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

  1. Feb 16, 2004 #1
    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?
     
  2. jcsd
  3. Feb 16, 2004 #2

    Evo

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    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.
     
  4. Feb 16, 2004 #3

    russ_watters

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    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.
     
  5. Feb 16, 2004 #4
    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
     
    Last edited by a moderator: Feb 16, 2004
  6. Feb 16, 2004 #5

    Phobos

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    how about this...try the experiment for yourself
     
  7. Feb 16, 2004 #6

    Evo

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    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
     
  8. Feb 16, 2004 #7

    FZ+

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    Wow.
     
  9. Feb 16, 2004 #8

    megashawn

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    I have to agree with Fz and Phobos

    I mean, is it really that hard to try this yourself?
     
  10. Feb 16, 2004 #9
    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 :smile:)
    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 :smile: … 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 !
     
  11. Feb 16, 2004 #10
  12. Feb 17, 2004 #11

    russ_watters

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    From the link:
    From my post:
    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:
    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:
     
    Last edited: Feb 17, 2004
  13. Feb 18, 2004 #12

    Phobos

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  14. Feb 18, 2004 #13

    LURCH

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    We had this same question a few months ago in one fo the Forums. I did the experiment myself, came up with consistant results. The cold water froze first, every time.
     
  15. Feb 18, 2004 #14

    Njorl

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    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
     
  16. Feb 20, 2004 #15

    Evo

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    For example: adding salt to water increases the time it takes for the water to reach the boiling point?
     
  17. Feb 20, 2004 #16

    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.
     
  18. Feb 20, 2004 #17

    Njorl

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

    Njorl
     
  19. Feb 20, 2004 #18
    I guess I don't understand why you mentioned that law after saying "however" (as if it is agianst his statement?).
     
  20. Feb 20, 2004 #19

    russ_watters

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    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.
     
    Last edited: Feb 20, 2004
  21. Feb 21, 2004 #20
    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:smile:), 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 ….

    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 …
     
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