🌡️ Phase Diagram

A phase diagram maps the thermodynamic conditions (pressure P and temperature T) under which phases of matter are stable. Phase boundaries are coexistence curves where two phases are in equilibrium. All three curves meet at the triple point — the unique (P,T) at which all three phases coexist. Above the critical point, liquid and gas become indistinguishable (supercritical fluid). Water has an anomalous negative-slope melting curve because ice is less dense than liquid water. 🇺🇦 Українська

Substance

Display

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SubstanceWater
Triple point T273.16 K
Triple point P611.7 Pa
Critical point T647.1 K
Critical point P22.1 MPa

Key Phase Boundaries

Melting curve (solid-liquid) — governed by the Clausius-Clapeyron equation dP/dT = ΔS/ΔV. For water, ΔV<0 (ice contracts on melting), giving a negative slope — unusual among substances. Sublimation curve (solid-gas) — follows the Antoine equation log P ≈ A − B/T. Below the triple point, solid sublimes directly to vapour. Vapour pressure curve (liquid-gas) — ends at the critical point (Tc, Pc). Above Tc, there is no distinction between liquid and gas; supercritical CO₂ is widely used as a green solvent in extraction processes.

About this simulation

This simulation renders a live pressure-temperature (P-T) phase diagram for water, CO₂, nitrogen or ethanol, computing the melting, sublimation and vaporisation coexistence curves directly from each substance's real thermodynamic data. The melting curve uses a linear Clausius-Clapeyron slope through the triple point, the sublimation and vaporisation curves use Antoine-style log₁₀P = A − B/T fits, and every pixel is coloured according to which phase (solid, liquid, gas, or supercritical fluid) is stable at that (T, P). A draggable crosshair lets you read off the exact phase at any point on the diagram.

🔬 What it shows

The three phase-coexistence curves — melting (solid/liquid), sublimation (solid/gas) and vaporisation (liquid/gas) — for the selected substance, all meeting at the triple point. Above the critical point (Tc, Pc), the liquid/gas boundary disappears and the region is shaded as supercritical fluid. Each substance's triple- and critical-point coordinates are displayed live in the stats panel.

🎮 How to use

Pick a substance (H₂O, CO₂, N₂ or EtOH) to reload its real Ttr, Ptr, Tc and Pc values and redraw the curves. Switch the pressure axis between Linear and Log P to see the huge pressure range near the triple point more clearly. Drag the P cursor and T cursor sliders, or move your mouse over the canvas, to place a crosshair and read the exact phase and coordinates in the info box below.

💡 Did you know?

Water's melting curve slopes backward (negative dP/dT) because ice is less dense than liquid water — squeezing ice at constant temperature can actually melt it. Almost every other substance, including CO₂ and nitrogen, has a positive melting slope because their solid phase is denser than the liquid, so the model uses a different sign of meltSlope for water than for the others.

Frequently asked questions

What is a triple point?

The triple point is the unique combination of temperature and pressure at which the solid, liquid and gas phases of a substance all coexist in equilibrium simultaneously. For water this occurs at exactly 273.16 K and 611.7 Pa — a value so precisely reproducible that it was historically used to define the kelvin itself. On the diagram it is the single point where the melting, sublimation and vaporisation curves all meet.

What happens at the critical point?

The critical point (Tc, Pc) marks the end of the liquid-vapour coexistence curve. Beyond this temperature and pressure, there is no longer a sharp interface between liquid and gas — the two phases become a single supercritical fluid with properties partway between both. For water this happens at 647.1 K and 22.06 MPa; for CO₂ it happens at a much more accessible 304.2 K and 7.38 MPa, which is why supercritical CO₂ is widely used industrially, for example in decaffeination.

Why does water's melting curve have a negative slope?

The Clausius-Clapeyron equation states dP/dT = ΔS/ΔV along a coexistence curve. For melting, ΔS is always positive (liquid is more disordered than solid), so the sign of the slope depends on ΔV, the volume change on melting. Water is one of the few common substances where ice occupies more volume than the liquid it melts into, making ΔV negative and therefore dP/dT negative — hence increasing pressure lowers water's melting point instead of raising it.

What is sublimation and when does it happen?

Sublimation is the direct transition from solid to gas without passing through a liquid phase. It occurs whenever the pressure is below the triple-point pressure, so the sublimation curve — modelled here with an Antoine-style equation log₁₀P = A − B/T — describes the vapour pressure of the solid phase at temperatures below Ttr. Dry ice (solid CO₂) sublimes at everyday atmospheric pressure because CO₂'s triple point (516.6 kPa) is far above 1 atmosphere, so liquid CO₂ cannot exist at normal pressure.

Why do phase diagrams often use a logarithmic pressure axis?

Vapour and sublimation pressures follow the Clausius-Clapeyron relation, which predicts an exponential dependence on temperature (P ∝ e^(−B/T)), so on a linear pressure axis the curve near the triple point is compressed into an almost invisible sliver while the curve near the critical point dominates the plot. Switching to a logarithmic P axis, as this simulation lets you do, spreads out the low-pressure region so the sublimation and low-temperature parts of the vaporisation curve become clearly visible alongside the high-pressure critical region.