Question

How can red phosphorus and white phosphorus be separated?

Answer 1

Phosphorus:

• Phosphorus is a p-block element with the atomic number 15.
• It comes under 15th group elements.

Allotropes of Phosphorus:

• There are two main forms of elemental phosphorus: Red Phosphorus and White Phosphorus.
• Red phosphorus is a chain of four tetrahedral grouped phosphorus atoms while white phosphorus is also made of 4 tetrahedral phosphorus atoms but an in-ring structure.
• Red phosphorus is more stable than white phosphorus.
• White phosphorus is soluble in organic solvents whereas red phosphorus cannot.

Separation of White Phosphorus:

• White phosphorus can be separated from red phosphorus by dissolving in caesium chloride $\left(\mathrm{CsCl}\right)$ and carbon disulphide $\left({\mathrm{CS}}_2\right)$.
• It can be treated with caustic alkalis like sodium hydroxide $\left(\mathrm{NaOH}\right)$ and potassium hydroxide $\left(\mathrm{KOH}\right)$ to form phosphine $\left({\mathrm{PH}}_3\right)$.

$\underset{\mathrm{White}\mathrm{phosphorus}}{{\mathrm{P}}_4}+\underset{\mathrm{Sodium}\mathrm{hydroxide}}{3\mathrm{NaOH}}+3\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}}\to \underset{\mathrm{Sodium}\mathrm{hypophosphite}}{3{\mathrm{NaH}}_2{\mathrm{PO}}_2}+\underset{\mathrm{Phosphine}}{{\mathrm{PH}}_3}$

• Red phosphorus does not react with caustic alkalis.

Separation of Red Phosphorus:

• By heating white phosphorus to $300°\mathrm{C}$ in the absence of air or by exposing it to sunlight, red phosphorus is formed.