In a binary system represented on a phase diagram, if you are given a composition located at a specific point within a two-phase region, how can you utilize the Lever Rule to determine the proportions of each phase when the critical point is approached?

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Seekh · Accepted Answer

When a composition lies inside a two‑phase region, the Lever Rule tells us that the fraction of each phase is proportional to the horizontal distance from the composition point to the phase boundary lines. First, draw a tie line through the point that meets the phase boundaries at the two coexisting phases; the distances along the tie line to each intersection give the lever arms. Divide the total length of the tie line by each lever arm to get the phase fractions, with the larger arm corresponding to the smaller phase fraction. As the composition approaches the critical point, the tie line shortens and the two lever arms become equal, so the phase fractions converge toward 50 % each, showing that the two phases become indistinguishable at the critical point. For example, if a tie line is 0.

In a binary system represented on a phase diagram, if you are given a composition located at a specific point within a two-phase region, how can you utilize the Lever Rule to determine the proportions of each phase when the critical point is approached?

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