What is the triple point in thermodynamics?

• A. The temperature at which a substance melts and boils at the same pressure.
• B. The point where three phases (solid, liquid, and gas) are in equilibrium.
• C. The temperature where a substance becomes plasma.
• D. The highest pressure for gas phase stability

Answer: (B)

The triple point is the condition where solid, liquid, and gas phases are in equilibrium at the same time. At a specific temperature and pressure, all three phases coexist, and the phase boundaries for solid–liquid, liquid–gas, and solid–gas meet. For water, this occurs at about 0.01°C and 611 pascals, a well-known reference point used to calibrate thermometers and define temperature scales. The other options describe phenomena not about three-phase coexistence: one speaks of melting and boiling at the same pressure, another points to ionized plasma, and the last to a high-pressure limit for gas stability.