The phase diagram for
C
O
2
is shown above. Assume that
C
O
2
is present at a pressure of 73.8 bar (gray line) at a temperature that corresponds to the solid/liquid phase transition. If the pressure is lowered a little while keeping the temperature constant, which of the following best describes what happens?

Question

The phase diagram for 
C
O
2
 is shown above.  Assume that 
C
O
2
 is present at a pressure of 73.8 bar (gray line) at a temperature that corresponds to the solid/liquid phase transition.  If the pressure is lowered a little while keeping the temperature constant, which of the following best describes what happens?

BlackTom AI · Accepted Answer

To approach the question, first note the phase diagram and the location described: a 73.8 bar pressure line (gray) that intersects the solid–liquid boundary at a temperature corresponding to the solid/liquid transition.
Option 1: The CO2 freezes, as more solid CO2 is formed. If you decrease the pressure slightly while holding temperature constant at a point on the solid–liquid boundary, you move to the left of the boundary into the solid region. This means the solid phase becomes more stable and the substance tends toward solidification (freezing). This aligns with the diagram where lower pressure at the same temperature places you in the solid area.
Option 2: The CO2 melts as more liquid CO2 is formed. Melting would occur if you increased pressure or moved across the boundary toward the liquid side at the same temperature. Since the scenario specifies lowering pressure, this would not happen here.
Option 3: The CO2 sublimes, as it is “dry ice.” Sublimation occurs when a solid turns directly into gas, which would require moving away from the solid–liquid boundary to conditions of the vapor region. Lowering pressure at the same temperature along the boundary does not place CO2 into the vapor domain.
Option 4: The solid and liquid CO2 remain in equilibrium at this pressure. Equilibrium at the boundary does not persist if you move slightly away from the boundary by changing pressure; lowering pressure moves you into the solid region, not maintaining a stable two-phase mixture at that fixed temperature.

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