Upper secondary · 12 min

The phase diagram of water

Rotate a three-dimensional p–T–V surface and project it onto the p–T plane: solid, liquid and vapour regions meet at the triple point and the liquid–vapour line ends at the critical point.

Goal

Read a p–T diagram: locate solid/liquid/gas domains, the triple point (0.01 °C, 611 Pa) and the critical point (374 °C, 22 MPa) of water.

Apparatus and reagents

Virtual 3D phase surface of water (p–T–V), reference values of the triple and critical points.

Procedure

Rotate the surface until the p and T axes are in the plane of the screen: the projection is the usual phase diagram.
Trace the three boundary lines — sublimation, fusion, vaporisation — to the point where they meet.
Follow the vaporisation curve upward: it stops rather than going to infinite pressure — the critical point.
Explain why ice can sublime below 611 Pa and why above 22 MPa liquid and vapour merge into one fluid.

What to observe

Three coexistence lines divide the plane and meet at the triple point, the only state where ice, liquid water and vapour coexist.
For water the fusion line leans slightly to the left (negative slope), a signature of ice being less dense than liquid water.

Explanation

The Clapeyron equation $\frac{\mathrm{d}p}{\mathrm{d}T}=\frac{\Delta H}{T\Delta V}$ gives the slope of each coexistence line from the enthalpy and volume changes. For water $\Delta V_{fusion}<0$ , so the ice line leans left; the vapour line ends where the liquid–vapour density difference vanishes at the critical point.

History of the experiment

Josiah Willard Gibbs founded the thermodynamics of phases in the 1870s; the triple point of water became the fixed point of the kelvin scale until 2019, when the kelvin was redefined through the Boltzmann constant.

Chemists behind it

Josiah Willard Gibbs William F. Giauque