QS Study

Phosphine (PH3) is the best known hydride of phosphorus.

Laboratory preparation: It is usually obtained by boiling white phosphorus with 30-40% solution of caustic soda in an inert atmosphere of CO2.

P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2 (Sodium hypophosphite)

Phosphine so obtained is impure. It is passed into an aqueous solution of hydrogen iodide, PH4I is formed. PH4I is heated with KOH or NaOH, pure phosphine is obtained.

PH3 + HI → PH4I

PH4I + NaOH → PH3 + NaI + H2O

Physical properties

Phosphine is colourless gas with rotten fish odour.

Chemical properties

  • Dissociation: Phosphine dissociates at about 723 K and gives red phosphorus.

4PH3 P4 →(23K)→ 6H2

  • Action of air: It burns with oxygen and produces phosphorus pentoxide.

4PH3 + 8 O2 → P4O10 + 6H2O


Smoke screens

When PH3 burns it produces smoke which is dense enough to serve as smoke screens.

Holme’s signal: Containers which have a perforated bottom and a hole at the top are filled with calcium phosphide and calcium carbide. These are thrown into the sea. Water enters the container through the bottom and reacts with calcium carbide and calcium phosphide to give acetylene and phosphine. Phosphine gets ignited spontaneously as it comes in contact with air and also ignites acetylene.

Thus a bright red flame is produced which is accompanied by huge smoke due to the burning of phosphine. This serves as a signal to the approaching ships.

  • Ca3P2 + 6H2O → 2 PH3 ↑ + 3Ca(OH)2
  • CaC2 + 2H2O → C2H2 ↑ + Ca(OH)2