In phosphorus processing, the conversion of yellow (white) phosphorus to red phosphorus is carried out at 250–450 °C. Under which atmosphere/condition is this transformation correctly performed in industrial retorts?

Introduction / Context:
Yellow (white) phosphorus is thermodynamically less stable and more reactive than red phosphorus. Converting it to the safer, polymeric red allotrope is an important conditioning step for matches, pyrotechnics, and certain fertiliser intermediates. The operating environment (presence or absence of oxygen) critically affects both yield and safety.

Given Data / Assumptions:

• Temperature range: 250–450 °C in retorts.
• Objective: allotrope conversion without oxidation/combustion.
• Industrial practice employs sealed vessels and inert purges.

Concept / Approach:
Yellow phosphorus readily oxidises in air and can ignite. The conversion to red phosphorus must therefore be performed without oxygen present—operationally described as “in the absence of air,” often implemented by an inert purge (e.g., nitrogen) to displace oxygen and moisture.

Step-by-Step Solution:

Verification / Alternative check:
Process descriptions specify inert atmospheres to prevent oxidation; this is operationally equivalent to maintaining an oxygen-free space.

Why Other Options Are Wrong:

• Oxidising atmosphere: would oxidise/ignite phosphorus.
• Reducing atmosphere: unnecessary and may introduce side reactions.
• Steam-rich: risks hydrolysis/oxidation issues.
• Inert atmosphere: often used, but the core requirement tested by many MCQs is “absence of air”.

Common Pitfalls:
Equating “inert” strictly with nitrogen presence; the principle is preventing oxygen access, i.e., air-free conditions.

Final Answer:
absence of air