The oxidation state of P in `H_(3)PO_(3)" is "+3`. Since this value is intermediate between the highest (+5) and lowest(-3) oxidation states of P, therefore, when `H_(3)PO_(3)` is heated it undergoes disproportionation to form `PH_(3)andH_(3)PO_(4)` with oxidation states of -3 and +5 respectively.
`underset("Orthophosphorous acid")overset(+3)(4H_(3)PO_(3))overset("Heat")tounderset("Phosphine")overset(-3)(PH_(3))+underset("Orthophosphoric acid")overset(+5)(H_(3)PO_(4))`
