Physical Chemistry : Factors Affecting Phase Interactions

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Factors Affecting Phase Interactions

Why is liquid water more dense than ice?

Possible Answers:

Water has metastable phases 

Water crystalizes with greater average space between molecules

Liquid water and solid ice have the same density

None of these

The slope of the solid/liquid phase line for water is positive

Correct answer:

Water crystalizes with greater average space between molecules

Explanation:

While water exhibits hydrogen bonding and has metastable phases, these don't have anything to do with the density of water. The slope of the solid/liquid line is in fact negative. This tells something about the relative densities of the phases. For a phase diagram, moving along a line of constant temperature (vertically on a phase diagram tells us something about the densities. Moving up along the line indicates an increase in density (this makes sense, since gasses are the least dense, and occupy the bottom portion of a phase diagram). If you move along a constant temperature line, you will see that water has a higher density than ice, since the ordering goes vapor, ice, then water. This is a result of the packing in solid ice.

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