When phosphorous acid is heated, it decomposes to form phosphoric acid and ?

Phosphorous acid (H₃PO₃) is an important oxyacid of phosphorus that exhibits both acidic and reducing properties. Upon heating, it undergoes thermal decomposition, a chemical process where heat breaks the compound into simpler substances.

When phosphorous acid is heated strongly, it partially oxidizes to phosphoric acid (H₃PO₄) while simultaneously reducing a portion of phosphorus to form phosphine gas (PH₃). The balanced chemical reaction for this process is:

4H₃PO₃ → 3H₃PO₄ + PH₃

This equation shows that four molecules of phosphorous acid decompose to yield three molecules of phosphoric acid and one molecule of phosphine gas.

The reaction occurs because phosphorous acid contains both P–H bonds and P–OH bonds. The presence of the P–H bond makes the compound a reducing agent. During heating, part of the phosphorus is oxidized (forming H₃PO₄) while another part is reduced (forming PH₃). This internal redox reaction is a unique property of H₃PO₃ among phosphorus oxyacids.

Phosphine (PH₃) is a colorless, highly toxic gas with a characteristic fishy or garlic-like odor. It is slightly soluble in water and burns in air, producing phosphorus pentoxide. Because of its toxicity and flammability, handling should be done with care.

Phosphoric acid (H₃PO₄), the other product, is a tribasic acid widely used in fertilizers, detergents, and the food industry.

This reaction demonstrates an important principle in inorganic chemistry—how different oxidation states of phosphorus can coexist and interconvert under suitable conditions.

Therefore, when phosphorous acid is heated, it decomposes into phosphoric acid and phosphine, showcasing a classic example of a redox decomposition reaction.