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Author Topic: Someone told me that Hot water is quicker to freeze than Cold water...  (Read 6348 times)

Offline Seany

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Is this true?

I would have thought the other way.. Because as the hot water cools, surely it would eventually reach the temperature of the cold water, and thus it would take longer?
Or is my friend  wrong?


 

Offline neilep

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Oooh Ooh ! *waving at seany*...this has been discussed here many times !!....and here's the answer !!...I can't remember !!...DOH !!

I think the conclusion was that it is true !..I will leave the science up to the science bods !..They live within the borders between each post ewe know !!
 

Offline lightarrow

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In my post of this thread (on a different but related subject) I discussed the question:
http://www.thenakedscientists.com/forum/index.php?topic=19070.0

When I say:

"Of course, in the right conditions that can happen, but to make a real comparison you should put the hot and the cold water in exactly the same conditions, excepting for their temperature"

I mean that, for example, if you put an open pan with boiling water in the freezer, some of the water will evaporate and will condense inside the freezer so you would remain with less water in the pan; with proper amounts (e.g. little thickness of the boiling water inside the pan) you will certainly have that this little amount of hot water which remains, freezes faster than a greater amount of cold water.
« Last Edit: 06/01/2009 14:12:16 by lightarrow »
 

Offline Madidus_Scientia

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The effect only happens under certain conditions, it's known as the mpemba effect.

http://en.wikipedia.org/wiki/Mpemba_effect
 

Offline Seany

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In my post of this thread (on a different but related subject) I discussed the question:
http://www.thenakedscientists.com/forum/index.php?topic=19070.0

When I say:

"Of course, in the right conditions that can happen, but to make a real comparison you should put the hot and the cold water in exactly the same conditions, excepting for their temperature"

I mean that, for example, if you put an open pan with boiling water in the freezer, some of the water will evaporate and will condense inside the freezer so you would remain with less water in the pan; with proper amounts (e.g. little thickness of the boiling water inside the pan) you will certainly have that this little amount of hot water which remains, freezes faster than a greater amount of cold water.

But if it was to be luke warm water, and very cold water.. The cold water would freeze first?
 

lyner

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Yes.
Of course, if you try it in a freezer, and the freezer is in different stages of  its cycle, each time you do the experiment, you may get the unexpected answer. Also, warm water may trigger the cooling pump to operate  sooner, which should also affect the fairness of the test..
But there's no thermal inertia in the simple setup you are implying.
 

Offline Seany

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Yes.
Of course, if you try it in a freezer, and the freezer is in different stages of  its cycle, each time you do the experiment, you may get the unexpected answer. Also, warm water may trigger the cooling pump to operate  sooner, which should also affect the fairness of the test..
But there's no thermal inertia in the simple setup you are implying.

But if the warm water and the cold water were to go into the freezer at the same time alongside each other, the cold water would definitely win?
 

Offline lightarrow

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Yes.
Of course, if you try it in a freezer, and the freezer is in different stages of  its cycle, each time you do the experiment, you may get the unexpected answer. Also, warm water may trigger the cooling pump to operate  sooner, which should also affect the fairness of the test..
But there's no thermal inertia in the simple setup you are implying.

But if the warm water and the cold water were to go into the freezer at the same time alongside each other, the cold water would definitely win?
It still depends on the conditions: if the water containers are closed, it's quite difficult that hotter water freezes faster...
« Last Edit: 07/01/2009 15:02:17 by lightarrow »
 

lyner

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lightarrow:
difficult = definite ?
 

Offline lightarrow

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lightarrow:
difficult = definite ?
lightarrow:
difficult = definite ?
No, wait a moment! If the water containers are closed, so that water cannot vaporize, hotter water cannot freeze faster than colder water. Please, tell me yes... :)
 

lyner

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Yes, yes. I agree. Don't panic!
Presumably some warm water could evaporate and leave less cold water to freeze. So, all the water wouldn't be frozen, necessarily and the test wouldn't be fair because you wouldn't be using the same area for heat loss in each case once the vapour was drifting around in the freezer.

The problem is, with questions like this one, is that they usually originate from a misconception and have a 'simple' answer which can correct the misconception and which corresponds to the straightforward situation. What follows is a load of replies from people who are trying to think of all the clever angles that could possibly be involved.

It's like the one about the boat in a bath with some ice in it. You throw the ice overboard and what happens to the level of the bathwater?
You can get either answer you want if you stretch the situation and the quantities involved. Whereas, the point of the question is surely to make a school-level pupil think about simple things like density and displacement - not anomalous expansion etc. etc.

Perhaps we should award stars to our answers. One star would be the straightforward answer and more stars involve more smartarseness. The guy who just wants a simple answer could stop right at the first one star answer and avoid confusion.

difficult = impossible? perhaps
 

Offline lightarrow

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Perhaps we should award stars to our answers. One star would be the straightforward answer and more stars involve more smartarseness. The guy who just wants a simple answer could stop right at the first one star answer and avoid confusion.
The problem is: who puts the stars? Someone could say,e.g., that your single star put in a post is wrong because that is not the simpler answer, because it's not an answer at all( :)), or the other way around. However, solved the problem of having an authoritative person to put the stars, it wouldn't be a bad idea, after all.

Quote
difficult = impossible? perhaps
It was you to say that, e.g., the fridge's engine could start faster, if you put hot water inside, instead of cold; however, yes, that "difficult" was intended a bit in ironical sense.
« Last Edit: 09/01/2009 22:45:18 by lightarrow »
 

lyner

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A star  voting system, perhaps.
 

Offline swedemick

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I saw a tv programme about Siberia. It was -50 C and this guy threw water up in the air. First he tried cold water that just fell on the ground and eventually froze to ice. Then he did the same thing with hot water. Amazingly the water froze to snow in midair and fell down as snow.
 

lyner

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I think this is a possible explanation.
The air humidity must have been very low. A lot of the warm water evaporated; the rest was cold enough to freeze and there was less of it so it froze quicker than the water which was already cold.
 

Offline lightarrow

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I think this is a possible explanation.
The air humidity must have been very low. A lot of the warm water evaporated; the rest was cold enough to freeze and there was less of it so it froze quicker than the water which was already cold.
Furthermore, it's possible that in those conditions even some of the vaporized water instantly condensed into snow.
 

lyner

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Yes - it's a matter of rate of heat transfer as well as temperature difference. Once the water molecules are dispersed, it can cool much quicker because it is in contact with so much more cold air.
 

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