Phosphorus acid is a chemical compound with the formula H₃PO₃. It is a colorless, syrupy liquid that is often used in fertilizers and as a reducing agent in various chemical reactions. Phosphorus acid is classified as a weak acid and can be produced by the hydrolysis of phosphorus trichloride or by the oxidation of phosphine. In addition to its agricultural applications, phosphorus acid is also used in the production of phosphonates, which are important in the synthesis of various chemicals. It plays a role in the food industry as a food additive and is sometimes used in cleaning products due to its ability to remove mineral deposits.