We must balance the chemical reaction
[tex]P_2O_3+H_2O\to H_2O+H_3PO_3[/tex]
Balancing equations consists of balancing the reactants and products of the formulas. To do this, coefficients are only added when required but the subscripts are not changed. When balancing chemical reactions we seek to comply with the Law of Conservation of Matter.
In this case, the balanced equation would be
[tex]P_2O_3+3H_2O\to2H_3PO_3[/tex]
