm
by angle.alpha
hot water freezes faster partly due to the fact that that it evaporates more leavng less water to freeze into ice.
it doesnt entirely explain it, wiki once again has other a variety of explanations.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Wed Jun 29, 2011 10:21 pm
by adhiluhur
angle.alpha wrote:wiki once again has other a variety of explanations
I also wiki'ed it but do you know that Wikipedia can be edited by anyone, including me and yourself?
Unfortunately, UNSW does not has article about this.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Wed Jun 29, 2011 10:48 pm
by angle.alpha
i learnt about this via science by email in 2010, which is run by CSIRO
here is the link to the experiment and explanation they gave. hopefully you'll find it more reliable and englightening than wiki.

http://www.csiro.au/helix/sciencemail/a ... inghot.htm

Re: Does Higher Temperature Water Freeze Faster?

Posted: Wed Jun 29, 2011 11:56 pm
by adhiluhur
angle.alpha wrote:i learnt about this via science by email in 2010, which is run by CSIRO
here is the link to the experiment and explanation they gave. hopefully you'll find it more reliable and englightening than wiki.
http://www.csiro.au/helix/sciencemail/a ... inghot.htm
I've read the article, however it still does not clearly explain why and what conditions must be fulfilled for Mpemba Effect took place.
Furthermore the arguments are weak, is this for high school students?
Usually high school information are less accurate compared to article intended for university, one of the example they say that there are three state of matter which are solid, liquid and gas. When in reality there are actually 6.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Wed Jun 29, 2011 11:59 pm
by adhiluhur
CSIRO wrote:...To get the best results, this water should be about 35 degrees Celsius. If it's too hot, add cold water in small amounts while stirring and checking the temperature with the thermometer.
CSIRO wrote:… In the hot water container a strong convection current forms due to the big temperature difference between the cold freezer and the hot water. This lifts the hot water to the surface where it rapidly cools. This cycle quickly removes heat from the water in the container until it is cold enough to freeze.
They shows that the initially hot water goes faster, but of course it also has farther to go. So whether it actually finishes first (that is, reaches 0°C first), is not clear from the above discussion. It should also be noted that the density of water reaches a maximum at 4° C. So below 4°C, the density of water actually decreases with decreasing temperature, and we will get a "cold top." This makes the situation even more complicated.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Thu Jun 30, 2011 12:18 am
by adhiluhur
Moreover, why should the water be around 35 C? Isn't it better to have, say 50 C? Furthermore, even though the higher temperature=stronger convention current, faster cooling is irrelevant because as the cooling taking place, the convention current become less and less strong. Its like saying cold water does boil faster than hot water because the rate of heating of a liquid depends on the magnitude of the temperature difference between the liquid and its surroundings (the flame on the stove, for instance) ;this is true argument but wrong statement. Because cold water only absorbing heat faster while it is still cold
As far as I know, there is no theoretical work supporting this explanation for the Mpemba effect.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Thu Jun 30, 2011 12:18 am
by adhiluhur
Supercooling (water freezes not at 0°C, but at some lower temperature) may be the answer. This would mean that the initially warmer water might freeze first because it would freeze at a higher temperature than the initially cooler water. Even if this true (I have neither knowledge nor understanding on how to reduce water temperature below 0 C in standard pressure, anyone can explain?), this would not fully explain the Mpemba effect, because we would still need to explain why initially warmer water supercools less than initially cooler water.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Thu Jun 30, 2011 12:21 am
by adhiluhur
A number of possible explanations for the effect have been proposed, but so far the experiments do not show clearly which, if any, of the proposed mechanisms the most important one is. While it is often to hear confident claims that X is the cause of the Mpemba Effect, such claims are usually based on guesswork, or on looking at the evidence in only a few papers and ignoring the rest. Of course, there is nothing wrong with informed theoretical guesswork or being selective in which experimental results you trust; the problem is that different people make different claims as to what X is.
Why hasn't modern science answered this seemingly simple question about cooling water?
Maybe a UNSW student should write this as a thesis material? lol.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Thu Jun 30, 2011 2:13 am
by angle.alpha
hahaha you're way too rigorous and analytical for me!! i just frankly have no idea :P can't wait see what joe is going to say. sorta reminds me of the phenomena of lightning - such a common occurence but yet one clearcut complete explanation. and also the "how do aeroplanes fly" with bernoulli/newton etc (thats sort of contentatious as well, and youre not going to get a single one answer. not even nasa!). also, a crookes radiometer, is explained by both Einstein's and Reynolds's forces although it still isn't clear which one is stronger.
maybe its true that many things don't have just one explanation...

Re: Does Higher Temperature Water Freeze Faster?

Posted: Thu Jun 30, 2011 10:41 am
by joe
Unfortunately, UNSW does not has article about this.
Hey, you didn't look very hard! There is indeed a page on Physclips devoted to this very problem.
http://www.animations.physics.unsw.edu.au/jw/Mpemba.htm

Apart from the very important artefacts, there are a couple of subtle effects involved - but go to that site for the details.

Joe

Re: Does Higher Temperature Water Freeze Faster?

Posted: Fri Jul 01, 2011 11:50 pm
by tagitables
Well, I learned something today.
If you want to make ices, use hot water.

Re: Does Higher Temperature Water Freeze Faster?

Posted: Mon Jul 04, 2011 8:47 pm
by joe
Well, I learned something today.
If you want to make ices, use hot water.
I hope that that was ironic. But just in case: it is usually better to make ice from cold water because
- it uses less energy and
- it is usually faster (if you avoid some of the artefacts mentioned in http://www.animations.physics.unsw.edu.au/jw/Mpemba.htm)

Joe