Question

Given below are four statements regarding the allotropes of Phosphorus. Which of them is/are incorrect?

• A. White Phosphorus is a waxy, molecular solid that has a garlic-like odour
• B. Red Phosphorus is more reactive than white phosphorus and it is soluble in carbon disulphide
• C. Red Phosphorus can be cut with a knife
• D. Red Phosphorus is a non-toxic, odourless polymeric solid at room temperature

Answer 1

Answer: (B), (C)

Red Phosphorus can be cut with a knife

First let us get the correct ones out of the way and then reason as to why the rest are incorrect shall we?

You will likely encounter some strange odours associated with white Phosphorus and one of the Phosphorus hydrides. Phosphine - $P{H}_3$ smells like a “rotten fish” while white Phosphorus has a garlic-like odour! Now that we have put aside the smelly stuff, what can you recollect about the textures of allotropes of Phosphorus?

White Phosphorus is a waxy solid at room temperature and so it can be cut with a knife. It melts close to ${30}^{\circ }C$ and this is a direct result of its highly strained molecular structure, which makes white Phosphorus much more reactive than its red counterpart. Red Phosphorus is a hard, odourless, non-toxic, polymeric solid which cannot be cut with an everyday kitchen knife.

Of the two allotropes, only white Phosphorus is soluble in $C{S}_2$.