Question

Mention some properties of orthophosphoric acid.

Answer 1

1) It has a melting point of 315 K and is a white crystalline solid. It has a deliquescent quality and turns into a colourless, syrupy liquid when moisture is absorbed.

2) It transforms into pyrophosphoric acid and then metaphosphoric acid upon heating.

\[\ce{2H3PO4 ->[520 K] \underset{acid}{\underset{Pyrophosphoric}{H4P2O7}} + H2O}\]

\[\ce{H4P2O7 ->[589 K] \underset{acid}{\underset{Metaphosphoric}{HPO3}} + H2O}\]

3) It is a tribasic acid and ionises in aqueous solution as

\[\ce{H3PO4 <=> H+ + H2PO^-_4}\]

\[\ce{H2PO^-_4 <=> H+ + HPO^{2-}_4}\]

\[\ce{HPO^{2-}_4 <=> H+ + PO^{3-}_{4}}\]

Its tribasic nature allows it to produce three different salt series, such as NaH₂PO₄, Na₂HPO₄, and Na₃PO₄.