12.2.5 Phase Diagrams

Question 1

Phase Diagrams

Answer 1

• A phase diagram is useful for predicting the phase of a substance at any pressure and temperature.
• At the triple point all three phases of a substance coexist, while beyond the critical point a substance is a supercritical fluid.
• The phase diagram for carbon dioxide (CO 2 ) illustrates its ability to sublimate, while the phase diagram for water (H 2 O) illustrates its unusual property of being less dense as a solid than a liquid.

Question 2

note

Answer 2

• A phase diagram is useful for predicting the phase of a substance at any pressure and temperature.
• In the example for water (H 2 O), the melting and
  boiling points at one atmosphere can be determined by dropping a line from the intersection of the dotted line and the transition points from solid to liquid and liquid to gas, respectively.
• At the triple point all three phases of a substance coexist, while beyond the critical point a substance is a supercritical fluid.
• The triple point represents the intersection of the three phase equilibria for a substance. At the pressure and temperature of the triple point for water, ice, water, and water vapor will coexist indefinitely.
• The critical point is a point on the liquid-gas equilibrium line past which the substance takes on properties of both a liquid and a gas. Beyond the critical point the substance exists as a supercritical fluid.
• The phase diagram for carbon dioxide (CO 2 ) illustrates its ability to sublimate, while the phase diagram for water (H 2 O) illustrates its unusual property of being less dense as a solid than a liquid.
• In the phase diagram for carbon dioxide (CO 2 ), the line for one atmosphere intersects with the solid-gas equilibrium line, not the solid-liquid line. Thus, under normal conditions, as temperature increases carbon dioxide will change from a solid to a gas without melting to produce a liquid first.
• In the phase diagram for water (H 2 O), the solid-liquid equilibrium has a negative slope. This means that as pressure increases for a temperature below the triple point, water will change from a solid to a liquid. Most substances show the reverse trend (and a solid-liquid equilibrium line with a positive slope).

Question 3

Which of the following is the best description of what the triple point is?

Answer 3

The triple point is the temperature and pressure in which all three phases (solid, liquid, and gas) can simultaneously exist in equilibrium

Question 4

Four different experiments are conducted on a sample of a substance. In each experiment, the temperature of the sample is slowly increased while the pressure is held constant. The four experiments were conducted at four different pressures. The verbal description of the temperature changes in one experiment is given below.

The sample began as a solid, completely filling the sample chamber. The pressure was maintained at 1.0 atm throughout the experiment. The temperature of the solid was slowly raised until it began to melt. Although heat was continuously applied to the melting sample, the temperature remained constant until the entire sample was melted. After the sample was completely melted, the space was filled with liquid sample. No vapor was present. The temperature of the liquid sample began to rise again and continued to rise until the sample began to boil. The temperature remained constant while the sample was boiling and immediately began to rise again as soon as the entire sample was converted to vapor.

Which experiment best matches the description?

Answer 4

Experiment C

Question 5

Early in the last century, county fairs and traveling shows would often set up a large block of ice sitting between two sawhorses. A piece of piano wire would be hung over the ice with a heavy weight attached to each end. As the hours went by, the wire would gradually move down through the ice. What would happen when the wire moved all the way through the ice?

Answer 5

The wire exerts substantial pressure on the ice. The boundary between the solid and liquid phases of water tilts backwards, so it is possible to cause the water to melt below the wire and refreeze above it. As the wire moves all the way through the block, the block is still one solid piece of ice

Question 6

What is the critical point?

Answer 6

The temperature and pressure above which the liquid and gas phases become indistinguishable

Question 7

The triple point for carbon dioxide is around 5.0 atmospheres, while the triple point for water is far below one atmosphere. What does this tell us about the behavior of water and carbon dioxide near atmospheric pressure?

Answer 7

Liquid CO2 cannot exist at one atmosphere so frozen carbon dioxide sublimates while ice melts

Question 8

At atmospheric pressure, ice (frozen water) and frozen CO2 behave differently. Which statement best describes the difference.

Answer 8

Frozen CO2 goes directly from solid to gas at atmospheric pressure.

Question 9

Iodine and frozen carbon dioxide pass directly from the solid phase to the gas phase. What is the phase change from solid to gas is called?

Answer 9

Sublimation

Question 10

Phase changes always involve large changes in what?

Answer 10

Internal energy

Question 11

For most substances the internal energy increase required to make the transition from liquid to gas is greater than the internal energy change required to make the transition from solid to liquid. Which of the following statements best explains why this is so?

Answer 11

The transition from liquid to gas requires overcoming more molecular bonds than the transition from solid to liquid.

Question 12

Which statement about the curves on the phase diagram is true?

Answer 12

• The curves divide the phase diagram into regions representing the solid, liquid, and vapor phases of a substance.
• The curves represent boundaries along which different phases of a substance can exist in equilibrium.
• The curves indicate temperatures and pressures at which the six different phase transitions can occur.