The reaction may be represented as:
(a) White phosphorus or Yellow phosphorus
(i) It is a soft waxy solid with garlic smell.
(ii) It is poisonous in nature.(iii) It turns yellowish on exposure to light. For this reason, it is also called yellow phosphorus.
(iv) It is not soluble in water but soluble in carbon disulphide.(v) It undergoes spontaneous combustion in air and produces greenish glow.
(vi) It exists as P4 molecules both in solid and vapor state. The four atoms in P4 molecule occupy the corners of regular tetrahedron as shown in Fig. 7(b) Red Phosphorus
It is prepared by heating white phosphorus to about 540 K in an inert atmosphere of nitrogen for several hours.
(i) It is a hard crystalline solid without any smell.
(ii) It is non-poisonous in nature.(iii) It is insoluble in water as well as in carbon disulphide.
(iv) It is more stable and relatively less reactive.(v) It consists of tetrahedral units of P4 linked to one another to constitute linear chains.
(c) Black Phosphorus
It is prepared by heating white phosphorus to about 470 K under high pressure of 1200 atmospheres in inert atmosphere.
(i) It has metallic lustre.
(ii) It is most inactive form of phosphorus.
(iii) It has a layer type structure in which each layer consists of phosphorus atoms.
Some physical properties of three forms of Phosphorus are given below.
| Property | White phosphorus | Red phosphorus | Black Phosphorous |
|---|---|---|---|
| Colour | White, but turns yellow on exposure | Dark red | Black |
| Colour | White, but turns yellow on exposure | Dark red | Black |
| State | Waxy solid, can be cut with knife | Brittle powder | Crystalline with greasy touch |
| Smell | Garlic smell | Odorless | |
| Density | 1.84 | 2.1 | 2.69 |
| Ignition temperature | 307 K | 533 K | 673 K |
| Melting point | 317 K | Does not melt | 860 K |
