Explain the chemical behaviour of oxoacids of phosphorus.

The chemical behaviour of oxoacids of phosphorus is determined by their oxidation states,the presence of $P-H$ bonds,and the number of $P-OH$ bonds.
$1.$ Disproportionation: Oxoacids with phosphorus in the $+3$ oxidation state tend to disproportionate into higher and lower oxidation states. For example,orthophosphorous acid $(H_3PO_3)$ on heating disproportionates to give orthophosphoric acid $(H_3PO_4)$ and phosphine $(PH_3)$:
$4H_3PO_3 \rightarrow 3H_3PO_4 + PH_3$
$2.$ Reducing Property: Acids containing $P-H$ bonds exhibit strong reducing properties. Hypophosphorous acid $(H_3PO_2)$ contains two $P-H$ bonds and acts as a strong reducing agent,for example,reducing $AgNO_3$ to metallic silver:
$4AgNO_3 + 2H_2O + H_3PO_2 \rightarrow 4Ag + 4HNO_3 + H_3PO_4$
$3.$ Basicity: The hydrogen atoms attached to oxygen in $P-OH$ bonds are ionisable and contribute to the basicity of the acid. Hydrogen atoms attached directly to phosphorus ($P-H$ bonds) are non-ionisable. Thus,$H_3PO_4$ is tribasic,$H_3PO_3$ is dibasic,and $H_3PO_2$ is monobasic.