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david.findlay
05-09-2003, 12:04 AM
Can anybody explain if it is true & why that hot water freezes quicker than water at ambient?

REL
05-09-2003, 05:12 AM
It is true.

Greater temp difference increase the rate of exchange, however that is not a good explanation.

The most credible answer actually makes sense. The water is going to freeze on the outer parts first, the internal core temp of the hot water will melt the outer shell which results in a phase change of the solid to a liquid. the heat givin up is specific heat, however the heat absorbed is latent heat which is greater than the specific heat of water. This increases the rate of the BTU's leaving the warmer water.

Pretty poor explanation on my part but it helped. A lot of info can be found if you google it.

bersaga
05-09-2003, 06:51 AM
Guys : Something from Google ! Have fun!

The ability of hot water to freeze faster than cold seems counter-intuitive as it would seem that hot water must first become cold water and therefore the time required for this will always delay its freezing relative to cold water. However experiments show that hot water (e.g. 90 °C) does often appear to freeze faster than the same amount of cold water (e.g. 18°C) under otherwise identical conditions
A number of explanations have been put forward but the most likely scenario is that the degree of supercooling is greater in initially-cold water than initially-hot water. The initially-hot water appears to freeze at a higher temperature (less supercooling) but less of the apparently frozen ice is solid and a considerable amount is trapped liquid water. Initially-cold water freezes at a lower temperature to a more completely solid ice with less included liquid water; the lower temperature causing intensive nucleation and a faster crystal growth rate. If the freezing temperature is kept about -6°C then the initially-hot water is most likely to (apparently) freeze first. If freezing is continued, initially-cold water always completely freezes before initially-hot water. Why initially-cold water supercools more is explained in terms of the clustering of water. Icosahedral clusters do not readily allow the necessary arrangement of water molecules to enable hexagonal ice crystal initiation; such clustering is the cause of the facile supercooling of water. Water that is initially-cold will have the maximum (equilibrium) concentration of such icosahedral clustering. Initially-hot water has lost much of its ordered clustering and, if the cooling time is sufficiently short, this will not be fully re-attained before freezing.
Experiments on low-density water around macromolecules has shown that such clustering processes may take some time It is also possible that dissolved gases may encourage supercooling by increasing the degree of structuring, by hydrophobic hydration, in the previously-cold water relative to the gas-reduced previously-hot water and increasing the pressure as gas comes out of solution when the water starts to crystallize, so lowering the melting point and reducing the tendency to freeze (see guestbook). Also, the presence of tiny gas bubbles (produced on heating) may increase the rate of nucleation, so reducing supercooling

Prof Sporlan
05-09-2003, 02:35 PM
This phenomena is referred to as the Mpemba effect.

bersaga
05-09-2003, 04:57 PM
Named after a Tanzanian boy!!!

Dan
06-09-2003, 03:57 AM
Basically a clever exception to the rule if you dig deep enough into it. Cold water freezes first. Period. Colder ice, by the way, doesn't thaw faster than warmer ice, either.

It becomes symantics.. faster, more quickly, etc. Certainly hot water cools more quickly than cold water exposed to freezing temperatures.

We are into brain teaser thinking here.

But equal water at 32.1 deg F compared to water at 32.0 deg F will freeze just a tad behind the colder water if you can measure it closely.

Everything else is a brain teaser. A fine time to think backwards and eliminate the differences... perhaps even Mpemba can be dealt with if we go backwards.

Argus
06-09-2003, 08:48 AM
Mpemba effect it is.

The lad noticed something that had evaded everyone else for years.

For the STAR PRIZE, suggestions, please, for a practical application.
________
F01 (http://www.bmw-tech.org/wiki/BMW_F01)

Prof Sporlan
06-09-2003, 02:38 PM
At first, the Mpemba effect would appear to violate Newton’s law of cooling:

dT/dt = k * (T – To)

where:
T = surface temperature, absolute
t = time
k = a constant
To = ambient temperature, absolute

But this law really only applies if we have some uniform surface temperature which is only affected by ambient temperature.

In the case of cooling water, we have all sorts of factors to consider, e.g., the size and shape of the container, viscosity and density changes, and the fact that hot water will rise to the surface during the cooling process.

Newton wasn’t wrong with his law of cooling. The Mpemba effect must be explained in terms of these other factors.

Is there a practical application for the Mpemba effect? Surely there is some utility for a manufacturer to freeze things faster is some process application.

Dan
06-09-2003, 03:11 PM
Surface area comes to mind. The size and shape of the container. I am willing to bet that designers and engineers have already taken some advantage of the Mpemba effect without having a person discovering and analyzing what is happening. But I bow my head to Mpemba for rationalizing something that was heretofore a tribal knowledge... an instinct. Willis Carrier falls into that category.

I'm still a skeptic, but only because I am a Newtonian at heart and have to be budged.

.
Is there a practical application for the Mpemba effect? Surely there is some utility for a manufacturer to freeze things faster is some process application



Surely there is some utility for a manufacturer to thaw things faster in some process, too. Beyond the concept of entropy, a process understood should be reversible

david.findlay
06-09-2003, 03:49 PM
Thank you gentlemen..........

Jasper
07-09-2003, 09:41 AM
Surely there is some utility for a manufacturer to thaw things faster in some process, too.

Microwave:D

Gary
07-09-2003, 07:09 PM
I keep telling myself that someday, when I have nothing better to do, I will try to sit down, sift through the technical mumbo-jumbo until I understand this phenomenon, and translate it into plain English (if possible).

Somehow, there is always something better to do. :D

Dan
08-09-2003, 02:41 AM
I keep telling myself that someday, when I have nothing better to do, I will try to sit down, sift through the technical mumbo-jumbo until I understand this phenomenon, and translate it into plain English (if possible).

Gary, I consider you the master of plain English. I think it is just a brain teaser. But Willis Carrier might have thought it a brain teaser when he saw fog when he exhaled.

I am prying my mind open, these days. By the way, isn't Mumbo Jumbo African language?

Gary
08-09-2003, 04:20 AM
Hmmmmm... Does the phrase "Mumbo Jumbo" have African origins? Maybe I should check that out someday.

Now you went and gave me yet another 'something better to do'... LOL

Rango
12-11-2003, 12:09 AM
This phenomena is mythical. Just try it at home. Did that 30 years ago. It doesn't work.

Rango
12-11-2003, 12:56 AM
This phenomena is mythical. Just try it at home. Did that 30 years ago. It doesn't work.

Brian_UK
12-11-2003, 11:43 PM
My Dad always told me that the reason hot water freezes quicker than cold is because the hot water goes straight through the hesitation period when turning to ice having travelled further down the themometer than cold water and doesn't need to slow down near the freeze point.

There, simple isn't it...it's the momentum theory ???

Apprentice
22-11-2003, 06:24 AM
This IS a myth. I cant get all technical about it, the formulai and all thjat jazz, but plainly stated...heat flows from hot to cold, and the greater the differential, the faster the transfer.

This is easily demonstrated and I've done it myself. I took two identical glasses, filled one with hot water, the other with cold water. I ran K type thermocouples from my dual temperature head one to each glass which I put in the freezer, and closed the door on the T-couples and watched the temps drop on the meter. The hot glass dropped temp faster than the cold until they both evened out a few deg above freezing, and both froze at the same time.

No believe? Try it yourself and see.

Bones
30-11-2003, 04:08 AM
Perhaps this is true, i seem to remember having a discussion about this at college... no body wanted to demonstrate to see exactly "what" happened, nor have i tryed it myself to see.

but i will see soon when i'm bored at work hehe ;)

according to the specific heat of water: 4.187 kj/kg
latent heat of fusion: 335 kj/kg
specific heat of ice: 2.11 kj/kg

1Kg water at ambiant say 35'c (were i live) frozen to -10

Q1=MC/\D : 1 * 4.187 * 35 = 146.5 watts
Q2=MLH : 1 * 335 = 335 = 335 watts
Q3=MC/\D : 1 * 2.11 * 10 = 21.1 watts

Ambient to -10
Q1+Q2+Q3 = 502.6 watts

Water at say 70'c to -10
Q1=MC/\D : 1 * 4.187 * 70 = 293.09 watts
Q2=MLH : 1 * 335 = 335 = 335 watts
Q3=MC/\D : 1 * 2.11 * 10 = 21.1 watts

Heated to 70'c to -10
Q1+Q2+Q3 = 649.19 watts

would appear that they would freeze fairly close together... would love to prove a mathematical formula wrong hehehe!

Apprentice
01-12-2003, 07:00 AM
Let us know of your results. IIRC, My hot water was around 120 F and the cold was around 60 F when I started. I didn't take notes or anything, me & my trade buddy did it one night at my house after a technical class we went to together where it came up in discussion.

Mark C
11-03-2004, 08:06 PM
Geeeezzzz..... http://www.mboxcommunity.com/images/Jester.gif All things such as surface areas, containers, etc. being equal...

212°F water must cool to 70°F before it freezes, which does take a little time.

Meanwhile, 70°F water has cooled a little bit during the same period of time, and is somewhat less than the temperature at which it started.

Then, the water that began at 212°F is a higher temperature than the cooler water. Consistantly!

Which will "win the race" to 32°F and freeze first?http://www.mboxcommunity.com/images/rolleyes.gif It IS proper to say that the hot water cools faster though. (The rate of cooling is higher.) This is the old "If a tree falls in the forest, does it make any noise?" question.

frank
11-03-2004, 10:22 PM
This is the old "If a tree falls in the forest, does it make any noise?" question.

Never heard that one before :)

Bones
12-03-2004, 07:53 AM
I think he means, the one "If a tree falls in the forrest and noone is there to hear it, does it still make a noise?"

or the one i got from the simpsons: "The sound of one hand clapping"

hehe, make you think?

chemi-cool
12-03-2004, 01:58 PM
the right frase is:

IF A TREE FALLS IN THE MIDDLE OF THE FOREST BUT NOBODY IS THERE TO WITNESS IT, DOES IT MAKE ANY NOISE?

there is one the same but with a bug.

didnt get the connection, please explaine.


chemi :confused:

Mark C
12-03-2004, 02:23 PM
"Semantics"

Gary
15-03-2004, 03:57 AM
Then there is, "If your wife didn't hear what you said, are you still wrong?"

shogun7
15-03-2004, 05:13 AM
A scenario suggested for the "hot water freezing" story is: the possibility of supercooling and snap-freezing by the hot water is because of having fewer nucleation points for the formation of crystals. Doesn't hot water lose disolved air and minerals in its container?
Roger:confused:

Mark C
16-03-2004, 04:32 PM
Originally posted by shogun7
A scenario suggested for the "hot water freezing" story is: the possibility of supercooling and snap-freezing by the hot water is because of having fewer nucleation points for the formation of crystals. Doesn't hot water lose disolved air and minerals in its container?
Roger:confused:

The answer lies in the logic, not in possibilities.

750 Valve
25-04-2004, 12:06 PM
Once did a service call on small 50kg per day ice machine at a dodgy chinese restaurant in the equally dodgy Kings Cross area in Sydney. Call was machine not making enough ice, turns out they had moved the machine from the bar to the kitchen and hooked it up to a hot water supply by accident, machine took around another 10 mins per cycle (cuber). Moved onto cold water supply and all was well... except for the rest of the place, should have been closed down - more roaches than customers. :eek:

Bones
25-04-2004, 12:53 PM
hehe, some are dodgy...

but cat sure does taste nice!

Peter Mitchell
15-10-2004, 12:12 AM
HI all
I am not convinced
The reason for the effect given by bersaga is flawed he suggests that hot water holds more disolved gas but infact more gas is held by cold water if the water is near to boiling then there would be not much gas in the water.
when dose the effect kick in. if you compared water at 90c with water at 89c would the 90c water freeze first or if it were a particulary cold frosty day and the cooler water was about 5c which would freeze first. I know which way i would bet
remembre there is per deg C only 4.19j/gramm of water of energy to remove before until the water freezes at around 0c and remember the higher the TD the quicker the heat exchange

Coolie
03-11-2004, 11:35 PM
I thought it was because the hot water was less dense than the cold water, therefore causing the hot water to freeze first. Not that technical, but made for a good enough explanation for me.

Prof Sporlan
04-11-2004, 02:37 AM
The factors which contribute to the Mpemba effect seem to be fairly well agreed upon, i.e.:

1. Higher temperature water may lose significantly more heat (and mass) thru evaporation.

2. Higher temperature water may lose more heat thru convection, due to the fact temperature will be less uniform.

3. The type and shape of the container will affect heat loss.

4. Colder water may require greater supercooling before it freezes.

The interesting aspect of this phenomenon is that while there is general agreement that the Mpemba effect exists, there is general disagreement over which of the above factors are most important. And if you don't have the conditions just right, you will not see the effect anyway.

Mpemba was making ice cream when he discovered this effect. But the effect has been verified with water

coolkev
05-11-2004, 08:23 AM
as it has already been pionted out at constant press pure water will only change state at 0 c or 32 f once latent heat is removed the question i feel is too do with shock coolers ie temprites which perform beter with an entering temp of 6-7 degrees rather than say 2-3 degrees a trade school teacher some 30 years ago explaind it as the heat transfer rate is greater with higher temp diff which in turn results in quicker cooling as liquid flows through cooler . too sum up its to do with inertia the heat flow is still quiker when product is almost down to refrig temp where the product that was close to ref temp the flow remains slower through out process. too warm entering and it still will be warmer upon leaving than cooler. the brewries sub scribe to this and is y u should not use same c'room for packaged beer and kegs c'room 2-3 deg keg room 6 degree

Graham
14-11-2004, 06:54 PM
And to think that I moan when the wife puts hot food in the fridge :)

jjpeterson
13-12-2004, 06:30 PM
Weight the ice cubes when they are frozen.
Cold water in an ice cube tray freezes.
Boiling water in an ice cube tray starts to evaporate.
By the time the originally boiling water cools to freezing temperature, there is less of it to freeze. Given loss of vlume in an ice cube tray the cubes have a higher surface to volume ratio an the they will freeze quicker.

To end up with the same amount of ice , one would need to start with a greater amount of hot water and it would take more time.

Ponca Dave
22-08-2008, 01:14 AM
Hot water gives up heat faster due to the TD.but it is behind in the race. It also has less density. It does rise don't it.
BTU's are BTU's. To be fair they must both have the same mass per volume. then I bet on the cold horse.

wambat
22-08-2008, 01:52 AM
Can anybody explain if it is true & why that hot water freezes quicker than water at ambient?

I believe that of all the theories’ that when water is heated, gases are driven out because their solubility in water is less at higher temperatures. Thus, when the hot water cools, it has less dissolved gas than water which was not heated, so it has a higher freezing point and freezes first.

frank
25-08-2008, 09:16 PM
Not a bad BUMP after 4 years.

Just goes to show that even the old posts are still read.

Phil K
09-03-2009, 12:07 AM
I think back in the old days before the frost free freezers were around an ice cube tray filled with hot water would indeed freeze faster than one filled with cold water.
This would happen because the hot tray would melt its way down into the freezer ice a ways creating a larger direct thermal transfer area to the old ice underneath it.
The tray with the cold water would tend to sit up on top of the ice and be surrounded mostly by air.

Its kind of the same way a beer shoved into snow will cool faster than one sitting in cold air!:cool:

bernard
09-03-2009, 07:19 PM
As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice. Theoretical calculations have shown that evaporation can explain the Mpemba effect if you assume that the water loses heat solely through evaporation [11]. This explanation is solid, intuitive, and evaporation is undoubtedly important in most situations. However, it is not the only mechanism. Evaporation cannot explain experiments that were done in closed containers, where no mass was lost to evaporation [12]. And many scientists have claimed that evaporation alone is insufficient to explain their results