In a phase diagram for CO_2, which point represents the critical point?

• A. The point where all three phase boundaries intersect
• B. The point where the liquid-gas boundary ends at high temperature and pressure
• C. The point at the lowest temperature and pressure on the diagram
• D. The point where the solid-liquid boundary meets the triple point

Answer: (B)

Verified step by step guidance

1. Understand that a phase diagram shows the states of matter (solid, liquid, gas) of a substance at different temperatures and pressures.
2. Identify the triple point on the diagram, which is where all three phase boundaries (solid-liquid, liquid-gas, and solid-gas) intersect, representing equilibrium of all three phases.
3. Recognize that the critical point is distinct from the triple point; it is located at the end of the liquid-gas boundary line at high temperature and pressure.
4. Know that beyond the critical point, the liquid and gas phases become indistinguishable, forming a supercritical fluid.
5. Therefore, the critical point corresponds to the termination of the liquid-gas phase boundary at high temperature and pressure on the phase diagram.