Integrated SequencePhysics Chemistry Organic Biology

Web Resources

London South Bank University - The Phase Diagram of Water
Excellent, in-depth treatment of the complex phase change behavior of water.

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Special points of emphasis

Intermolecular Forces


The States of Matter

Chemical Thermodynamics and the Equilibrium State


Water has some extremely unusual physical properties, such as having a solid state which is less dense than its liquid state. Being a polar molecule with the positive pole occupied by hydrogen, the intermolecular forces among water molecules are characterized by hydrogen bonding. Water has abnormally high melting and boiling points due to the power of its molecules to attract one another strongly due to hydrogen bonds. Because each molecule has two hydrogen atoms and two unshared electron pairs, each can form four hydrogen bonds. Ice crystals have an open structure to allow for maximum hydrogen bonding. This accounts for the relatively low density of ice. Picture it. As water freezes, the molecules rotate, pushing away a bit from each other, to maximize hydrogen bonding. (Liquid water at atmospheric pressure reaches its greatest density at 4oC.)

The phase diagram of water has some interesting characteristics. Because normal ice melts with a decrease in volume, the melting point decreases with increases in pressure. A decreased melting point means that at higher pressure, the liquid water will become more thermodynamically favored over the solid. Can you use 'free energy' in a sentence explaining why?

Increased pressure increased the relative enthalpy (and thus the free energy) of the higher volume (lower density) side, shifting the phase equilibrium toward the lower volume liquid. If a question on the MCAT gets you thinking about changes in relative free energy, see if you can't use Le Chatelier's principle to relieve the mental pain. The increase in pressure produces a change in the position of equilibrium that relieves the disturbance, a shift toward the lower volume side, liquid water.

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