Phosphorus acid can act both as oxidising agent as well as reducing agent while phosphoric acid is only an oxidising agent. Explain.
Or
Phosphorus acid undergoes disproportionation reaction but phosphoric acid does not. Explain.

Explanation

The behaviour is linked with the oxidation state of phosphorus atom in the two acids. In phosphorus acid `(H_(3)PO_(3))`, the oxidation state of P is + 3. It is in a position to increase its oxidation state (acts as reducing agent ) as well as decrease it (acts as oxidising agent). The oxidation state of phosphorus in phosphoric acid `(H_(3)PO_(4))` is + 5. Since it cannot increase it beyond 5, it cannot act as a reducing agent. However, it can behave as oxidising agent by decreasing its oxidation state.
We can also say that `H_(3)PO_(3)` undergoes disproportination while `H_(3)PO_(4)` does not.
`H_(3)overset(+3)(PO_(3)) overset("heat")to overset(-3)(PH_(3))+3H_(3)overset(+5)(3H_(3)PO_(4))`