Chemistry · Lesson

Phase changes, phase diagrams and critical behaviour

Read a phase map as a set of equilibrium conditions. Distinguish phase regions, coexistence boundaries, the triple point and the end of the liquid–vapour boundary.

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By the end, you can

Identify phase regions and coexistence curves on a qualitative P–T diagram.
Explain latent heat during equilibrium phase change.
Distinguish triple-point and critical behaviour without treating a schematic as numerical property data.

Changing phase changes an arrangement

Melting changes solid to liquid; freezing reverses it. Vaporization changes liquid to gas, and condensation reverses it. Sublimation changes solid directly to gas, while deposition changes gas directly to solid. The appropriate path depends on pressure as well as temperature.

During an equilibrium phase change of a pure substance at fixed pressure, energy transfer can change the phase amounts while temperature stays approximately at the coexistence value. This latent heat is different from sensible heating that raises temperature within one phase. Mixtures, changing pressure and nonequilibrium conditions can behave differently.

A pressure–temperature map locates stable phases

The horizontal axis increases temperature and the vertical axis increases pressure. A region identifies the equilibrium phase under those conditions. A coexistence curve separates neighbouring regions: on the curve, the two phases can coexist in equilibrium. Crossing it along a chosen pressure or temperature path can change which phase is stable.

The diagram here is qualitative and refers to ordinary-water behaviour over a simplified range. Its drawing coordinates have no numerical kelvin or pressure scale. It omits higher-pressure ice phases. Use a measured or validated property diagram when a numerical temperature or pressure is needed.

A map of phases and coexistence

Away from the coexistence curves, labels indicate a single stable phase in the simplified map. On a curve, the two named phases can coexist at equilibrium. Three curves meet at the triple point. The liquid–vapor distinction ends at the critical point. The shaded supercritical criterion requires both T > Tc and p > pc.

The negative melting slope belongs to ordinary ice Ih in its stated range; it is not a rule for every water-ice phase. Positions and slopes are exaggerated for clarity, and this map cannot supply a numerical melting or boiling point.

What the marked features mean
Feature	Meaning
Triple point	Solid, liquid and gas can coexist at equilibrium.
Critical point	Endpoint of liquid–vapor coexistence.
Ordinary-ice melting curve	Higher pressure corresponds to lower melting temperature on this displayed branch.
Supercritical criterion	Both temperature and pressure exceed their critical values; the dotted guides are reference markers.

An original schematic pressure–temperature map for pure water at equilibrium. Solid, liquid and gas regions meet along three coexistence curves at a triple point. The liquid–gas curve ends at a critical point. The ordinary-ice melting curve leans toward lower temperature at higher pressure. No quantitative axis scale or high-pressure ice phases are shown.

Two special points have different meanings

The triple point joins three coexistence curves: solid, liquid and vapour can coexist in equilibrium. For ordinary water, the current best estimate of triple-point temperature is 273.16 K, and the recommended pressure is approximately 611.657 Pa. The temperature is no longer an exact SI defining value.

The critical point is the endpoint of the liquid–vapour coexistence curve. Beyond it, that liquid–vapour phase boundary no longer exists. Ordinary water’s adopted reference values are T꜀ = 647.096 K and p꜀ = 22.064 MPa. A supercritical-fluid setting normally requires both T > T꜀ and p > p꜀; exceeding temperature alone is not the full criterion.

A phase path can differ between substances

Ordinary ice is less dense than liquid water. Over its ordinary ice–liquid coexistence range, increasing pressure favours the denser liquid, giving a negative melting-curve slope on the P–T diagram. This is a scoped property of the familiar ice phase, rather than every ice phase at every pressure.

Carbon dioxide at ordinary atmospheric pressure has no stable liquid phase: warming solid carbon dioxide there leads to sublimation. Its familiar temperature near −78 °C concerns sublimation at that pressure, not an atmospheric-pressure melting point. Name the substance and pressure before interpreting any phase-change number.

Worked examples

Interpret a named pressure path

At ordinary atmospheric pressure, solid carbon dioxide is warmed. Should its transition near −78 °C be labelled melting?

Identify the substance as carbon dioxide and keep atmospheric pressure explicit.
At this pressure, liquid CO₂ is not the stable intervening phase.
The solid changes directly into gas.

No. This is sublimation under the stated pressure conditions, rather than melting to liquid.

Try it yourself

On a pure-substance coexistence curve, how many bordering phases can coexist?
Show the answer and reasoning
Two.
A triple point is the special intersection where three phases can coexist.
Must temperature rise throughout every equilibrium phase change at fixed pressure?
Show the answer and reasoning
No.
For a pure substance, energy can change phase amounts at approximately constant coexistence temperature.
Is T > T꜀ alone enough for the usual supercritical-fluid criterion?
Show the answer and reasoning
No.
The stated setting also requires p > p꜀.
Why is ordinary water’s ice–liquid melting curve negatively sloped in the simplified range?
Show the answer and reasoning
The liquid is denser than ordinary ice.
Higher pressure favours the denser liquid; the qualification excludes other high-pressure ice phases.

What to carry forward

Phase maps describe equilibrium at stated pressure and temperature.
Coexistence curves and guide lines have different meanings.
Triple and critical points are distinct.
Latent heat can change phase amounts without raising temperature.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.