Liquid–liquid extraction selectivity – temperature effect and plait point Which of the following statements about solvent selectivity in liquid–liquid extraction is/are correct?

Introduction / Context:
In liquid–liquid extraction, selectivity (or separation factor) quantifies how preferentially a solute partitions between two immiscible phases. Correct statements about selectivity determine whether extraction is feasible and how sensitive performance is to temperature and phase behavior near the critical solution composition (plait point).

Given Data / Assumptions:

• Selectivity S is the ratio of distribution coefficients or relative solubilities.
• Plait point is where the two liquid phases become identical in composition.
• Typical organic–aqueous systems show temperature-dependent partitioning.

Concept / Approach:
When S = 1, the solute shows no preference for either phase, making separation impossible. As temperature increases, distribution coefficients of many systems move closer together, reducing selectivity (though exceptions exist). At the plait point, the two phases are indistinguishable; selectivity becomes 1 by definition since there is no phase contrast to exploit for separation.

Step-by-Step Solution:

Verification / Alternative check:
Ternary phase diagrams and tie-line data show diminishing partition differences as T approaches critical solution temperature; S tends to unity at the plait point.

Why Other Options Are Wrong:
Any single-statement choice omits other valid facts, while denying any of (a)–(c) contradicts core phase-equilibrium principles.

Common Pitfalls:
Treating statement (b) as universal; it is a frequent trend but not an absolute rule—always consult system-specific data when designing extractions.

Final Answer:
all (a), (b) and (c)