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 becomes supercriticalSolid CO2CO_2 sublimates and then becomes supercriticalSolid CO2CO_2 melts and then evaporatesSolid CO2CO_2 sublimates

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BlackTom AI · Accepted Answer

To tackle this question, I’ll walk through what the CO2 phase diagram implies when a solid sample at low temperature is heated at a fixed high pressure up to 100 °C.
Option 1 says the solid melts and then becomes supercritical. In CO2’s phase diagram, at high pressures like 100 bar, the solid does not first melt into a liquid before heating to 100 °C; instead, it sublimates directly from solid to gas. This makes the claim that melting occurs incorrect.
Option 2 states that the solid CO2 sublimates and then becomes supercritical. This aligns with the known CO2 behavior: at sufficiently high pressure, heating a solid CO2 causes sublimation to gas, and as temperature rises past the substance’s critical temperature (about 31 °C) under high pressure (here 100 bar), CO2 enters the supercritical region. Therefore, this option correctly describes the sequence: solid → gas via sublimation, then gas becoming supercritical once the critical point is crossed.
Option 3 claims that the solid melts and then evaporates. Evaporation refers to a liquid phase turning to gas, but there is no stable liquid CO2 at most of the relevant high-pressure conditions because CO2 transitions from solid directly to gas (sublimation) before any liquid phase becomes stable; thus this option misstates the phase sequence.
Option 4 reduces the process to merely sublimation, omitting the subsequent transition into the supercritical region at 100 bar and 100 °C. While sublimation is indeed part of the process, the temperature and pressure quickly place CO2 into the supercritical state, so this option is incomplete.
In summary, the reasoning reflects how the path starts with sublimation at 100 bar and then, as temperature climbs to 100 °C, crosses the critical point, yielding a supercritical CO2 fluid. The misstatements in the other choices arise from assuming melting under high pressure, mislabeling phase transitions (evaporation after a liquid, or omitting the supercritical stage), or ignoring the phase boundaries present in the diagram.

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