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

Dec 5, 2016

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

For example:

$6 {\text{HCl" + 2"Al"(s) -> 2"AlCl"_3 + 3"H}}_{2} \left(g\right)$

If you suddenly write $6 \text{HCl}$ as $3 {\text{H"_2"Cl}}_{2}$, then what you've done is replace $\text{HCl}$ with a new molecule whose formula is ${\text{H"_2"Cl}}_{2}$, and this molecule may or may not exist. This one doesn't.

The new chemical formula of ${\text{H"_2"Cl}}_{2}$ literally tells you that there are two hydrogen atoms and two chlorine atoms in this hypothetical molecule, but $\text{HCl}$ only had one of each.

Likewise, $3 {\text{H"_2"Cl}}_{2}$ is also not the same as $3 {\text{H"_2 + 3"Cl}}_{2}$.