Physical Chemistry : Phase Interfaces and Surfaces

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:

None of these

Liquid water and solid ice have the same density

Water crystalizes with greater average space between molecules

Water has metastable phases 

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.

Example Question #1 : Phase Interfaces And Surfaces

Which of the follow are false?

I. Three phases can coexist at the triple point.

II. It is not possible for ice to transition directly to water vapor without become a liquid first.

III. The slope of the ice/water phase curve is positive

IV. Distinct phases are not achievable above the critical point

Possible Answers:

II and III

IV only

III and IV

I only

II and IV

Correct answer:

II and III

Explanation:

Condition I is true. The only place three phases can coexist is at the triple point.

Condition II is false. Water in fact can undergo deposition and move from the solid phase to the gas phase directly. This occurs at pressures below 0.006atm.

Condition III is false. A peculiar fact of the water phase diagram is the slope of the solid/liquid line. For most phase diagrams, the slope is sharply positive, but that of water is negative.

Condition IV is true. No distinct phases exist above the critical point. 

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