What point on a phase diagram represents the pressure and temperature at which solid, liquid, and gas coexist under equilibrium conditions?

What point on a phase diagram represents the pressure and temperature at which solid, liquid, and gas coexist under equilibrium conditions?

• A. The critical point
• B. The boiling point
• C. The triple point
• D. The melting point

Answer: (C)

The point being tested is the one where all three phases—solid, liquid, and gas—are in equilibrium at the same time. On a phase diagram, which plots pressure versus temperature, this is a unique intersection where the boundaries between solid–liquid, liquid–gas, and solid–gas meet. At that precise pressure and temperature, the solid, liquid, and gas can coexist; small changes in conditions will shift the system to favor two of the phases instead of all three. By contrast, the critical point marks where liquid and gas become indistinguishable and you can have a supercritical fluid, the boiling point is the temperature at which a liquid's vapor pressure equals the external pressure, and the melting point is the temperature at which a solid turns into a liquid at a given pressure. For water, this all-three-phases coexistence occurs at about 0.01°C and 0.006 atm, illustrating the concept.

The point being tested is the one where all three phases—solid, liquid, and gas—are in equilibrium at the same time. On a phase diagram, which plots pressure versus temperature, this is a unique intersection where the boundaries between solid–liquid, liquid–gas, and solid–gas meet. At that precise pressure and temperature, the solid, liquid, and gas can coexist; small changes in conditions will shift the system to favor two of the phases instead of all three. By contrast, the critical point marks where liquid and gas become indistinguishable and you can have a supercritical fluid, the boiling point is the temperature at which a liquid's vapor pressure equals the external pressure, and the melting point is the temperature at which a solid turns into a liquid at a given pressure. For water, this all-three-phases coexistence occurs at about 0.01°C and 0.006 atm, illustrating the concept.