**Ternary Phase Diagram with Phase Envelope**

This Demonstration shows a ternary phase diagram with a phase envelope. This diagram is used to represent the phase behavior of mixtures of three components that are only partially miscible over a range of compositions so that phase separation occurs. The black point, which can be moved to any location within the triangle by clicking on that location, represents the overall composition of the mixture. Each corner of the triangle corresponds to a pure component. The mass fraction of a component in the mixture is read off the axis that is the same color as that component. These axes are labeled by drawing a line through the point; this line is parallel to the base of the triangle that is opposite the corner corresponding to that pure component. "View phases" mode identifies the one-phase region and the two-phase region (phase envelope). In "view diagram" mode, gray tie lines are shown in the two-phase region when the "show tie lines" option is selected. The endpoints of each tie line correspond to the compositions of the two phases in equilibrium (α and β phases), and these compositions are shown when the "show α phase" and "show β phase" options are selected. Points along the left of the phase envelope boundary belong to the α phase, and the points along the right belong to the β phase. The point at which the α phase switches to the β phase on the phase envelope is the plait point, which is found by following the tie lines up until a tie line is tangent to the phase envelope boundary. The point of intersection is the plait point. Ternary phase diagrams can also be drawn with mole fractions instead of mass fractions.