Here's why:

* Balancing chemical equations means making sure the number of atoms of each element on the reactant side (left side) of the equation is equal to the number of atoms of that element on the product side (right side).

* Subscripts in a chemical formula indicate the number of atoms of that element in a molecule. Changing a subscript would change the identity of the molecule itself, which is not allowed when balancing equations.

* Coefficients are the numbers placed in front of chemical formulas. These are used to adjust the number of molecules of each substance in the reaction, allowing for a balanced equation.

Here's an example:

Unbalanced: H₂ + O₂ → H₂O

Balanced: 2H₂ + O₂ → 2H₂O

We added coefficients (2 in front of H₂ and 2 in front of H₂O) to balance the equation. We did not change the subscripts within the formulas (like changing H₂O to H₂O₂).