Which one of the following is formed (mainly) when red phosphorus is heated in a sealed tube at 803 K?

Solution

Chemistry ma’am · 3 steps

Match each allotrope to its starting material: red phosphorus at 803 K in a sealed tube gives α-black.

1. Two black forms. Black phosphorus has two forms, α and β. They come from different starting allotropes under different conditions, so the starting material tells you which one forms.

   Allotrope formed	Starting material	Conditions
   Red phosphorus	White phosphorus	573 K, inert atmosphere
   α-black phosphorus	Red phosphorus	803 K, sealed tube
   β-black phosphorus	White phosphorus	473 K, high pressure

2. Why not white. Heating converts phosphorus towards more stable, more polymeric forms. Red phosphorus does not go back to the reactive, tetrahedral P₄ white form. Yellow phosphorus is only impure white phosphorus, so it is ruled out too.
3. So. Red phosphorus heated in a sealed tube at 803 K gives mainly α-black phosphorus, option (d).

The idea to keep. Remember the pairs: red to α-black (803 K, sealed tube) and white to β-black (473 K, high pressure). The starting allotrope decides which black form you get.

Answer from the NTA final answer key. Solution by PrepWiser.