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Phase Diagrams

High schoolChemistryPure-substance P-T phase map with IAPWS water saturation and declared approximate sublimation and CO2 boundaries

Pressure-temperature map

Liquid

Drag the orange probe. Arrow keys on the map change temperature and logarithmic pressure. Boundaries are model coexistence curves, not amounts of each phase.

19031844557370010^210^310^410^510^610^7Temperature (K)Pressure (Pa), logarithmic
SolidLiquidGasSupercritical fluid

Measurement notebook

Change the setup, test a prediction and save the result. Up to 12 snapshots are kept.

About this experiment

Move a pressure-temperature probe through solid, liquid, gas and supercritical regions. Snap to the triple and critical points and compare the negative melting slope of ordinary ice with the positive model slope for carbon dioxide.

Learning goals

  • Read a logarithmic pressure axis and identify coexistence curves.
  • Distinguish triple points from critical points.
  • Explain why liquid carbon dioxide is unavailable at one atmosphere.

How to use it

  1. 1At one atmosphere, heat water across its vapour boundary.
  2. 2Snap to the water triple point and identify the three adjoining regions.
  3. 3Switch to carbon dioxide and compare sublimation at one atmosphere with liquid formation above the triple pressure.
  4. 4Inspect the critical point and move into the supercritical region.

The model

A constant-latent-heat Clausius-Clapeyron form approximates the selected boundaries.
The melting curve is a declared local linear approximation.
Both conditions define the supercritical region in this teaching map.

Assumptions

  • Water liquid-vapour saturation uses the IAPWS six-coefficient correlation between its triple and critical temperatures.
  • CO2 liquid-vapour saturation uses an effective constant latent heat anchored to its triple and critical points. Its sublimation approximation also passes through the normal sublimation point 194.65 K.
  • Water sublimation and both melting curves are simplified. High-pressure ice polymorphs, metastability, nucleation hysteresis and phase amounts are omitted.
  • A coexistence boundary identifies permissible equilibrium phases; temperature and pressure alone do not determine their proportions.

Try this in class

  1. Q1Why does a critical point terminate a liquid-vapour coexistence line?
  2. Q2Why is the slope of the ordinary ice melting boundary negative?

Frequently asked questions

Are every boundary and phase transition here precision property data?

No. The water saturation correlation is a reference relation, while the other boundaries are explicitly anchored teaching approximations.

Does crossing a line compute how quickly the substance changes phase?

No. The diagram classifies equilibrium regions. Latent heat transfer and transition kinetics are not simulated.