What does the triple point represent in a phase diagram?

What does the triple point represent in a phase diagram?

• A. The temperature at which a liquid turns to a gas at a fixed pressure.
• B. The condition where solid, liquid, and gas coexist in equilibrium.
• C. The pressure at which a gas becomes a liquid.
• D. The point at which a solid becomes a gas directly.

Answer: (B)

The triple point is the specific combination of temperature and pressure where solid, liquid, and gas can all exist at once in balance. On a phase diagram, it’s where the solid–liquid, liquid–gas, and solid–gas boundaries meet, so all three phases are in equilibrium together. For water, this occurs at about 0.01°C and 611 pascals, illustrating that each substance has its own unique triple point. Outside this point, only one or two phases are stable, so you don’t see all three phases coexisting. The other descriptions refer to transitions between just two phases (melting/boiling, condensation, or sublimation) at specific conditions, not the simultaneous balance of all three.

The triple point is the specific combination of temperature and pressure where solid, liquid, and gas can all exist at once in balance. On a phase diagram, it’s where the solid–liquid, liquid–gas, and solid–gas boundaries meet, so all three phases are in equilibrium together. For water, this occurs at about 0.01°C and 611 pascals, illustrating that each substance has its own unique triple point. Outside this point, only one or two phases are stable, so you don’t see all three phases coexisting. The other descriptions refer to transitions between just two phases (melting/boiling, condensation, or sublimation) at specific conditions, not the simultaneous balance of all three.