Question

Why can't I change the subscripts in a molecule when I balance a chemical reaction?

Answer 1

Because you change the integrity of the compound you're looking at.

For example:

`6"HCl" + 2"Al"(s) -> 2"AlCl"_3 + 3"H"_2(g)`

If you suddenly write `6"HCl"` as `3"H"_2"Cl"_2`, then what you've done is replace `"HCl"` with a new molecule whose formula is `"H"_2"Cl"_2`, and this molecule may or may not exist. This one doesn't.

The new chemical formula of `"H"_2"Cl"_2` literally tells you that there are two hydrogen atoms and two chlorine atoms in this hypothetical molecule, but `"HCl"` only had one of each.

Likewise, `3"H"_2"Cl"_2` is also not the same as `3"H"_2 + 3"Cl"_2`.