Phosphorous acid
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.