Pick the wrong statement — Identify the incorrect assertion among the following heat-transfer/phase-change statements.

Introduction:
Quick conceptual checks are common in exams to ensure familiarity with definitions and order-of-magnitude criteria in heat transfer and separation processes. Hydraulic (equivalent) diameter, convection criteria, and distinctions between unit operations are foundational ideas.

Given Data / Assumptions:

• Hydraulic (equivalent) diameter Dh = 4A/P for internal flows.
• Forced vs. natural convection dominance gauged by Gr/Re^2.
• Evaporation vs. distillation distinction is understood.

Concept / Approach:

For a square duct with side x, area A = x^2 and wetted perimeter P = 4x, hence Dh = 4A/P = 4x^2/(4x) = x. Therefore, stating Dh = 4x is incorrect. The criterion Gr/Re^2 indicates whether buoyancy (natural convection) competes with or dominates forced convection; small values imply forced convection dominance. Distillation involves vapor–liquid equilibrium separation by relative volatility, distinct from simple evaporation (removing solvent).

Step-by-Step Solution:

Verification / Alternative check:

Textbook examples routinely derive Dh = x for a square duct; published correlations use Dh in Reynolds/Nusselt numbers accordingly.

Why Other Options Are Wrong:

Here 'wrong' applies only to (b). Options (a), (c), and (d) are correct statements; (e) is a reinforcing truth explaining (a).

Common Pitfalls:

Mixing up Dh with the actual diameter in circular pipes; for non-circular passages always use Dh = 4A/P.

Final Answer:

For a square duct of side x, the equivalent diameter for heat transfer equals 4x.