Phase diagrams and partially miscible liquids (ternary systems): A plait point is the point on a binodal (solubility) curve where the tie line shrinks to a point. For a ternary liquid system that contains two pairs of partially miscible liquids, how many plait points will appear on its phase diagram?

Introduction / Context:
Ternary liquid–liquid phase diagrams often display regions of partial miscibility. Each partially miscible binary pair produces a binodal curve terminating at a critical mixing point known as the plait point. Understanding how many plait points exist helps interpret separation behavior and solvent selection strategies.

Given Data / Assumptions:

• Ternary liquid system (three components).
• Exactly two binary pairs are partially miscible; the third pair is completely miscible.
• Standard triangular composition diagram at constant temperature and pressure.

Concept / Approach:
In ternary diagrams, each binary pair that is partially miscible contributes a binodal loop along the corresponding binary edge that extends into the ternary field. Each such binodal has a critical solution point at which the tie lines coalesce to a single composition—this is the plait point. Therefore, the number of plait points equals the number of partially miscible binary pairs in the system, provided their immiscibility persists at the diagram conditions.

Step-by-Step Solution:
Identify partially miscible pairs: count = 2.Recognize that each partially miscible pair contributes one plait point where tie lines vanish.Therefore, total plait points in the ternary diagram = 2.

Verification / Alternative check:
Reference ternary maps show one plait point for every partially miscible binary edge; systems with three such pairs (e.g., water–phenol–hydrocarbon at certain T) show three plait points, confirming the one-to-one correspondence.

Why Other Options Are Wrong:
0 or 1 contradict the presence of two partially miscible pairs; 3 would require all three binary pairs to be partially miscible, which is not the case here.

Common Pitfalls:

• Confusing a plait point with a critical solution temperature; here we consider a fixed T–P diagram.
• Assuming a single plait point governs the whole ternary field irrespective of how many binary pairs are immiscible.

Final Answer:
2