A phase diagram is a visual representation of phases and their most stable ranges of pressures and temperatures; phase diagrams are substance-specific.
As an example, a general phase diagram for water is shown above. Regions bordered by lines indicate that water at a particular pressure and temperature can be classified as water vapor, liquid water, or ice. The triple point indicates the conditions at which all three states coexist in equilibrium. The critical point indicates the end of the liquid-gas boundary; beyond that temperature and pressure, water will exist as a gas only. The boundaries in the graph represent the point where bordering states exist in equilibrium.
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Key Points
• Boundaries in phase diagrams indicate equilibrium points for bordering states
• Phase diagrams are substance-specific
Key Terms
Triple Point: Point where all three states in a diagram coexist in equilibrium.
Critical Point: Point where the boundary of two states ends; one states typically dominates.