Phase Diagrams of Ternary Systems

Consider a ternary system of water, trichloromethane, and ethanoic acid. Water and trichloromethane are partially miscible, while ethanoic acid is fully miscible with both.

The phase diagram is shown as an equilateral triangle, with each apex representing a pure component. Each edge represents a binary system, and any interior point means the presence of all three components.

The binodal curve encloses the two-phase region and marks the compositions at which phase separation occurs in a ternary system.

Adding ethanoic acid to the water-trichloromethane mixture is shown by the line joining a₁ to the ethanoic acid apex.

At point a₂, the system still contains two liquid phases. Because ethanoic acid increases mutual solubility, the trichloromethane-rich phase, a₂″, contains more water, and the water-rich phase, a₂′, contains more trichloromethane.

As the composition shifts from a₂ to a₃, the trichloromethane-rich layer becomes smaller, and beyond a₄, the mixture becomes a single homogeneous phase.

The plait point P represents the composition where the two liquid phases become identical.