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How do temperature and pressure determine the phase of a pure substance?
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liquid
Figure 1: Phase diagram of argon.
The phase of a pure substance is determined by its temperature and pressure, which dictate whether the substance exists as a solid, liquid, or gas. These relationships are mapped on a phase diagram, where areas represent single phases and lines represent conditions where two phases coexist in equilibrium.12
- Phase Boundaries: The vapour-pressure curve separates the liquid and gas regions, while the melting curve represents the boundary between solid and liquid. Crossing these lines results in a distinct phase change.12
- Equilibrium Points: The triple point is the unique condition where all three phases coexist. The critical point marks the end of the vapour-pressure curve; beyond this point, the liquid and gas phases become indistinguishable, forming a single fluid state.12
- Thermodynamic Influence: The slope of these phase boundaries is governed by the Clapeyron equation, which relates the change in pressure and temperature to the latent heat and volume change associated with the phase transition.3