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Homework Help: Chem - freeze drying & equilibrium

  1. Sep 27, 2006 #1
    So I have to use Le Chateliers principle to expalin why low pressure is used in the freeze-drying process (eg in making instant coffee)

    So here is what I know so far:
    in freeze drying, they turn solid water right into a gas basically removing all moisture from the food so it keeps for a very long time.
    Heat and pressure determine which phase the water is in.
    In freeze drying they freeze the water in the food and then lower the pressure to turn the frozen water straight into gas skipping the liquid phase completely. Then the gas is sucked out of the chamber leaving your freeze dried food!

    So why do they use low pressure to do this? My answer:
    increasing pressure reduces volume which would cause the ice to melt (why?) Added pressure favours the liquid phase (again why?) So from these statements it is obvious that low pressure is used to keep the ice from melting into water. It is much easier to remove all moisture from the food if the ice turns directly into gas and is sucked out of the chamber. Also, when the food is frozen first, the water molecules seperate from the food molecules when they turn to ice. Then the water molecules can be removed seperately and the food tastes the same when it is rehydrated.

    Thanks :)
  2. jcsd
  3. Sep 27, 2006 #2
    It's all about hydrogen bonds. H2O atoms need a certain space between them in order to form the 4 hydrogen bonds in the crystal lattice. In liquid phase at low temperatures, the molecules are able to get closer to each other.
  4. Sep 27, 2006 #3
    Okay, but I am not sure how LeChatelier's principle fits in here. Anyone have any ideas?
    The only thing I can think of is that the freeze drying process is kind of exploiting how a system reacts to different temps and pressures by adjusting. The adjustment would be consistent with LeChatelier I think.
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