Chemical Properties and Structure of Phosphorous Oxoacids

To determine the correct statement(s) regarding the oxoacids of phosphorus, we analyze each option individually:

1. Analysis of Statement (A): When orthophosphorous acid ($\text{H}_3\text{PO}_3$) is heated, it undergoes a disproportionation reaction to form orthophosphoric acid ($\text{H}_3\text{PO}_4$) and phosphine ($\text{PH}_3$): $4\text{H}_3\text{PO}_3 \xrightarrow{\Delta} 3\text{H}_3\text{PO}_4 + \text{PH}_3$

   • In $\text{H}_3\text{PO}_3$, the oxidation state of phosphorus is $+3$.
   • In $\text{H}_3\text{PO}_4$, the oxidation state of phosphorus is $+5$.
   • In $\text{PH}_3$, the oxidation state of phosphorus is $-3$. Since phosphorus simultaneously undergoes oxidation (from $+3$ to $+5$) and reduction (from $+3$ to $-3$), this is a disproportionation reaction. Thus, Statement (A) is correct.

2. Analysis of Statement (B):

   • In $\text{H}_3\text{PO}_3$, phosphorus is in the $+3$ oxidation state, which is an intermediate oxidation state for phosphorus (group 15 element). Therefore, it can be oxidized further to its highest oxidation state of $+5$. Thus, $\text{H}_3\text{PO}_3$ acts as a reducing agent due to the presence of the $\text{P}-\text{H}$ bond.
   • In $\text{H}_3\text{PO}_4$, phosphorus is already in its maximum oxidation state of $+5$. Since it cannot be oxidized any further, $\text{H}_3\text{PO}_4$ cannot act as a reducing agent. Thus, Statement (B) is correct.

3. Analysis of Statement (C): The structural formula of orthophosphorous acid ($\text{H}_3\text{PO}_3$) contains one $\text{P}=\text{O}$ double bond, two $\text{P}-\text{OH}$ single bonds, and one $\text{P}-\text{H}$ single bond.

   • Only the hydrogen atoms attached to the electronegative oxygen atoms ($-\text{OH}$ groups) are ionizable as protons ($\text{H}^+$) in aqueous solution.
   • Since there are two $-\text{OH}$ groups present in $\text{H}_3\text{PO}_3$, it can furnish two $\text{H}^+$ ions per molecule in water, making it a dibasic acid (not a monobasic acid). Thus, Statement (C) is incorrect.

4. Analysis of Statement (D): The electronegativity of phosphorus ($\approx 2.19$) is nearly equal to that of hydrogen ($\approx 2.20$). As a result, the $\text{P}-\text{H}$ bond is non-polar and does not dissociate to yield $\text{H}^+$ ions in aqueous media. Thus, the hydrogen atom bonded directly to phosphorus in $\text{H}_3\text{PO}_3$ is non-ionizable in water. Thus, Statement (D) is correct.

Conclusion: The correct statements are A, B, and D.