Question

Red P can be obtained from white P by:
A.Heating it with a catalyst in an inert atmosphere
B.Distilling it in an inert atmosphere
C.Dissolving it in carbon disulphide and crystallizing
D.Melting it and pouring liquid in water

Answer 1

Hint:To answer this question, you must recall the various allotropic forms of phosphorus and their properties. White phosphorus and red phosphorus are both allotropic forms of phosphorus. White phosphorus exists as a ${P_4}$ unit while red phosphorus consists of multiple${P_4}$ units joined together.

Complete step by step answer:
Phosphorus has three allotropic forms, namely, white phosphorous, red phosphorus and black phosphorus. All these forms are interconvertible.
White phosphorus is the most simple of all the allotropes consisting of a ${P_4}$ unit. White phosphorus is obtained when phosphate rock is heated with coke or sand in an electric furnace. It is white waxy and soft solid and can be cut with a knife. It is extremely poisonous and causes diseases like Phossy jaw to people coming in contact.
Red phosphorus is a non- poisonous, hard, crystalline and odorless solid. It has an iron grey lustre and is made up of many ${P_4}$ units joined together. Thus, we can prepare red phosphorus from white phosphorus. This conversion is achieved by heating white phosphorus at a temperature of around $573K$ in an inert atmosphere, that is, out of contact of air in presence of a catalyst. The catalyst used is either carbon dioxide gas or iodine gas.
\[{P_4}(white)\xrightarrow[{C{O_2}/{I_2}}]{{573K}}{P_4}(red)\]

Hence, the correct answer is A.

Note:
Phosphorus is represented by the chemical symbol P. It has atomic number 15 and is a member of group 15 and period 3 of the periodic table. It is a non- metal in the p block and is highly reactive.
Phosphorus has 3 electrons in its $3p$ orbital and can thus form three bonds in the ground state. So each phosphorus forms three bonds and the molecule of phosphorus exists as ${P_4}$. It exists in the gaseous state as white phosphorus as ${P_4}$ molecule which has a tetrahedral shape.