Why does phosphorous acid form only two series of salts, even though the molecule contains three hydrogen atoms?

Oxyacids are compounds that contain hydrogen, oxygen, and another element (usually a nonmetal). An important aspect of oxyacid chemistry is understanding the structure of the oxyacid molecule and how it relates to acidity. Phosphorous acid, H₃PO₃, is an example of an oxyacid where the central phosphorus atom is bonded to oxygen and hydrogen atoms.

In the case of H₃PO₃, the molecule has a specific structure in which not all hydrogen atoms are created equal. This molecular structure affects which hydrogen atoms can dissociate to form hydrogen ions (H⁺). Understanding the specific bonding patterns and dissociation possibilities within an oxyacid like phosphorous acid is crucial for predicting its chemical behavior, such as its ability to form salts with bases.

Hydrogen ion dissociation refers to the process by which hydrogen atoms in a molecule detach and form hydrogen ions, H⁺. This typically occurs in the aqueous solution where the hydrogen atoms are attached to the more electronegative oxygen atoms, known as hydroxyl groups (-OH).

Dissociable vs. Non-dissociable Hydrogen

In phosphorous acid, only the hydrogen atoms in the -OH groups can dissociate to produce hydrogen ions. The third hydrogen atom, which is bonded directly to the phosphorus atom (P-H), does not dissociate under normal conditions. The ability to release hydrogen ions and form new compounds is a cornerstone of acid-base chemistry and is what leads to the formation of various salts when acids react with bases.

Salt formation is a chemical process where an acid and a base react to form a salt and usually water. This process often involves the replacement of hydrogen ions from the acid with cations from the base. In oxyacids like phosphorous acid, only the dissociable hydrogen atoms, which are from the hydroxyl groups, participate in forming salts.

Because phosphorous acid contains two hydroxyl groups, it can form two series of salts depending on how many of the hydrogen atoms are replaced. For example, if both dissociable hydrogen atoms are replaced, a hydrogen phosphite (HPO₃^2-) salt is formed. On the other hand, if only one is replaced, it forms a dihydrogen phosphite (H₂PO₃^-) salt. These reactions are essential in the study of chemistry as they show the reactivity of acids and the formation of new compounds with a wide range of applications.