Here's why:
* Subscripts indicate the number of atoms of each element within a molecule or compound. Changing a subscript changes the chemical formula itself, altering the identity of the substance. For example, H₂O is water, but H₂O₂ is hydrogen peroxide – two completely different compounds.
* Balancing equations involves adjusting the number of molecules or formula units of each reactant and product. This is done by placing coefficients in front of the chemical formulas.
Example:
Let's balance the equation for the combustion of methane:
Unbalanced: CH₄ + O₂ → CO₂ + H₂O
Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O
* We used coefficients (2 in front of O₂ and 2 in front of H₂O) to ensure that the number of atoms of each element is the same on both sides of the equation.
* We did not change the subscripts within the formulas (CH₄, O₂, CO₂, H₂O).
In summary:
* Subscripts define the chemical composition of a substance and cannot be changed to balance an equation.
* Coefficients are used to adjust the number of molecules/formula units of each reactant and product, thus balancing the equation.
