When balancing a chemical equation, you can change the coefficients because they indicate the number of molecules or moles of each substance involved in the reaction. Adjusting the coefficients allows you to maintain the same ratio of reactants to products and to ensure that the law of conservation of mass is upheld. This means that the total number of atoms of each element must be the same on both sides of the equation.
On the other hand, changing the subscripts in a chemical formula alters the actual identity of the substance. For example, altering the subscript in water (H2O) to H2O2 would change it from water to hydrogen peroxide, which are two entirely different compounds. This would not only change the nature of the chemical reaction but would also lead to an incorrect representation of the substances involved. Therefore, while coefficients can be varied to balance an equation, subscripts must remain fixed to accurately represent each chemical’s structure.