Question

A \( \mathrel{\mathop{\kern0pt\longrightarrow} \limits_{}^{573\,K}} \) Red phosphorus \( \mathrel{\mathop{\kern0pt\longrightarrow} \limits_{under\,pressure}^{heat\,;\,803\,K}} \) B

Red phosphorus is obtained by heating "A" at 573 K, and can be converted to "B" by heating at 803 K under pressure.

A and B, respectively, are

• A. \( \beta \)-black phosphorus and white phosphorus.
• B. white phosphorus and \( \beta \)-black phosphorus.
• C. \( \alpha \)-black phosphorus and white phosphorus.
• D. white phosphorus and \( \alpha \)-black phosphorus.

Hint:

White phosphorus heated at 573 K in an inert atmosphere gives red phosphorus; red P heated under high pressure at 803 K gives \(\alpha\)-black phosphorus.

Answer 1

The Correct Option is D

Step 1: Red phosphorus is prepared by heating white phosphorus at about 573 K in an inert atmosphere (CO₂ or N₂), so A = white phosphorus.
Step 2: Heating red phosphorus at 803 K under high pressure converts it into \(\alpha\)-black phosphorus, so B = \(\alpha\)-black phosphorus. (\(\beta\)-black P is obtained by heating white P at 473 K under high pressure.)
Answer: (D) white phosphorus and \(\alpha\)-black phosphorus