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Freezing water of different starting temp.

  1. Aug 20, 2012 #1
    I have questions to a classic experiment : you have 2 equal containers filled with water. IN one the temperature of the water is 98° Celsius, in the other the water has 25° Celsius. If you put both in the freezer the one with the hotter water freezes first.
    Now i do understand that one looses its temperature quicker due to the evaporating water (hotter water evaporates more then lukewarm water), but at some point both will have the same temperature, so why is one still loosing its temperature quicker ? Is it cause it has lost more water, so there is less to freeze ? So little water can´t have that much consequences, can it ?
     
  2. jcsd
  3. Aug 20, 2012 #2
  4. Aug 24, 2012 #3
  5. Aug 24, 2012 #4

    fluidistic

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    If you have 2 recipees, one with say 100 ml of water and the other with 90 ml of water at the same temperature and you start to freeze them. It is not obvious at all that the 90 ml will freeze first. I've noticed that the fewer amount of water you have, the lesser it will reach in temperature before freezing (overcooling). I could never avoid over cooling (even with vibrations) when freezing water.
    This could imply that evaporation of water actually enlarge the time it will take for the water to freeze. (I did not test for high temperatures).
    For very small drops of water, I could go as low as -17°C easily without them to freeze. In fact I could not get them to freeze at 0°C or even close to it. The bigger drops would freeze before the smaller ones (around -4°C was common).
    This effect is something I've never read when reading about the Mpemba effect. On the contrary, they all seemed to assume that a lesser quantity of water would freeze before a greater quantity. So they say that evaporation helps the hot water to freeze before the colder water. Well, I do not trust this at all. I've played enough in the lab with water and I'm sure that this is wrong. :smile:
     
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