H3PO3, known as phosphorous acid, is a chemical compound composed of three hydrogen atoms, one phosphorus atom, and three oxygen atoms. It is a colorless, syrupy liquid that is often used in various industrial applications, including the production of fertilizers and as a reducing agent in chemical reactions. Phosphorous acid is classified as a weak acid and can act as a diprotic acid, meaning it can donate two protons (H+) in solution. It is important in the synthesis of phosphonates and plays a role in the phosphorus cycle, which is essential for plant growth and development.