Phosphorus and its oxygen compounds

When phosphorus burns in enough oxygen, phosphorus(V) oxide forms (formula P₄O₁₀, usually written P₂O₅): 4P + 5O₂ → 2P₂O₅. It is a white, strongly water-absorbing (hygroscopic) acidic oxide; with hot water it gives orthophosphoric acid: P₂O₅ + 3H₂O → 2H₃PO₄. Orthophosphoric acid H₃PO₄ is a colourless crystal (melts at ≈ 42 °C), dissolves well in water and is a tribasic acid of medium strength; it dissociates in steps: H₃PO₄ ⇌ H⁺ + H₂PO₄⁻ ⇌ 2H⁺ + HPO₄²⁻ ⇌ 3H⁺ + PO₄³⁻. So it forms three series of salts: dihydrogenphosphates (NaH₂PO₄), hydrogenphosphates (Na₂HPO₄) and phosphates (Na₃PO₄). All dihydrogenphosphates and the phosphates of alkali metals and ammonium dissolve in water; Ca₃(PO₄)₂ is insoluble in water but dissolves in strong acid. In the laboratory the acid is made by Ca₃(PO₄)₂ + 3H₂SO₄ → 3CaSO₄ + 2H₃PO₄. The qualitative test for the phosphate ion is silver nitrate, which gives a yellow precipitate of Ag₃PO₄: Na₃PO₄ + 3AgNO₃ → Ag₃PO₄↓ + 3NaNO₃. Phosphorus is needed for life: bone is Ca₅(PO₄)₃OH, tooth enamel Ca₅(PO₄)₃F, and the energy carrier ATP contains it; academician A. Fersman called it “the element of life and thought”. Only the teacher works with strong acids and P₂O₅.