Phase Diagrams of Water and Carbon Dioxide
Interactive pressure-temperature phase diagrams for water and carbon dioxide. Click or tap anywhere to identify the phase at that temperature and pressure; points on a curve read as the two-phase equilibrium, and the triple and critical points announce themselves. Vapor curves derive from the course anchor points by Clausius-Clapeyron fits; fusion lines are anchored linear fits.
Water (H₂O)
Carbon dioxide (CO₂)
| Substance | Triple Point | Critical Point | Behavior at 1 atm | Fusion-Line Slope |
|---|---|---|---|---|
| Water (H2O) | 0.01 °C, 0.61 kPa | 374 °C, 22089 kPa | Melts at 0 °C, boils at 100 °C | Negative (ice is less dense than liquid water) |
| Carbon dioxide (CO2) | −56.6 °C, 518 kPa | 31 °C, 7370 kPa | Sublimes at −78.5 °C (no liquid at 1 atm) | Positive (solid denser than liquid, the usual case) |
Important Notes
- Reading the diagram: each curve is a two-phase equilibrium; crossing a curve is a phase transition. The three curves meet at the TRIPLE POINT (all three phases coexist); the liquid-gas curve ENDS at the CRITICAL POINT, beyond which liquid and gas merge into a supercritical fluid.
- Dry ice sublimes at 1 atm because CO₂'s triple-point pressure (518 kPa, about 5.1 atm) sits ABOVE atmospheric pressure: at 1 atm there is no temperature at which liquid CO₂ exists. Liquid CO₂ appears only above 518 kPa.
- Water's fusion line leans slightly LEFT (negative slope): higher pressure melts ice, because ice is less dense than liquid water. CO₂'s fusion line leans right, the usual case.
- The pressure axis is logarithmic; it spans from below 1 kPa to beyond the critical pressures.
- Water's vaporization curve below 100 °C follows the course vapor-pressure table (see the Water Vapor Pressure reference); the 100 °C to critical stretch, both sublimation curves, and CO₂'s vaporization curve are Clausius-Clapeyron fits between the course anchor points. The fusion lines are linear fits through the 1-atm transition points, with CO₂'s melting slope anchored to the NIST melting curve. ChemWhiz problems supply their own values in the problem statement.
Source: Vaporization and sublimation curves: Clausius-Clapeyron fits between the ChemWhiz course anchors (graded Topic 18 triple/critical points and transition temperatures; ΔHvap = 40.7 and ΔHfus = 6.02 kJ/mol), with water below 100 °C following the course vapor-pressure table. Fusion lines: linear fits through the course 1-atm transition points; the CO₂ melting slope is anchored to the NIST-documented melting curve (NIST IR 8608)