Intermediate Topic 269 of 1102 States of Matter & Solutions
Phase Diagram
A plot of pressure versus temperature showing regions of stable phases and their boundaries.
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In-depth explanation of Phase Diagram
A phase diagram is a graph of pressure against temperature that shows which phase of a substance is stable under each set of conditions. The diagram has regions for solid, liquid and gas, separated by boundary curves where two phases coexist in equilibrium. The sublimation, vaporization and fusion curves meet at the triple point, and the vaporization curve ends at the critical point. Reading the diagram tells you the stable phase at any pressure and temperature, and crossing a boundary corresponds to a phase transition. Phase diagrams differ among substances, which is why carbon dioxide and water behave so differently.
Going deeper: Phase Diagram
Key ideas: Reading steps: locate the temperature and pressure point and read the region. Moving horizontally at constant pressure gives heating curves, vertically gives compression. The normal melting and boiling points are where the 1 atm line crosses the boundaries. Along a boundary, two phases coexist, and the slope follows the Clausius-Clapeyron equation.
Common mistakes and connections: Students think boundary lines mean the substance is in a mix of all states at that temperature. Only on the line, two phases coexist. Another error is reading temperature on the pressure axis. Also, there are no phases for a single component beyond the critical point. Related to triple point, critical point, supercritical fluids and the water anomaly.
Visualization of Phase Diagram
A phase diagram plots pressure against temperature; lines mark equilibria between solid, liquid and gas, and regions show the stable phase. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Carbon dioxide has no liquid phase at 1 atm, so solid CO2 sublimes directly to gas, as seen with dry ice.
Frequently asked questions about Phase Diagram
What is Phase Diagram?
A plot of pressure versus temperature showing regions of stable phases and their boundaries.
Is Phase Diagram a beginner, intermediate or advanced topic?
Phase Diagram is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Phase Diagram?
Study these earlier topics in States of Matter & Solutions first: Heating Curve, Sublimation, Deposition.
What comes after Phase Diagram?
The next topics in States of Matter & Solutions are Triple Point and Critical Point.
Can I practise Phase Diagram with quizzes?
Yes. The interactive lesson includes a self-check question on Phase Diagram, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Phase Diagram fits in States of Matter & Solutions
Phase Diagram is topic 50 of 109 in States of Matter & Solutions.