Give the disproportionation reaction of $H_3PO_3$.

(N/A) On heating,orthophosphorus acid $(H_3PO_3)$ disproportionates to give orthophosphoric acid $(H_3PO_4)$ and phosphine $(PH_3)$.
The balanced chemical equation is:
$4H_3\overset{+3}{P}O_3 \xrightarrow{\Delta} 3H_3\overset{+5}{P}O_4 + \overset{-3}{P}H_3$
In this reaction,the phosphorus atom in $H_3PO_3$ (oxidation state $+3$) is simultaneously oxidized to $H_3PO_4$ (oxidation state $+5$) and reduced to $PH_3$ (oxidation state $-3$).