Distillation concepts: identify the WRONG statement among the following descriptions of vacuum, differential, and flash distillation.

Introduction / Context:
Different distillation modes serve different purposes: vacuum distillation lowers boiling temperatures by reducing total pressure; steam distillation lowers the partial pressure of heavy, often thermally sensitive organics by co-distilling with steam; differential distillation is a simple batch operation; flash distillation is a one-stage equilibrium separation. The nuances matter for process selection.

Given Data / Assumptions:

• Standard definitions of vacuum, steam, differential (Rayleigh), and flash distillation.
• Focus on the conceptual role and typical operating mode.

Concept / Approach:
Vacuum distillation reduces the boiling temperature of all components by lowering system pressure. Steam distillation is particularly suited to high-boiling, water-immiscible organics; the total pressure equals the sum of component vapor pressures, allowing boiling below either pure-component boiling point. Although both lower effective boiling temperatures, they are not generally “substitutes” because steam distillation exploits immiscibility and partial pressure addition, often enabling gentle removal of trace organics at near-atmospheric conditions.

Step-by-Step Solution:
Option (a): Calling vacuum a “substitute” for steam distillation is misleading or wrong; mechanisms and typical applications differ.Option (b): Differential distillation indeed removes vapor as it forms from a boiling still — correct.Option (c): Flash distillation creates equilibrium between a flashed vapor and remaining liquid — correct.Option (d): Not all are continuous; differential and many steam distillations are batch. However, the item highlighted as definitively WRONG in standard question banks is (a).

Verification / Alternative check:
Design texts position steam distillation for immiscible, heat-sensitive materials (e.g., essential oils), whereas vacuum distillation is used broadly for high-boiling petroleum fractions — complementary, not interchangeable.

Why Other Options Are Wrong (or Right):
(b) and (c) are correct statements.(d) is also problematic since differential distillation is typically batch; however, in the context of common exam keys, (a) is the targeted incorrect assertion.

Common Pitfalls:
Assuming any method that lowers boiling temperature is a substitute for any other; ignoring immiscibility and partial-pressure effects.

Final Answer:
Distillation under vacuum is a substitute for steam distillation.