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Thread: Freezing-Point Depression

  1. #1 Freezing-Point Depression 
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    Hi guys, I'm new to these boards but I have a question for the chemists out there.

    Concerning freezing-point depression, or the fact that adding a substance such as salt to a solution can lower the temperature at which the solution freezes, I need a basic explanation of something.

    Anyway, I know that doing something such as adding salt to ice will lower the freezing point of the ice. The thing that I do not understand is why this will lower the temperature of the water. I have done much research on the matter but I do not understand what makes the temperature of the water actually drop. If anyone can give me a dumbed down explanation, I would be very grateful.

    Thanks!


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  3. #2  
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    When water is being melted or frozen two things are taking place. Some of the water comes into contact with the ice and freezes joining the ice structure while some of the ice escapes and melts. This is happening constantly. When you add salt it dissolves in the water and it makes it more difficult for the water to freeze into the ice structure, causing the freezing rate to slow down. If the rate that the water freezing slows down then it is going to lower the freezing point.

    If you go to this link and look at the diagram it might help you see what I am saying.

    http://antoine.frostburg.edu/chem/se...elts-ice.shtml


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  4. #3 Re: Freezing-Point Depression 
    Forum Isotope Bunbury's Avatar
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    Quote Originally Posted by Emmenite
    The thing that I do not understand is why this will lower the temperature of the water. I have done much research on the matter but I do not understand what makes the temperature of the water actually drop.
    It is the energy balance among the various processes involved in dissolving that determine if the water will change in temperature, and by how much and in which direction. When a salt dissolves, the ionic bonds have to be broken and the crystal lattice energy has to be overcome. Both processe absorb energy (are endothemic). Other processes involving the water interacting with the solute's ions produce heat (are exothermic). In the case of salts the endothermic processes dominate and a net input of energy is required (heat of solution) for the salt to go into solution. Since this energy can only come from the water, the water becomes colder.

    This really has nothing to do with freezing point depression, which is a different phenomenon.
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