Chemistry › Section J · Liquids and solids › Topic J.4

Phase diagrams

A phase diagram is a map that tells you whether a substance is solid, liquid or gas at any temperature and pressure. With it you can explain why dry ice never melts on a table, why freeze-drying works, and why water behaves so differently from almost everything else.

In this topic

  • A map of phases
  • Lines, the triple point and the critical point
  • Water's phase diagram
  • Why water's melting line slopes backwards
  • Carbon dioxide's phase diagram
  • The critical point and supercritical fluids
  • Worked example: warming CO₂ at two pressures
  • Worked example: compressing water vapour
  • Common mistakes to avoid
  • Key takeaways

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