TNK LAB

P–T Phase Diagrams

Every point on this chart is a (T, P) condition — hover or click anywhere to see which phase the substance is in there. The three curves are coexistence lines: cross one and the substance changes phase. They meet at the triple point, where all three phases coexist, and the liquid–gas line ends at the critical point, beyond which liquid and gas are no longer distinct.

Water
CO₂
N₂

Inspected Point

Hover or click the diagram →

Reference Points

TP

How These Curves Are Built

The sublimation and vaporization curves come from the Clausius–Clapeyron equation with the latent heat treated as constant over each segment,
ln(P) = ln(Pref) − (L/R)(1/T − 1/Tref)
fit through well-documented reference points (triple point, critical point, and — where the substance actually has one at 1 atm — its normal boiling point). The solid–liquid line is drawn schematically as a steep straight segment through the triple point: real fusion curves are nearly vertical (melting point barely shifts with pressure), so getting its exact curvature right matters far less than getting its correct direction — leaning left for water's famous density anomaly, right for everything else.
This is a simplified model, not a lab-precision equation of state — it's tuned to hit the real triple, critical, and (where they exist) boiling points, but the curves between those landmarks are approximate.
Why can't CO₂ be a liquid in the open air?
CO₂'s triple point sits at 5.1 atm — above 1 atm. Follow the P=1 atm line on its diagram and you cross straight from solid to gas without ever passing through the liquid region: that's why dry ice sublimes instead of melting. Liquid CO₂ only exists above 5.1 atm, which is why fire extinguishers and soda cartridges that hold liquid CO₂ are pressurized containers.

Water — P–T Phase Diagram

Solid
Liquid
Gas
Supercritical fluid
Coexistence curve
Dashed = 1 atm reference