Allotropy of phosphorus: Conversion of yellow (white) phosphorus to red phosphorus is carried out by heating in covered retorts at what temperature range (°C) in the absence of air?

Introduction / Context:
Phosphorus exists in several allotropes—white (yellow), red, and black—each with distinct structures and properties. Converting the highly reactive, toxic white form to the safer red form is an industrially important step for matches, flame retardants, and reagents. This conversion is a solid-state rearrangement triggered by controlled heating without oxygen.

Given Data / Assumptions:

• Feed: white (yellow) phosphorus.
• Operation performed in covered retorts, i.e., an oxygen-free (inert) environment.
• Objective: obtain red phosphorus safely.

Concept / Approach:
White phosphorus consists of molecular P4 units with high ring strain, making it pyrophoric. Heating at moderate temperatures allows atoms to re-polymerize into a more stable, polymeric red phosphorus network. Oxygen must be excluded to prevent combustion and oxidation to phosphorus pentoxide.

Step-by-Step Solution:
Identify the typical industrial range for the white→red transformation.Safe and effective conversion occurs around 250–400 °C under inert/oxygen-free conditions.Higher temperatures risk sublimation or different chemistry; lower temperatures are too slow.Hence, select 250–400 °C.

Verification / Alternative check:
Materials processing texts and MSDS data list 250–400 °C as the practical window for converting white to red phosphorus in sealed vessels, often with inert gas sweeping to remove traces of white phosphorus vapor.

Why Other Options Are Wrong:
50–80 °C: far too low; white phosphorus remains unchanged.

800–900 °C and 1000–1200 °C: unnecessarily high and potentially hazardous; may lead to volatilization or undesired products.

Common Pitfalls:

• Allowing oxygen ingress; even traces can lead to fire.
• Overheating beyond recommended range, increasing pressure and hazard.

Final Answer:
250–400