H3PO3, on heating, undergoes disproportionation reaction to form PH3 and H3PO4. The
oxidation numbers of P in H3PO3, PH3, and H3PO4 are +3, −3, and +5 respectively. As the oxidation number of the same element is decreasing and increasing during a particular reaction, the reaction is a disproportionation reaction.
