Arsenous acid, also known as arsenious acid, is a chemical compound. It is also an acid. Its chemical formula is As(OH)3. It has arsenic and hydroxide ions in it. The arsenic is in its +3 oxidation state.
Arsenous acid is a colorless solution. It is a weak acid. It reacts with bases to make arsenite salts. It is highly toxic. It cannot be made in a crystalline form because it turns into arsenic trioxide and water again. It is similar to phosphorous acid, although it is not a strong reducing agent.