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Allotropes of Phosphorus

The existence of phosphorus in nature in different physical form but possessing similar chemical properties is known as allotropes of phosphorus. The allotropes of phosphorus are:

  1. Red phosphorus
  2. White phosphorus

Besides these other known allotropes of phosphorus found in nature are:

  1. Black phosphorus
  2. Scarlet phosphorus
  3. Violet phosphorus

1. White Phosphorus

The phosphorus obtained by connecting vapours of phosphorus under water is known as white phosphorus. It is highly reactive as well as toxic. It is most common allotrope of phosphorus.

Physical Properties

  • Colorless solid that becomes yellow in light so known as yellow allotrope.
  • Garlic smell.
  • The melting point is 44°C.
  • Highly poisonous & used as rat poison.

Chemical Properties

  1. It reacts with air & gently gets oxidized to form phosphorus pentoxide (P2O5) that glows at night.

$$P_4+5O_2\longrightarrow 2P_2O_5$$

  1. It reacts with caustic soda (NaOH) producing phosphine gas on heating & produces sodium hydrogen phosphate as the side product.

$$P_4+3NaOH+3H_2O\longrightarrow PH_3+3NaH_2PO_2$$

  1. It reacts with concentrated HNO3 to give phosphoric acid.

$$P_4+20HNO_3\longrightarrow 4H_3PO_4+20NO_2+4H_2O$$


  • Used in match industry.
  • Used as rat poison.
  • Used to prepare phosphine gas.

2. Red Phosphorus

The phosphorus obtained by heating white phosphorus at 250°C at an inert atmosphere for some hour is called red phosphorus.


  • Dark red powder
  • Odorless
  • Less toxic and less reactive
  • Don’t react with NaOH
  • Don’t oxidize with air.
  • With halogen, it form trihalide & pentahalide

$$P_4+6Br_2\longrightarrow 4PBr_3$$

$$P_4+6 I_2\longrightarrow 4PI_3$$

$$P_4+10Cl_2\longrightarrow 4PCl_5$$


  • Used in match industry.
  • Used to prepare HBr & HI gases.

Laboratory preparation of phosphine gas

In the lab, phosphine gas is prepared by heating white phosphorus with the solution of NaOH or KOH.

$$P_4+3NaOH+3H_2O\longrightarrow PH_3+3NaH_2PO_2$$

laboratory preperation of phosphine
Fig:laboratory preperation of phosphine


White phosphorus is taken in a flask with the concentrated solution of caustic soda (NaOH). The flask is fitted with a delivery tube and inlet valve to provide the inert atmosphere. Air is removed out from the flask. Now, the flask is gently heated & phosphine gas with a trace amount of diphosphine (P2H4) is evolved & as each bubble escape into the air, it burns into flame forming vortex ring of the white smoke of phosphorus pentoxide (P2O5). The combustion of gas is due to diphosphine so the produced phosphine gas should be passed through freezing mixture where diphosphine condenses but phosphine does not. Finally, pure phosphine is collected into the jar by downward displacement of water.

Physical properties

  • Colourless gas with the rotten fish smell.
  • Toxic in nature.
  • Slightly soluble in water & heavier than air.

Chemical properties

  • Decomposition: It decomposes to give red phosphorus & hydrogen.

$$4PH_3\longrightarrow P_4+6H_2$$

  • Basic nature: It is weaker base than ammonia.

$$PH_3+H_20\longrightarrow PH_4+OH^-$$

Weak basic nature of phosphine (PH3) then ammonia (NH3) is due to the vacant orbital of phosphorus & lone pair of the electron that cannot be donated easily.

$$PH_3+HX\longrightarrow PH_4X (Phosphonium\space halide)$$

$$PH_3+HCl\longrightarrow PH_4Cl (Phosphonium chloride)$$

  • Combustion: It burns with air to form phosphorus pentoxide& phosphoric acid.

$$2PH_3+4O_2\longrightarrow P_2O_5+ 3H_2O (Phosphorous pentoxide)$$

$$PH_3+2O_2\longrightarrow H_3PO_4 (Phosphoric\space acid)$$

  • Reducing agent: It reduces chlorine to chloride.

$$PH_3+4Cl_2\longrightarrow PCl_5 + 3HCl$$

It reduces metal ions like Cu2+, Ag+, Au3+ to respective metals.

$$3CuSO_4+2PH_3\longrightarrow Cu_3P_2\downarrow+3H_2SO_4$$

$$Cu_3P_2\longrightarrow 6Cu+P_4$$

$$3AgNO_3 +PH_3\longrightarrow Ag_3P+3HNO_3$$

$$4Ag_3P\longrightarrow 12Ag+P_4$$


  • Used in formation of smoke during war.
  • Used in production of Holme’s signal (signal in sea journey during emergency)

Orthophosphoric acid


  • In the lab, Phosphoric acid is prepared by heating rock phosphate with concentrated H2SO4.

$$Ca_3(PO_4)_2+3H_2SO_4\longrightarrow 2H_3PO_4+3CaSO_4$$

  • It can be prepared by treating phosphorus pentaoxide (P2 O5) with hot water.

$$P_2O_5+3H_2O\longrightarrow 2H_3PO_4$$

  • It can also be prepared by hydrolysis of PCl5

$$PCl_5+4H_2O\longrightarrow H_3PO_4+5HCl$$

Physical properties

  • White crystalline solid.
  • Weak acid & fairly soluble in water.

Chemical properties

  1. Action with heat: when phosphoric acid is heated. It produces pyrophosphoric acid at 250-260°C & metaphosphoric acid at 300°C by losing water molecules.

$$2H_3PO_4\longrightarrow H_4P_2O_7\longrightarrow HPO_3$$

  1. Acidic properties: It is tribasic acid due to ionization of 3 hydrogen atom lean with oxygen. It forms 3-series of salt corresponding to 3-stage of ionization.

$$H_3PO_4↔H^+ + H_2PO_4^-$$

$$H_2PO_4^-↔H^+ +HPO_4^-$$

$$HPO_4↔H^+ + PO^-$$

Therefore, it’s salt with NaOH are:

  1. NaH2PO4 {sodium dihydrogen phosphate}
  2. Na2HPO4 {disodium hydrogen phosphate}
  3. Na3PO4 {sodium phosphate}
  4. Reaction with AgNO3: A yellow ppt. of silver phosphate is obtaied.

$$3AgNO_3+H_3PO_4\longrightarrow Ag_3PO_4+3HNO_3$$


  • Used to manufacture fertilizer.
  • Used to make phosphate salt.
  • Used to prepare medicine, detergent etc.


pulse, Tracy. Introduction to chemistry. u.s.a: flex book, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. Kathmandu: Vidhyarthi Pustak Bhandar, 2012.

  • Phosphoric acid is weaker base than ammonia. 
  • Weak basic nature of phosphine (PH3) then ammonia (NH3) is due to the vacant orbital of phosphorus & lone pair of the electron that cannot be donated easily. 
  • White phosphorus is used to prepare phosphine gas.
  • The phosphorus obtained by heating white phosphorus at 250°C at an inert atmosphere for some hour is called red phosphorus.

Questions and Answers

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Rojina Poudel

What does White phosphorus and NaOH{anhydrous} when heated give?

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how red p

how red phosphrous is changed into white phosphorus?

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