Phosphoric acid, H ₃ PO ₃ is a colorless weak acid. For phosphoric acid the chemical formula is H ₃ PO ₄ . It contains one phosphorus atom, four oxygen atoms, and three hydrogen atoms. It is different from phosphorous acid, H ₃ PO ₃ since it contains four oxygen atoms. For phosphoric acid, three OH groups are bonded to the phosphorus atom so it is a tribasic group with basicity of three. Basicity is the number of hydrogen atoms present in a chemical compound. And here the three hydrogen atoms are strongly bonded with an electronegative atom of oxygen so it is acidic. While the basicity of phosphorus acid is only two. As all the hydrogen atoms are acidic it is dissociated completely in water except at low pH. When all the three hydrogen ions have been removed the compound obtained is called orthophosphates.