Give the disproportionation reaction of `H_(3)PO_(3)`.

Explanation

Orthophosphorous acid (or phosphorus acid) disproportionate to produce orthophosphoric acid (or phosphoric acid) and phosphine on heating.
`underset("acid")underset("Phosphorus")(4H_(3)PO_(4))overset("Heat")rarr underset("acid")underset("orthophsphoric")(3H_(3)PO_(4))+underset("Phosphine")(PH_(3))`
Thus, `H_(3)PO_(3)` (oxidation state `+3`) oxidizes to give `H_(3)PO_(4)` (oxidation state `+5`) and reduced to produce phosphine (oxidation state `-3`).