# Why is it incorrect to balance a chemical equation by changing the subscripts?

The number of subscripts for each element in a chemical formula is like a form of spelling. Each chemical formula has its own unique set of elements and subscripts. For example, $\text{H"_2"O}$ is the formula for water, and ${\text{H"_2"O}}_{2}$ is the formula for hydrogen peroxide, a very different substance from water. For another example, ${\text{CO}}_{2}$ is the formula for carbon dioxide, and $\text{CO}$ is the formula for carbon monoxide, also a very different substance from carbon dioxide. So, if you change the subscripts in the chemical formulas in a chemical equation, you are actually changing the substances themselves.
So if I write the coefficient 2 in front of the formula for water, $\text{H"_2"O}$, so that it looks like $\text{2""H"_2"O}$, then I multiply the coefficient of 2 times the subscript 2 for hydrogen, and the subscript 1 for oxygen (No subscript is understood to be 1.).This gives me 4 atoms of hydrogen and 2 atoms of oxygen.