[lightarrow]

I am not trying to prove the phenomenon.  It has already been proven!  I am trying to find out "WHY" it works that way.  Thanks for comments.  Joe L. Ogan

No, you don't have to prove anything, you only have to answer the question I asked in my previous post.
About the explanations of the convective currents: ok, very nice. But then it's not "hot water freezes faster than cold water", it's: "moving water freezes faster than stationary water".
It's like if I said: "brunettes get more money from their husbands than blondes", but then one says: "ah yes, but *those* brunettes have much more rich husbands"  []

Are we talking about physics or about cabaret here?

[Joe L. Ogan (OP)]

I see that you are never going to give up on this topic, lightarrow.  Where have you been that you have never heard about this before?  It is not a new topic.  It has been around for years and years.  Thanks for comments.  Joe L. Ogan

[Geezer]

Are we talking about physics or about cabaret here?

I'll come if you have any free tickets. Who's in the lineup?  []

[lightarrow]

I see that you are never going to give up on this topic, lightarrow.  Where have you been that you have never heard about this before?  It is not a new topic.  It has been around for years and years.  Thanks for comments.  Joe L. Ogan

It's just this to make me nervous: people shoud understand, after some time. Another thing that makes me nervous is the fact people seems to become more and more prone to be convinced by false science year after year. Will we arrive to the point that everyone will believe on its personal theory, and science will be totally deleted? Science must be precise, otherwise it would be better to throw it away.

[Joe L. Ogan (OP)]

Do you think that this is false science?  Have you read Mpemba?  Ask some of the other guys.  It appears that most are familiar with the topic.  Thanks for comments.  Joe L. Ogan

[lightarrow]

Yes, I think it's false science, and dangerous, since people around seems to learn less and less from schools and so it's more easy to influence with wrong facts.

[rosy]

Lightarrow.

I think you are a bit too sure of yourself. There are lots of perfectly plausible explanations why, under some circumstances, a container of water that started out hotter might end up freezing faster than an otherwise identical container that started out colder.

I don't know, from personal experience, whether such conditions actually exist, but you are trying to apply thermodynamics in an inappropriately simplified form for the situation described.

If the water were at the same temperature throughout the container (top to bottom), it would be valid to assume that the container with the higher temperature must pass through a state with the same temperature as the cooler one started at, that this would take time, and that therefore the hotter sample must take longer to cool.

However, since water does -not- cool at a uniform temperature throughout its bulk, due to convection currents and to the fact that the rate at which heat is lost need not be constant in all directions (and in a freezer will certainly not be so).

No-one is claiming that hot water always freezes faster than cold water, but that under certain circumstances it could really is not the immediately impossible paradox it looks at first sight.

Whether or not the products of modern schooling are excessively inclined to believe "false science" is completely both irrelevant and uncalled for in this context*.

Your attitude to this question smacks, to me, of not knowing enough about the potential complexities of this situation to realise you might want to consider them.

*I don't know where you come from (ETA - having looked at your profile, it appears you're from/currently based in Italy), but my mother, now in her 50s, studied no science at school (England) except for human biology after she was 14. I doubt my brother, now 21, who studied at least some physcis, chemistry and biology compulsorily to 16 knows less science.. even if he doesn't really know enough to do anything meaningful with it.

[tommya300]

What is false science?
How is it that many independently proved facts, can be considered false?
Designating conditions of an experiment, recording events throughout the experiment, providing the results that reoccurs in redundancy over and over again.

[lightarrow]

Lightarrow.

I think you are a bit too sure of yourself. There are lots of perfectly plausible explanations why, under some circumstances, a container of water that started out hotter might end up freezing faster than an otherwise identical container that started out colder.

I know, infact I wrote some myself if you noticed.

I don't know, from personal experience, whether such conditions actually exist, but you are trying to apply thermodynamics in an inappropriately simplified form for the situation described.

No, it'e exactly the opposite: *they* are oversimplifying things. If one said: "under some specific circumstances a pot with hot water can freeze faster than another one with cool water" it would be *very different*. Instead, it's false science to say *hot water freezes faster than cool water*. I know that you don't appreciate all this difference, but there is. The first reason that comes to my mind is that people can be confused and brought to think that hot water freezes faster than cool one, in general and believe me, with especially with the help of internet, the risk is very high. The net has enormously amplified the exchange of informations, and at the same time has mixed the "pure" with the "impure".

I have some teaching experience and I continuously see people who should be cultured, for example journalists, say such stupid things that a child, 30 years ago, could have said: "what are you talking about? Are you crazy?" Who or what is responsible for this?

If you don't believe me, wait some other years and then you'll realize better. I talk with university students of technical faculties, that can tell you amazing things about what they studied, but ask them a simple question of physics, mathematics, chemistry on which they have to make a simple reasoning and you'll put them under embarassement. Some examples:
"if you put a piece of lead on water, will it float or will it go down?" No answer. "How much is it 10 percent of 100?" No answer. "What happens if you mix sodium carbonate with nitric acid? Does reactions happen? Which ones?" No answer.
I'm not kidding! I asked them personally such kinds of question; in my job I have to teach sometimes to new arrived. They know a lot of things, but they cannot put together the simpler ones. And I'm talking of tens of young people.
It can be a problem limited to here in Italy, don't know, I hope in UK it's different. The problem is they teach too much things at school that people becomes totally incapable to reason by its own.

If the water were at the same temperature throughout the container (top to bottom), it would be valid to assume that the container with the higher temperature must pass through a state with the same temperature as the cooler one started at, that this would take time, and that therefore the hotter sample must take longer to cool.
However, since water does -not- cool at a uniform temperature throughout its bulk, due to convection currents and to the fact that the rate at which heat is lost need not be constant in all directions (and in a freezer will certainly not be so).
No-one is claiming that hot water always freezes faster than cold water,

You *think* that no-one is claiming that, but people with poor knowledge and, mostly, poor ability to reason on its own is not able to understand that simple statement you have done.

[Geezer]

Ahem  [], if I might, if we try to address the original question, the answer is no. Thermodynamics still wins (and I think that's Lightarrow's point).

If we ask the question like this;

"Is it possible that under a certain set of very specific circumstances it appears that a mass of warmer water can freeze more quickly than the same mass of colder water?"

I think the answer is maybe. It's not obvious that a satisfactory experiment has been devised that proves the point one way or another.

If someone can take any arbitrary number of molecules of water at different temperatures and demonstrate that the warmer water always crystallizes first, I'll be a believer. Until them, I'm sticking with thermodynamics (and I'm not buying into the idea that we can use cold fusion as a source of energy either).   

[tommya300]

maybe this is possible Geezer?

Producing a miniature weather pattern:
Steamy energetic water, in  a container, producing water vapors, is placed in a freezer.
The surface of the water in the container is cooling as the vapors escape.
But not all water vapors are lost, some condense and circulate above in a cycle to attempt to gain a lattice crystal structure, precipitates, dropping to the surface and transfers the exchange of heat.
When the water surface reaches its highest density the lattice crystallization begins at the condensed vapor level  and begins to displace the warm water below it. As this happens water begins to freeze at the surface first.
It is the vapors of the hot water creating an additional heat sink.
Note it is slushy not solid ice.
The other room temperature water does not have the abundance of energetic water vapors to do this extra heat sink exchange. 

This is the most logical explanation that I have seen in regard to the phenomenon.  Have you performed experiments to substantiate your conclusions?  If so, I believe you should write it up and submit it for consideration.  This is not a new subject.  I remember reading about it many years ago.  Thanks for comments.  Joe L. Ogan

[Geezer]

maybe this is possible Geezer?

Producing a miniature weather pattern:
Steamy energetic water, in  a container, producing water vapors, is placed in a freezer.
The surface of the water in the container is cooling as the vapors escape.
But not all water vapors are lost, some condense and circulate above in a cycle to attempt to gain a lattice crystal structure, precipitates, dropping to the surface and transfers the exchange of heat.
When the water surface reaches its highest density the lattice crystallization begins at the condensed vapor level  and begins to displace the warm water below it. As this happens water begins to freeze at the surface first.
It is the vapors of the hot water creating an additional heat sink.
Note it is slushy not solid ice.
The other room temperature water does not have the abundance of energetic water vapors to do this extra heat sink exchange. 

I sort of see what you are saying Tommy. I'd like to see as few molecules of water as possible involved. I have no doubt that there are all kinds of interesting stratification and transfer effects going on when there is a large volume of water.

If we are going to say that warm water freezes faster than cold water, it would have to be true in all cases, even when there are only a few molecules of water involved.

The other thing I'd like to see is some accurate method of determining the actual thermal energy removal rate. Puting the whole thing into a freezer does not provide any data regarding the rate of energy transfer, which I think is very important, particulaly as we are changing the state of the water.

The energy transfer will consist of two components. The first simply cools the water to 0C, and the second removes the energy to allow the state change to occur. The second component is much greater than the first, particularly when we are starting off with water that's only a few degrees above freezing, so other small variables might be swamping the data associated with the first component.

[tommya300]

Since the question can be restated to represent a particular condition for each sampled model.
Freezing solid was not a condition.
Just to achive an ice crystal and to which system would show a sign to this affect.
Water vapor in abundance can be visually seen. Some supercooling must be occuring with the vapors.

Frost on the side of the container will develop and drop which will help the cooling process, speed things up with thermal sink or contact.
As soon as the surface of the water reaches 4 degrees C it is at its max density displacing an other density of water below it. What happens to the water above this thin layer?
I think the trick is is to prove the water at the surface is at 4 deg C without disturbing the medium.

When it came to freezing a cooked meal, I was told to bring it to room temperature before freezing, Why?
I was told to prevent early signs of freezer burn, frost developing on the food from the hot vapors.

I thought that hot water can cool quicker then cool water as it approaches the freezing point but  the cool water sample will freeze solid before the warmed water sample. At least that is what I can remember

[Geezer]

But if you can't determine a specific condition, it's imossible to run a meaningful experiment.

For example, when is a lake "frozen"?

When the first ice crystal appears, but how would you know?
Or is it when the entire lake is frozen solid - not sure when you'd know that either.
And, if we go back to the original observation, how would you know icecream is frozen?

Of course, as there is only a physical change involved, we could try running the experiment in the other direction.

If the observation is true, I'd expect that we'd observe some interesting non-linearity in temperature if we applied a constant amount of thermal energy to ice. (It's probably a lot simpler to run that experiment too.)

[JP]

I think this is a very good example of how models of a system work in physics.  Usually, if you're trying to model a system you start with the simplest possible model based on known physics.  Then, if the experiments match its predictions, you keep the model.  If experiments disagree with it, you start looking for ways in which you might have oversimplified things, and you introduce a more complex model (still base on known physics).  The problem I have is when people take the failure of their first model to mean that they've discovered some completely new physics.  I don't think that's where Joe was going with this question.

The problem here is that there is a simple and very intuitive model: cooling rate depends only on temperature of the water.  This model is apparently wrong, according to experiments.  If you think about it, it's not so hard to accept that it could be wrong.

Consider a simple example of two identical metal balls.  I heat the center of one up, while I heat up the surface of the second in a way that they both have the same average temperature.  Then I leave them to cool.  The one with the heated surface will cool faster, since all its heat is right at the surface where it can escape to the surrounding air easily.  In the second ball, the heat has to flow form the center outwards until it reaches the surface, then it can escape.  The problem is that you need to use heat transfer models here, not just an ideal-gas-law-type equation.  

Modeling the water is much more complicated since you probably need to account for convection currents, which depend on the temperature of the water, along with the different cooling rates on each surface of the water's container.   This isn't violating thermodynamics, but its a more realistic model than just assuming cooling rate depends on temperature alone.  It's also extremely hard to compute, which is why I think there isn't a satisfactory answer yet.  You also have to know the initial conditions of the water--I don't know if assuming an initially uniform temperature distribution, but a container with an open top is enough to cause this effect.  Maybe I'll try an experiment this weekend.   []

[JP]

By the way, the title of the post, "Will hot water freeze faster than cold waver?" is a bit misleading since it makes it sounds like it always happens.  The question Joe asks in the first post isn't misleading, since he asks if it's possible for it to happen.  Yes, it is possible, but no--it can't always happen.

[tommya300]

By the way, the title of the post, "Will hot water freeze faster than cold waver?" is a bit misleading since it makes it sounds like it always happens.  The question Joe asks in the first post isn't misleading, since he asks if it's possible for it to happen.  Yes, it is possible, but no--it can't always happen.

Will hot water ever freeze faster than cold water.  If it does, why does it freeze faster than cold water?  Thanks for comments.  Joe L. Ogan

JP isn't this his original post?

Where does he say, "if it's possible for it to happen"?

I see that different conditions affect the model! Promoting different results

[Geezer]

Ah, but we're not talking about cooling the water. The question was about freezing, but it's not very clear what was meant by that term.

If it means the entire mass of water becomes frozen, the discussion about localized effects may be less important. If it means when the first ice crystal forms then localized effects are more important.

If we are only interested in the rate of thermal energy transfer while the water is still liquid, it's quite simple to include an agitator in the water sample so that it has fairly uniform temperature. In fact, as we (hopefully) agree that pure water only starts to freeze at 0C (at STP), we can then leave out the freezing bit altogether, and focus on the energy that has to be removed to lower the temperature to 0C.

I'm reasonably confident that if we run that experiment we will observe that the amount of heat that has to be extracted is always consistent with the initial temperature of the sample.

If we want to get a bit more sophisticated, if our apparatus removes heat at a constant rate, we can even determine a "freezing point" when the water is starting to freeze. When that happens, the temperature of the sample will remain almost constant because of the very large amount of energy that has to be removed for water molecules to change state.

BTW, if we don't allow the water sample to be mixed/stirred/agitated, how do we even know what its temperature is?

[tommya300]

But if you can't determine a specific condition, it's imossible to run a meaningful experiment.

For example, when is a lake "frozen"?

When the first ice crystal appears, but how would you know?
Or is it when the entire lake is frozen solid - not sure when you'd know that either.
And, if we go back to the original observation, how would you know icecream is frozen?

Of course, as there is only a physical change involved, we could try running the experiment in the other direction.

If the observation is true, I'd expect that we'd observe some interesting non-linearity in temperature if we applied a constant amount of thermal energy to ice. (It's probably a lot simpler to run that experiment too.)

You can not observe the full surface of a lake that would take some doing.
Bit a cup of hot water in  a freezer can be observerd. I am sure you can see a small crust of ice forming in an open lid in the freezer.
Joe wanted to know why it happens.

[tommya300]

Ah, but we're not talking about cooling the water. The question was about freezing, but it's not very clear what was meant by that term.

If it means the entire mass of water becomes frozen, the discussion about localized effects may be less important. If it means when the first ice crystal forms then localized effects are more important.

If we are only interested in the rate of thermal energy transfer while the water is still liquid, it's quite simple to include an agitator in the water sample so that it has fairly uniform temperature. In fact, as we (hopefully) agree that pure water only starts to freeze at 0C (at STP), we can then leave out the freezing bit altogether, and focus on the energy that has to be removed to lower the temperature to 0C.

I'm reasonably confident that if we run that experiment we will observe that the amount of heat that has to be extracted is always consistent with the initial temperature of the sample.

If we want to get a bit more sophisticated, if our apparatus removes heat at a constant rate, we can even determine a "freezing point" when the water is starting to freeze. When that happens, the temperature of the sample will remain almost constant because of the very large amount of energy that has to be removed for water molecules to change state.

BTW, if we don't allow the water sample to be mixed/stirred/agitated, how do we even know what its temperature is?

You don't disturbing the model will affect the model, but can you agree that the hot water has more convection currents going on in the hot water and the vapors above the surface, then the cooler water sample. Can there be a vapor barrier between the water and the vapors in both system models?
Can the system having the more vapors condense and supercool more mass and drop into the water breaching the vapor barrier, in short time period?


Is it easier to prove ice will always start to melt at 0 deg C,  then it is to prove it will always freeze at 0 deg C?