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Water phase diagram

Discussion in 'MCAT Study Question Q&A' started by unsung, Jun 17, 2008.

  1. unsung

    unsung 10+ Year Member

    Mar 12, 2007
    I've been staring at this phase diagram of water and I still can't figure it out. Why is the slope of the solid/liquid equilibrium line negative? (For most other substances like CO2, etc. the slope slants right, i.e. is positive.)

    I know the negative slope is supposed to explain a lot of peculiarities of water, such as why ice floats on top of water (density of the solid is less), and why water actually expands as it freezes. But, I can't for the life of me see how.

    This is from the chemguide website someone else suggested, referring to the equilibrium between ice(s) <-> water(l).
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    "If you have this equilibrium and increase the pressure on it, according to Le Chatelier's Principle the equilibrium will move to reduce the pressure again. That means that it will move to the side with the smaller volume. Liquid water is produced."

    .I don't get why the side with the smaller volume has less pressure? From PV = nRT, if V decreases, doesn't P increase? Also, I'm not sure what pressure they're referring to... the vapor pressure exerted by ice vs water ?

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  3. Quantum Chem

    Quantum Chem 2+ Year Member

    Jun 10, 2008
    College Park, Md
    The slope is negative, I believe, because of hydrogen bonding being stronger in the liquid than in the solid. The clausius-clapeyron equation gives an approximation to the slope. I'm not going to try to to explain it because I disliked thermo p. chem and explaining the equation is probably beyond the MCAT since it involves calculus and differential equations.
  4. physics junkie

    physics junkie 5+ Year Member

    Nov 20, 2006
    There are plenty of ways to look at it but here is one that I'm going to try:

    When water is in ice forum it forms a rigid lattice structure that is orderly. This agrees with the second law of thermodynamics which says that entropy goes to zero as temperature goes to zero. Since entropy is a "measure of disorder" it makes sense that a solid structure would have less disorder than a liquid. The positive hydrogens and negative oxygens are actually closer in this lattice structure than they are in liquid water but because of the way the H2O molecules are arranged you get a smaller number of h-bonds forming between a molecule and its neighbors thus decreasing the total strength of h-bonding on any H2O molecule.

    When water is in liquid form the oxygens and hydrogens are h-bonding like crazy. Although individual h-bonds are weak and they are made and broken many times a second there is a net effect that causes the water molecules to stick closely together in liquid form. A physics professor put it to me like this and I have never been able to verify it so the numbers might be off: If you add 0.5 liters of ethanol to 0.5 liters of water you get 0.98 liters of liquid because the water hydrogen bonds the ethanol so strongly it sucks up 0.02L of volume. At the time he said the amount of pressure you would need to apply to ethanol to shrink it's volume that much is on the order of 28,000psi.

    H-bonding is a big deal.
    Last edited: Jun 18, 2008
  5. SKation


    Jan 15, 2014

    Did anybody find a way to explain this? I'm really confused as well I can just memorize the fact that a negative slope means that the solid phase is less dense but I want to understand how to decipher that on a phase diagram. If anybody could explain that would be great!

  6. Jepstein30

    Jepstein30 2+ Year Member

    Jun 29, 2011
    There are two great explanations right before your post.
  7. SOOyC52H1Sof

    SOOyC52H1Sof 2+ Year Member

    Jan 10, 2013
    The Ideal Gas Equation PV = nRT should really not be used for fluids, especially water. Instead you look at the phase diagram and see what the changes would be.

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