What freezes faster in an ice tray, cool or hot water?

[timejim]

I have heard this argument all my life. Some say put hot water in an ice tray and it will freeze quicker than cool water. I don't follow the logic to that way of thinking. Do you?

What is the right answer?

 

[Evo]

Originally posted by timejim
I have heard this argument all my life. Some say put hot water in an ice tray and it will freeze quicker than cool water. I don't follow the logic to that way of thinking. Do you?

What is the right answer?

Depending on the circumstances, water that has been heated/boiled "can" freeze faster than cold water. There are a lot of variables so you can't just say yes. It has to do with circulation, evaporation, if the water was previously boiled air would have been removed etc...

At least, this is my understanding.

 

[russ_watters]

All other conditions being equal, cold water will freeze faster.

Think about it - before the hot water freezes, it'll have to cool down to the temperature of the cold water. After that, freezing times are the same. So the time (and energy) it takes the hot water to freeze is the time it takes the cold water to freeze plus the time it takes for the hot water to cool to the temperature of the cold water.

 

[MacTech]

Originally posted by russ_watters
All other conditions being equal, cold water will freeze faster.

Think about it - before the hot water freezes, it'll have to cool down to the temperature of the cold water. After that, freezing times are the same. So the time (and energy) it takes the hot water to freeze is the time it takes the cold water to freeze plus the time it takes for the hot water to cool to the temperature of the cold water.

most common answer and misconception, you forget the other things that happen with cooling of water. hot water will lose more of its mass as it evaporates from moving the heat energy to a system that has no heat and when it does this it will lose mass. Also evaporation will take all that heat energy out of the system. This net loss is enough to overtake the time it takes to freeze the cold water. Other parts of the system also play roles, like motion of water, content of the gas and etc.

References:
"Hot water freezes faster than cold water. Why does it do so?",
Jearl Walker in The Amateur Scientist, Scientific American,
Vol. 237, No. 3, pp 246-257; September, 1977.

"The Freezing of Hot and Cold Water", G.S. Kell in American
Journal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969.

http://www.urbanlegends.com/science/hot_water_freezes_faster.html
http://math.ucr.edu/home/baez/physics/General/hot_water.html

 

[Phobos]

how about this...try the experiment for yourself

 

[Evo]

Originally posted by russ_watters
All other conditions being equal, cold water will freeze faster.

Think about it - before the hot water freezes, it'll have to cool down to the temperature of the cold water. After that, freezing times are the same. So the time (and energy) it takes the hot water to freeze is the time it takes the cold water to freeze plus the time it takes for the hot water to cool to the temperature of the cold water.

Actually, hot water "can" freeze faster than cold water, it is referred to as the "Mpemba effect". It's one of those weird science things. MacTech also referred to the link below, but there are tons of other references. I was going from memory earlier.

http://math.ucr.edu/home/baez/physics/General/hot_water.html

 

[FZ+]

Wow.

 

[megashawn]

I have to agree with Fz and Phobos

how about this...try the experiment for yourself

I mean, is it really that hard to try this yourself?

 

[eagleone]

Haven’t read referral links,
Now, thefirst thing that’s on my mind is “over cooling” (damn, again I don’t know the right term )
of the water. It makes sense, cold water would over cool so not freeze (temperature would fall to quickly (under the freezing point), and there wouldn’t be time for molecules to rearrange to crystal structure), then in case of hot water you wouldn’t have so cold joining surface, and freezing would be possible (God I don’t understand myself, hope u will understand something … brains way to tell me go to sleep”)…

Funny, by my model you would have to worm up cold water to freeze it.


Yo, mister administrator, “show printable version” then -> "Show all X posts from this thread on one page" does Not work !

 

[member 5645]

http://www.straightdope.com/classics/a2_098b

 

[russ_watters]

Originally posted by MacTech
most common answer and misconception, you forget the other things that happen with cooling of water.

http://math.ucr.edu/home/baez/physics/General/hot_water.html

From the link:

What's wrong with this proof is that it implicitly assumes that the water is characterized solely by a single number -- the average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30° C, it may look very different than the initially cooler water (at a uniform 30° C) did at the start. Why? Because the water may have changed when it cooled down from a uniform 70° C to an average 30° C. It could have less mass, less dissolved gas, or convection currents producing a non-uniform temperature distribution.

From my post:

All other conditions being equal...

There is always a way to weasel a scientific question into a trick question via nitpicking. So many people do it on this board, I've taken to always qualifying my responses and being very specific about the domain under which my answer applies: The link talks about changes. Changes don't fit with "all other conditions being equal." The link is titled (as Evo pointed out) "Can hot water freeze faster than cold wate?" You can certainly construct an experiment designed to make the unequal appear equal and thus give a counterintuitive result. But be reasonable and be consistent with reality.

Nice link, phat:

The results? The cold water froze about 10 or 15 minutes faster than the hot water, and there was no detectable difference between the boiled water and the other kind. Another old wives' tale thus emphatically bites the dust. Science marches on.

It also explains how the Scientific American article, while scientifically accurate, does not fit with what we observe in our freezers on a day to day basis:

I found that cold water (38 degrees Fahrenheit) froze faster than hot water out of the tap (125 degrees F). I chose these two temperatures because (1) they were pretty much what the average amateur ice-cube maker would have readily available and (2) I couldn't find a mercury thermometer that went higher than 125 degrees.

Jearl, who is not afflicted with penny-pinching editors like some of the rest of us, was able to get his mitts on a thermocouple that could measure as high as the boiling point, 212 degrees F. He found that water heated to, say, 195 degrees would freeze three to ten minutes faster than water at 140-175 degrees. (There were differences depending on how much water was used, where the thermocouple was placed, and so on.)

 

[Phobos]

Originally posted by phatmonky
http://www.straightdope.com/classics/a2_098b

...the inspiration for my previous response.

 

[LURCH]

Originally posted by Phobos
how about this...try the experiment for yourself

We had this same question a few months ago in one fo the Forums. I did the experiment myself, came up with consistent results. The cold water froze first, every time.

 

[Njorl]

I never thought there was much controversy about this. It is the flipside that is more problematic. Which boils faster, hot or cold water.

For freezing, the biggest secondary impact (after initial temperature) will be dissolved solids. They will actually tend to reinforce the obvious outcome. The hot water will freeze even more slowly than expected.

In boiling, the boiling of the hot water will be slowed by dissolved solids. The larger amount of dissolved gasses in the cold water, as they are released, will tend to act as points of turbulence necessary for boiling. These factors compete with the starting temperature difference rather than enforce it. This makes it a harder question to answer.

Njorl

 

[Evo]

Originally posted by Njorl
In boiling, the boiling of the hot water will be slowed by dissolved solids. The larger amount of dissolved gasses in the cold water, as they are released, will tend to act as points of turbulence necessary for boiling. These factors compete with the starting temperature difference rather than enforce it. This makes it a harder question to answer.

Njorl

For example: adding salt to water increases the time it takes for the water to reach the boiling point?

 

[PrudensOptimus]

Originally posted by russ_watters
All other conditions being equal, cold water will freeze faster.

Think about it - before the hot water freezes, it'll have to cool down to the temperature of the cold water. After that, freezing times are the same. So the time (and energy) it takes the hot water to freeze is the time it takes the cold water to freeze plus the time it takes for the hot water to cool to the temperature of the cold water.

I agree with Russ walters. However, from the 2nd law of thermodynamics: heat tends to travel in the fassion of from hot object to cold.

 

[Njorl]

Originally posted by Evo
For example: adding salt to water increases the time it takes for the water to reach the boiling point?

It is more precise to say that it raises the boiling point. All else being equal, it will take longer to boil.

Njorl

 

[member 5645]

Originally posted by PrudensOptimus
I agree with Russ walters. However, from the 2nd law of thermodynamics: heat tends to travel in the fassion of from hot object to cold.

I guess I don't understand why you mentioned that law after saying "however" (as if it is agianst his statement?).

 

[russ_watters]

Originally posted by PrudensOptimus
I agree with Russ walters. However, from the 2nd law of thermodynamics: heat tends to travel in the fassion of from hot object to cold.

Yes, but hotter objects also have more heat they need to lose. Net result: more time to get to the same end point.

Relay race analogy: your opponents team has 2 runners running 100m each. Your team has 3 runners running 100m each. If your 2nd and 3rd runners are equal in speed to the other team's two runners. Does it matter that your 1st runner is the fastest in the world? No matter how fast he runs, his time will be the margin by which you lose.

 

[eagleone]

Of course you’re right, but they aren’t wrong (except if they are saying, “it always freezes faster”).
I’ve gave an example of phenomena that could explain this anomaly (I’ve read one of those pages later, and proper term is super cooling), and I see no obstacles why effect, using this phenomena, couldn’t be achieved. And yes, you probably do need special conditions, if not so, everyone would be familiar with this strange effect (and don’t forget that we’re dealing here with weird H2O molecule ). Anyway, I remember I’ve done it, temperature difference wasn’t so big (maybe 15C), but cold water hit -5C in liquid form, and hotter froze normally at 0C ….

For example: adding salt to water increases the time it takes for the water to reach the boiling point?

Yes, salt decreases entropy, lowers freezing and increases boiling temperature (as many other substances – that’s why when you take a look at ice cube you see the bubbles in the core and it looks shady, but periphery parts (that froze first) are clear free from impurities). Same effect is used to prepare silicon for that computer on your desk …

 

[Chuck-GoBears]

Think inside the box

Inside the refrigerator, that is. In the olden days, instruction manuals advised the user to put hot water in the ice tray to speed up the freezing process. It worked then but not today. The difference is that today's refrigerators are self-defrosting. In the 1940's there was often a layer of ice between the ice tray and the metal cooling surface. Because ice and air do not conduct heat very well, putting hot water in the ice tray allowed the tray to melt through the ice. Improving contact increased the heat transfer rate and the hot water froze faster.

 

[TVP45]

I went to the true expert on this - a Zamboni operator. He cannot tell the difference in time, but the hot water gives clearer, harder ice.

 

[BobG]

I went to the true expert on this - a Zamboni operator. He cannot tell the difference in time, but the hot water gives clearer, harder ice.

Different issue. You don't want a thin layer of ice laid on top of the old because the new layer of ice will just chip off. You want to melt the old ice so the new water becomes part of the old layer, just with a smoother surface.

 

[BobG]

Inside the refrigerator, that is. In the olden days, instruction manuals advised the user to put hot water in the ice tray to speed up the freezing process. It worked then but not today. The difference is that today's refrigerators are self-defrosting. In the 1940's there was often a layer of ice between the ice tray and the metal cooling surface. Because ice and air do not conduct heat very well, putting hot water in the ice tray allowed the tray to melt through the ice. Improving contact increased the heat transfer rate and the hot water froze faster.

Did the manual explain how to remove the ice trays from the freezer after both the water in the ice tray and the layer of ice in the freezer both froze?

Defrosting the freezer once in a while was a lot better option than using hot water. Doh! What am I thinking?! Back when I was single and living in cheap apartments with non-self-defrosting refrigerators, defrosting was something you did when the ice built up so thick you couldn't squeeze an ice tray into it anymore (or, in my case, it meant it was time to start looking for a new apartment).

 

[fourier jr]

wouldn't it depend on "faster?" hot water would cool at a faster rate (Newton's law of cooling) but cold water is already cold, so it would take less time to freeze, wouldn't it?

 

[TVP45]

Different issue. You don't want a thin layer of ice laid on top of the old because the new layer of ice will just chip off. You want to melt the old ice so the new water becomes part of the old layer, just with a smoother surface.

But, they don't put a thin layer of ice on the old. They put a thin layer of water.

 

[tribdog]

I tried the experiment myself, but my results weren't what I was expecting. Here's what I learned. 1. A video camera in your freezer can't take pictures of water freezing because it's dark in there. 2. The temperature of the water is a major factor on how bad it hurts when you spill it on yourself.
Hope this helps

 

[TVP45]

But at least you settled the question of whether the light goes out when you close the door.

 

[ZapperZ]

Talk about resurrecting a very old thread...

This issue, which is more popularly known as the Mpemba effect, has been discussed several times here. For a thread that has all the relevant discussion and references, please refer to

https://www.physicsforums.com/showthread.php?t=205920

Zz.

 

[BobG]

But, they don't put a thin layer of ice on the old. They put a thin layer of water.

You missed the point. Hot water melts more of the old ice than cold water does, making a more seamless transition between the old ice and the new water. In other words, it creates "clearer, harder ice" just as the Zamboni operator said. Of course it takes longer to freeze than cold water, but getting good results are important enough that he's never even really thought about the time issue.