What is your explanation for the Mpemba effect?

a reply to: Arbitrageur

That's pretty much what I meant.
What's the difference between the 2 charts? (Besides the data, that is.)

originally posted by: intrptr
And I don't mean that to insult you, I respect you and your content here, just surprised you would be tangled by some guy claiming he proved it again and again, defying physics, in this case.

I don't think you've read his paper or understand his results, but they don't defy the physics I know in any way, look at the graph I just posted showing the hot water freezes first. The trick is that temperature alone is not the determinant of when water freezes if supercooling can take place.

originally posted by: Phage
a reply to: Arbitrageur

That's pretty much what I meant.
What's the difference between the 2 charts? (Besides the data, that is.)

"We show two typical cooling curves in Fig. 21."

Nothing different that I can tell except they are two different trials, and this aspect of variability of supercooling results is hinted at in the video which suggests that perhaps in one trial a piece of dust might be present and in another trial it might not be present which may speed up (or not) the formation of ice in supercooled water, causing inconsistent results which is my interpretation of these graphs though the video was talking about generalities and not this specific experiment.

a reply to: Arbitrageur

I understand supercooling, thats different.

a reply to: Arbitrageur

Rather than speeding up, it looks more like freezing was delayed for cool water, in the upper chart. Time for warm was pretty similar in both.

So. Tricky experiment to execute. Seems like it should be so easy.

originally posted by: intrptr
a reply to: Arbitrageur

I understand supercooling, thats different.

According to the author of one paper, it's not different but an integral part of the explanation of the mpemba effect in his experiments.

A search for the Mpemba effect: When hot water freezes faster then cold water

An explanation for why hot water will sometime freeze more rapidly than cold water is offered. Two specimens of water from the same source will often have different spontaneous freezing temperatures; that is, the temperature at which freezing begins. When both specimens supercool and the spontaneous freezing temperature of the hot water is higher than that of the cold water, then the hot water will usually freeze first, if all other conditions are equal and remain so during cooling. The probability that the hot water will freeze first if it has the higher spontaneous freezing temperature will be larger for a larger difference in spontaneous freezing temperature. Heating the water may lower, raise or not change the spontaneous freezing temperature. The keys to observing hot water freezing before cold water are supercooling the water and having a significant difference in the spontaneous freezing temperature of the two water specimens. We observed hot water freezing before cold water 28 times in 28 attempts under the conditions described here.

Emphasis mine to address your comment.

originally posted by: Phage
a reply to: Arbitrageur

Rather than speeding up, it looks more like freezing was delayed for cool water, in the upper chart. Time for warm was pretty similar in both.

So. Tricky experiment to execute. Seems like it should be so easy.

There does seem to be a consensus about that much, that the experiments can be trickier to carry out than one might expect.

a reply to: Arbitrageur

These charts exactly demonstrate my post.

For further evidence, notice the converse effect below the latent heat line.

The cold water warms at a greater rate than the warm water.

a reply to: abe froman

The cold water warms at a greater rate than the warm water.

Nah, it's pretty much instantaneous in both cases. That's because of the phase change.

a reply to: Phage

It's easy to see if you look at the chart.

The evidence is pretty clear.

Look, everybody go get two (not glass) cups and fill one with hot water and one with cold.

Toss them in your freezer.

Post results.

originally posted by: abe froman
Look, everybody go get two (not glass) cups and fill one with hot water and one with cold.

Toss them in your freezer.

Post results.

Sure, if you want to try it, these are the suggested guidelines I used though I didn't try varying my freezer temperature:
Mpemba effect

New Scientist recommends starting the experiment with containers at 35 and 5 °C (95 and 41 °F) to maximize the effect. In a related study, it was found that freezer temperature also affects the probability of observing the Mpemba phenomenon as well as container temperature.

Theories not talked about
1. If it was equal volume not mass, hotter water(lets excuse 1-4C exception) is less dense, meaning less absolute water molecules. at boiling water appears to have 5% less mass from my quick google.

2. Air convection currents outside the containers. Hotter water will make hotter air which will create movement in the air, creating an air currents pushes more cold air to the container. The effect is much weaker with cold water.
2b This also creates air currents inside closed containers, creating a faster heat exchange.

3. hotter objects emit more radiation.

Convection inside the container make 2 and 3 more powerful, pushing hotter water to the outside of the container more often letting 2 and 3 occur with greater intensity than standing still water.

There is my attempt at cooling acceleration.

I am wondering if a simple non-water mechanic is being missed here.

Most freezers have ambient temperature sensors in them. Putting in a sample of hot water will raise the ambient temperature in the freezer enough to make the pump/heat exchanger / whatever actually work harder to dump that heat, the effect of which will be an overall reduction of temperature in the freezer. Even though it will eventually level off, freezers are well insulated and that would take longer than it does for the unit to become colder (hence freezing the hot water faster)

a reply to: Arbitrageur

Water is less dense in the solid phase than in the liquid phase. The molecules are farther apart when solid than when liquid. So perhaps the thermal expansion allows the heat of hot water to dissipate faster than cold water. It probably has to do with crystallization under the extreme stress of freezing. The crystals are obviously forming faster when the water is hot -but are they the same crystal structure? If I had to design an experiment I would look to determining the crystal structure of both after freezing - I would also want to observe the crystallization process - maybe under an electron microscope. If the crystals are different then perhaps the density is also different - so do all the standard measurements on both samples.
That's where I would start anyway.

Could it be as simple as the hot water causes the freezer to kick into the cooling cycle quicker as the thermostat reacts to a higher overall temperature? Like shifting gears, the water is pulled towards cooling faster (higher rate of change). Putting both hot and cold into the freezer at the same time would not be a valid control.

a reply to: charlyv


So. Outdoors on a cold, cold night?

originally posted by: Phage
a reply to: charlyv


So. Outdoors on a cold, cold night?

Exactly! (tonight would work - Boston )

So, I did that experiment. 2 paper cups of water , equally filled half way, 1 hot and the other, cold. Both from the respective dispensers of a hot/cold water cooler. Set a foot apart in snow during a N'orEaster we had here tonight. Results, were the hot cup had a skim of ice over it before the cold, if it means anything... but both did not freeze, most likely from being in snow. Not good enough for science but interesting none the less.

Observation granularity 5 minutes between seeing start of ice on top the 'hot' cup, and coming back and both frozen on top.

originally posted by: charlyv
Could it be as simple as the hot water causes the freezer to kick into the cooling cycle quicker as the thermostat reacts to a higher overall temperature? Like shifting gears, the water is pulled towards cooling faster (higher rate of change). Putting both hot and cold into the freezer at the same time would not be a valid control.

originally posted by: MasterAtArms
I am wondering if a simple non-water mechanic is being missed here.

Most freezers have ambient temperature sensors in them. Putting in a sample of hot water will raise the ambient temperature in the freezer enough to make the pump/heat exchanger / whatever actually work harder to dump that heat, the effect of which will be an overall reduction of temperature in the freezer. Even though it will eventually level off, freezers are well insulated and that would take longer than it does for the unit to become colder (hence freezing the hot water faster)

charlyv and MasterAtArms, you are both suggesting the hot water might cause the freezer to try to lower temperature in response and this is a plausible hypothesis for why the freezer temperature might vary, however I don't understand how this might cause the hot water to freeze first. If the freezer gets colder then both hot and cold water would just get colder faster.

a reply to: charlyv
If the supercooling explanation for the mpemba effect is correct, it seems to me that conducting the experiment outside where there are potentially a lot of airborne particles that could trigger the freezing when they hit the water may greatly reduce your chances of getting supercooling, unless you use sealed containers as has been done in some experiments.

originally posted by: jellyrev
Theories not talked about
...
There is my attempt at cooling acceleration.

Not talked about in the thread, but more or less talked about in the video in the opening post so those are plausible considerations which may help to explain the apparent paradox. Good job if you came up with that without watching the video.

originally posted by: Phantom423
a reply to: Arbitrageur

Water is less dense in the solid phase than in the liquid phase. The molecules are farther apart when solid than when liquid. So perhaps the thermal expansion allows the heat of hot water to dissipate faster than cold water. It probably has to do with crystallization under the extreme stress of freezing. The crystals are obviously forming faster when the water is hot -but are they the same crystal structure? If I had to design an experiment I would look to determining the crystal structure of both after freezing - I would also want to observe the crystallization process - maybe under an electron microscope. If the crystals are different then perhaps the density is also different - so do all the standard measurements on both samples.
That's where I would start anyway.

These are interesting questions which I don't recall seeing in the sources I reviewed. What it made me think of is a proposed explanation that doesn't seem that likely to me about hydrogen bonds being involved. There's no doubt about hydrogen bonds having significant observable effects, but I'm not convinced nor do I think the scientific community is convinced that they play a role in the Mpemba effect. If the model suggesting that was more predictive I might change my mind, but it gets in to the density issue:

Why Hot Water Freezes Faster Than Cold—Physicists Solve the Mpemba Effect

Now Xi and co say hydrogen bonds also explain the Mpemba effect. Their key idea is that hydrogen bonds bring water molecules into close contact and when this happens the natural repulsion between the molecules causes the covalent O-H bonds to stretch and store energy.

But as the liquid warms up, it forces the hydrogen bonds to stretch and the water molecules sit further apart. This allows the covalent molecules to shrink again and give up their energy. The important point is that this process in which the covalent bonds give up energy is equivalent to cooling.

In fact, the effect is additional to the conventional process of cooling. So warm water ought to cool faster than cold water, they say. And that’s exactly what is observed in the Mpemba effect.

These guys have calculated the magnitude of the additional cooling effect and show that it exactly accounts for the observed differences in experiments that measure the different cooling rates of hot and cold water.

Maybe except that simple thermodynamics also suggests that the hot water will cool faster because of the larger temperature difference, and apparently I'm not the only one unconvinced by the explanation of Xi and co:

But while Xi and co’s idea is convincing, it is not quite the theoretical slam dunk that many physicists will require to settle the question. That’s because the new theory lacks predictive power—at least in this paper.

Xi and co need to use their theory to predict a new property of water that conventional thinking about water does not. For example, the shortened covalent bonds might give rise to some measurable property of the water that would not otherwise be present. The discovery and measurement of this property would be the coup de grâce that their theory needs.

So while these guys may well have solved the riddle of Mpemba effect, they will probably need to work a little harder to convince everyone. Nevertheless, interesting stuff!