🧊Phase Diagram Explorer - Water & CO₂ Phase Diagrams, Triple Point, Clausius–Clapeyron

How to use Phase diagrams & phase transitions

1. 1Choose water, carbon dioxide or nitrogen, or define your own substance from its triple point, critical point, heat of fusion and volume change
2. 2Press and drag the state point to read the phase, degrees of freedom and the melting, boiling or sublimation point at that pressure
3. 3Pick a process path and play it to watch melting, boiling, sublimation or a continuous trip around the critical point
4. 4Use the other tabs to get latent heat from two points, boiling points at altitude or in a pressure cooker, and van der Waals isotherms

FAQ

Why does water's melting line lean to the left?

The Clapeyron equation gives dP/dT = ΔH/(TΔV). Ice is less dense than water, so melting shrinks the volume (ΔV ≈ −1.63 cm³/mol) and the slope is negative, about −13.5 MPa/K. Pressure therefore lowers the melting point. Most substances, including CO₂ and N₂, have a denser solid and a positive slope. The line is very steep, though: lowering the melting point of ice by 1 °C takes about 133 atmospheres.

Why doesn't dry ice melt into a liquid?

The triple-point pressure of CO₂ is 5.18 atm (518 kPa), and liquid CO₂ cannot exist below it. Heating dry ice at 1 atm moves the state point across the sublimation curve, so it turns straight into gas at −78.5 °C (194.7 K). To get liquid CO₂ you need more than 5.18 atm, such as the roughly 6 MPa inside a cylinder at room temperature.

How hot is the water in a pressure cooker?

The absolute pressure inside is local air pressure plus the gauge reading. At a typical 100 kPa gauge the absolute pressure is about 201 kPa and water boils at about 120 °C; at 50 kPa gauge it is about 111 °C. At high altitude the air pressure is lower, so the same gauge pressure gives a lower boiling point, and the calculator here accounts for altitude.

What is a supercritical fluid?

Above both the critical temperature and pressure (374 °C and 22.06 MPa for water, 31 °C and 7.38 MPa for CO₂), liquid and gas have the same density and there is no boiling interface. The result is a supercritical fluid: it diffuses like a gas and dissolves things like a liquid, which is why supercritical CO₂ is used to decaffeinate coffee and extract flavors. By going around the critical point, a gas can turn into a liquid without ever boiling.

Why does the van der Waals isotherm have a loop?

Below the critical temperature, a vdW isotherm has a stretch where dP/dV > 0. That part is mechanically unstable, so a real fluid splits into liquid and vapor at the saturation pressure. The Maxwell equal-area rule places this horizontal line so the areas enclosed above and below it are equal. The vdW equation gets the critical point and coexistence right qualitatively, but not quantitatively: for CO₂ at 280 K it predicts 5.3 MPa versus about 4.2 MPa in reality.