phase diagram

Meaning: Each region of the graph is one phase. A line marks conditions where two phases coexist at equilibrium, and crossing it is a phase change. The three lines meet at the triple point, the single T and P where solid, liquid and gas coexist. The liquid–gas line is the vapor pressure curve and ends at the critical point, beyond which the substance is a supercritical fluid. The normal boiling point and normal melting point are where the corresponding lines cross 1 atm.

Equation: ln(P₂/P₁) = −(ΔHvap/R)(1/T₂ − 1/T₁), the Clausius–Clapeyron relation for the liquid–gas line. P is vapor pressure (Pa), T is temperature (K), ΔHvap is molar enthalpy of vaporization (J/mol), R = 8.314 J/(mol·K).

On the MCAT: A passage gives a diagram and asks for the phase at a given T and P. You may be asked to trace a path: a horizontal line is heating at constant pressure, a vertical line is compressing at constant temperature. Expect questions on how the boiling point shifts when pressure changes, and on why a substance such as CO₂ sublimes at 1 atm (1 atm lies below its triple point pressure).

Common confusion: Students expect every solid–liquid line to lean right. It does for most substances, but water's leans left because ice is less dense than liquid water, so raising pressure can melt ice. Also, a phase diagram is not a heating curve, which plots temperature against heat added at fixed pressure.