What happens when `H_(3) PO_(3)` is heated ?

Explanation

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) and H_(3)PO_(4)` with oxidation states of -3 and +5 respectively.
`underset("Orthophosphorous acid")(4H_(3)overset(+3)(PO_(3))) overset("Heat") rarr underset("Phosphine")overset(-3)(PH_(3))+underset("Orthophosphoric acid")overset(+5)(H_(3)PO_(4))`