Phosphorous acid

Phosphorous acid is a chemical compound. It is an acid. Its chemical formula is H3PO3. It contains hydrogen and phosphite ions.
Properties.
It is a white solid. It absorbs water. It is a strong reducing agent. It can be deprotonated twice. When it is heated, it turns into phosphoric acid and phosphine. It is toxic. Salts of phosphorous acid is called phosphites. It reacts with mercury(II) chloride to make mercury(I) chloride. If it still is kept, it reacts with mercury(I) chloride to make mercury.
Preparation.
It is made by reacting phosphorus(III) oxide with water. It is also prepared by reaction of phosphorus trichloride with water, which produces hydrochloric acid too. 
Uses.
It is used as a reducing agent. It is also used to kill certain bacteria that grow on plants.