Question

When balancing equations, which numbers are you allowed to change? why only these?

Answer 1

Let’s say that you are asked to balance the equation
H₂ + Cl₂ → HCl

You would immediately put a 2 in front of the HCl and write
H₂ + Cl₂ → 2HCl

But why can’t you write
H₂ + Cl₂ → H₂Cl₂?

This is also a balanced equation. However, we use the formulas in equations to represent elements and compounds. If we put a number (a coefficient) in front of the formula, we are simply using a different amount of the same substance. If we change the subscript in the formula, we are changing the substance itself.

Thus, HCl represents a molecule that contains one H atom attached to one Cl atom. H₂Cl₂ would represent a molecule in which two H atoms and two Cl atoms are somehow bonded together to give a new particle containing four atoms. Since the original equation listed HCl as the product, we are not answering the question that was asked.

BOTTOM LINE
When balancing equations, we can change only the coefficients in front of the formulas. We are not allowed to change the subscripts in the formulas.