Question at position 7 Consider the phase diagram for CO2CO_2 below. If a sample of CO2CO_2 at 100 bar and -100°C is heated at constant pressure to 100°C, what happens? Consider the phase diagram for CO2CO_2 below. If a sample of CO2CO_2 at 100 bar and -100°C is heated at constant pressure to 100°C, what happens? Solid CO2CO_2 melts and then evaporatesSolid CO2CO_2 melts and then becomes supercriticalSolid CO2CO_2 sublimatesSolid CO2CO_2 sublimates and then becomes supercriticalSingle choice
A
Solid 𝐶 𝑂 2 melts and then evaporates
B
Solid  𝐶 𝑂 2 melts and then becomes supercritical
C
Solid 𝐶 𝑂 2  sublimates
D
Solid 𝐶 𝑂 2 sublimates and then becomes supercritical

Question

Question at position 7 Consider the phase diagram for CO2CO_2 below. If a sample of CO2CO_2 at 100 bar and -100°C is heated at constant pressure to 100°C, what happens? Consider the phase diagram for CO2CO_2 below. If a sample of CO2CO_2 at 100 bar and -100°C is heated at constant pressure to 100°C, what happens? Solid CO2CO_2 melts and then evaporatesSolid CO2CO_2 melts and then becomes supercriticalSolid CO2CO_2 sublimatesSolid CO2CO_2 sublimates and then becomes supercriticalSingle choice

A

Solid 𝐶 𝑂 2 melts and then evaporates

B

Solid  𝐶 𝑂 2 melts and then becomes supercritical

C

Solid 𝐶 𝑂 2  sublimates

D

Solid 𝐶 𝑂 2 sublimates and then becomes supercritical

BlackTom AI · Accepted Answer

To approach this question, first recall the CO2 phase diagram and how phase changes occur at a fixed pressure as temperature increases from a low starting point.
Option 1: 'Solid CO2 melts and then evaporates' This would imply that after melting into a liquid, the substance then turns into a gas without becoming a supercritical fluid. At 100 bar, once CO2 melts into liquid, increasing the temperature further keeps it as liquid until reaching the liquid–gas boundary. Since the pressure is well above the critical pressure, further heating does not produce a separate gas phase at temperatures below the critical temperature; instead, the liquid will approach the supercritical region, not simply evaporate. Thus this path misses the liquid phase that exists at high pressures and temperatures up to the critical point.
Option 2: 'Solid CO2 melts and then becomes supercritical' This matches the expected sequence at 100 bar: at high pressure CO2 can exist as a liquid up to its critical temperature (about 31°C). As temperature rises past the critical temperature while pressure remains above the critical pressure, the substance transitions from liquid to a supercritical fluid. Therefore melting into a liquid followed by passage into the supercritical state is the correct sequence for heating at constant high pressure.
Option 3: 'Solid CO2 sublimates' Sublimation implies a direct solid-to-gas transition. At a high fixed pressure of 100 bar, the CO2 is far from the sublimation conditions, which occur at much lower pressures. The phase diagram at 100 bar shows a path that favors melting first (solid to liquid) rather than sublimation, so this option does not describe the actual sequence.
Option 4: 'Solid CO2 sublimates and then becomes supercritical' This combines sublimation with a subsequent supercritical transition, but the initial step of sublimation is inconsistent with being at 100 bar, where the solid would more plausibly melt before any gas phase is approached. The sublimation-first path does not align with the high-pressure phase boundaries and thus is incorrect.
In summary, under a constant pressure of 100 bar, raising the temperature from −100°C initiates melting of solid CO2 into a liquid, and as temperature continues to rise past CO2’s critical temperature while staying above the critical pressure, the system enters the supercritical regime. The sequence described in option 2 best captures this progression, whereas the other options fail to reflect the high-pressure phase behavior of CO2.

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