What happens at the triple point on a phase diagram?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

argon phase diagram — liquid Figure 1: Phase diagram of argon.

On a phase diagram, the triple point represents the unique condition of temperature and pressure at which the solid, liquid, and gaseous phases of a substance coexist in equilibrium.12

At this specific junction, the three curves that define the boundaries between these phases meet. While the phase diagram generally shows areas where only one phase can exist or lines where two phases exist in equilibrium, the triple point is the singular state where all three phases are present simultaneously. The specific values for temperature and pressure at the triple point vary significantly depending on the substance.23