The properties of phosphorous acid ($H_3PO_3 $) are described as
phosphoric acid, chemical formula $H_3PO_3 $, also known as Phosphorous Acid. Its properties are unique and widely used.

In terms of its properties, phosphorous acid is white crystalline, hygroscopic, and easily soluble in water, alcohols, etc. This substance has medium and strong acidity, and can be ionized in steps in aqueous solution to release hydrogen ions. Its acidity is due to the easy dissociation of hydrogen from the hydroxyl group in the molecular structure. And phosphorous acid is reductive, because phosphorus is in the + 3 valence state, it can be oxidized to a higher valence state. In case of strong oxidants, such as potassium permanganate, etc., oxidation reactions can occur, and the phosphorus valence state increases, and it is oxidized by itself.

As for the application, in the chemical industry, it is often used as a reducing agent. In organic synthesis, it can reduce specific functional groups, such as formaldehyde, ketone, etc. to alcohol compounds, and the reaction conditions are mild and the selectivity is good. In chemical plating, phosphite and its salts can provide electrons to reduce and deposit metal ions on the surface of the matrix, forming a uniform and dense coating, which enhances the corrosion resistance and wear resistance of materials. In agriculture, phosphite and its salts can be used as fertilizers. Appropriate application can supplement phosphorus for plants, and it has a certain bactericidal and disease-preventing effect. It can activate the plant's own defense mechanism, enhance the plant's resistance to diseases and insect pests, and improve crop yield and quality. In the field of water treatment, phosphoric acid can be used as a water quality stabilizer to complex with metal ions in water to prevent scaling and corrosion and maintain stable operation of the system.

In summary, phosphoric acid plays an important role in many fields due to its unique properties, and is of great significance to the progress of industrial production, agricultural development and related fields.