Final answer:
Changing a subscript in a chemical formula changes the substance the formula represents. Subscripts indicate the actual number of atoms of each element in a molecule, whereas coefficients indicate the number of molecules.
Step-by-step explanation:
Changing a subscript in a correctly written chemical formula changes the formula so that it no longer represents the compound it previously represented. Subscripts in a chemical formula indicate the number of atoms of each element in a molecule and changing them alters the chemical identity of the substance.
For instance H₂O (water) and H₂O₂ (hydrogen peroxide) differ significantly in their chemical properties due to the different subscripts. Therefore when balancing chemical equations one should only change the coefficients which state the number of molecules involved, not the subscripts which are fixed for a given compound.