What happens when
(A)`PCl_(5)` is heated.
(B)`PCl_(5)` is reacted with heavy water.
( C)`H_(3)PO_(3)` is heated.

Explanation

`H_(3)PO_(3)`, on heating, undergoes disproportionation reaction to form` PH_(3)` and `H_(3)PO_(4)`. The oxidation numbers of P in `H_(3)PO_(3)`, `PH_(3)`, and `H_(3)PO_(4)` 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.
`underset((+3))underset("Orthophosphrous acid") (4H_(3) PO_(3)) overset(Delta)to underset((+5))underset("Orthophosphoric acid") (3H_(3) PO_(4)) + underset((-3))underset("Phosphile")(PH_(3))`