Phase Diagrams of Ternary Systems

Consider a ternary system, which is composed of three components: water (W), ethanoic acid (E), and trichloromethane (T). Here, Ethanoic acid (E) is fully miscible with both water (W) and trichloromethane (T), meaning it can mix entirely with either of them. However, water and trichloromethane have partial miscibility, meaning they can only mix to a certain extent, beyond which two separate phases will form.

The phase diagram of a ternary system is represented as an equilateral triangle, where each apex represents a pure component. The edges of the triangle represent binary systems (two-component systems), while a point inside the triangle indicates the presence of all three components.

A single phase is created when enough E is added to the binary system of (W,T). This is represented by the line joining point a₁ to E on the phase diagram.

At point a₂, the solution still has two phases, but the composition of each phase has changed. The phase that was primarily T now has more W, and the primary W phase now has more T. This is due to the E enhancing the solubility of both components in each other. As the composition shifts from point a₂ to a₃, we see a trace of the formation of the T-rich layer. Adding more E and moving towards point a₄ transitions the system into a single phase beyond point a₄.

The plait point P is a unique point on the diagram where the compositions of the two equilibrium phases are identical. This means that the system is in equilibrium at this point, and the proportions of water, ethanoic acid, and trichloromethane are the same in both phases.