In industrial inorganic chemistry and allotropy of phosphorus: How can red phosphorus be converted back into the more reactive white (yellow) phosphorus during processing? Select the method that achieves this allotropic transformation safely under controlled conditions.

Introduction:
Phosphorus exists in several allotropes with distinct structures and reactivities. White (yellow) phosphorus is highly reactive and waxy, while red phosphorus is polymeric and comparatively stable. Understanding how to interconvert these forms is essential in match manufacture, pyrotechnics, and phosphorus handling safety.

Given Data / Assumptions:

• We start with red phosphorus (polymeric, less reactive).
• Goal: obtain white phosphorus (molecular P4, more reactive).
• Industrial methods must avoid oxidation and uncontrolled combustion.

Concept / Approach:
Red phosphorus can revert to white phosphorus by thermal treatment that breaks the polymeric network into discrete P4 molecules. Because white phosphorus forms as a molecular vapor, the practical route is to volatilize phosphorus and then condense the vapor under inert or reducing conditions and without oxygen. This distillation-like approach separates the allotropes based on phase behavior.

Step-by-Step Solution:
Heat red phosphorus in an oxygen-free environment to generate phosphorus vapor (predominantly P4).Prevent exposure to air to avoid oxidation and ignition.Condense the vapor in water or an inert medium to collect white phosphorus as a solid.Therefore, the correct transformation method is vaporisation followed by condensation.

Verification / Alternative check:
Reference processing notes show that white phosphorus is commonly stored under water to avoid oxidation; its formation via vapor condensation is a standard route from red phosphorus when required.

Why Other Options Are Wrong:

• Heating in presence of light: Risks photochemical oxidation/ignition; not the controlled conversion route.
• Melting under pressure: Does not directly produce white phosphorus from red; melting conditions are unsafe and inappropriate.
• Oxidation in dry air: Produces oxides, not a different allotrope.
• None of these: Incorrect because vaporisation followed by condensation is valid.

Common Pitfalls:
Confusing oxidation (a chemical change) with allotropy (a structural change). The transformation requires oxygen exclusion and careful temperature control to avoid combustion.

Final Answer:
vaporisation followed by condensation.