[VISUAL] Shown above is the phase diagram of a pure substance. The substance under the conditions co — States of Matter Chemistry Question

STEPS:

1. Identify the Phases in the Diagram: A standard phase diagram is divided into three primary regions. The region at high pressures and low temperatures (top left) represents the solid phase; the region at intermediate pressures and temperatures (middle/top) represents the liquid phase; and the region at high temperatures and low pressures (bottom right) represents the gas phase.
2. Determine the Starting Phase at Point X: Locate Point X on the diagram, which sits at approximately $100^\circ\text{C}$ and slightly above $4\text{ atm}$. By observing the boundaries, Point X is located in the region below the liquid-gas equilibrium curve (the boiling/vaporization line), meaning the substance starts as a gas.
3. Trace the Path of Constant Pressure Cooling: The problem specifies cooling the substance while keeping pressure constant. On the diagram, "cooling" corresponds to moving horizontally to the left (decreasing temperature) while staying at the same vertical height (constant pressure).
4. Identify the First Phase Transition: As the substance cools from $100^\circ\text{C}$ at constant pressure, the horizontal path crosses the first boundary line (the vaporization curve) at approximately $75^\circ\text{C}$. Moving from the gas region into the middle region marks a transition from gas to liquid (condensation).
5. Identify the Second Phase Transition: Continuing to cool the substance further to the left, the path crosses the second boundary line (the melting/freezing curve) at approximately $52^\circ\text{C}$. Moving from the middle region into the far-left region marks a transition from liquid to solid (freezing).
6. Locate the Final State: Cooling concludes at $40^\circ\text{C}$. At this temperature and the original pressure, the substance is located firmly within the solid region of the phase diagram.
7. Conclusion: The full sequence of phases encountered during the cooling process is gas to liquid to solid, which corresponds to option A.

WHY_OTHERS_WRONG:

• B) gas to solid to liquid: This suggests the substance transitions directly to a solid and then melts while cooling, which contradicts the layout of the phase regions.
• C) solid to liquid to gas: This sequence describes the phases encountered if the substance were being heated at constant pressure, rather than cooled.
• D) liquid to solid to gas: This sequence is physically impossible for a pure substance under these conditions; cooling cannot result in a transition back into a gas at high pressure.
• E) liquid to gas to solid: This suggests the substance was initially a liquid and briefly became a gas during cooling, which is not supported by the starting location of Point X or the direction of the temperature change.