Question

what happens when is H3PO3 is heated

Answer 1

H3PO3, on heating, undergoes disproportionation reaction to form PH3 and H3PO4. ... As the oxidation number of the same element is decreasing and increasing during a particular reaction, the reaction is a disproportionation reaction.

Answer 2

Hey !!

Orthophosphorous acid on heating disproportionates to give orthophosphoric acid and phosphine.

                  4H₃PO₃ --(HEAT)--> PH₃ + 3H₃PO₄

Hope it helps you !!