Question

What if a friend was balancing chemical equations by changing the values of the subscripts instead of using the coefficients? How would you explain to your friend that this was the wrong thing to do?

Short answer

The meaning of the chemical reaction changes if the subscripts are used in a chemical formula while balancing the chemical equation. So, coefficients should be used.

Step-by-step solution

Definition of formula

Subscript to an element in a chemical formula represents the number of atoms of that element present in a single molecule. If the number of atoms in a molecule changes, it results in a completely different molecule.

Explanation

For example, consider the unbalanced equation:

${\text{H}}_{\text{2}}{\text{+O}}_{\text{2}}\to {\text{H}}_{\text{2}}\text{O}$

Here, the water molecule has one oxygen and two hydrogen atoms. Its chemical formula is ${\text{H}}_{\text{2}}\text{O}$. If the subscript of oxygen is changed to 2, the formula becomes ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$. Each molecule of this compound has two oxygen atoms and two hydrogen atoms. It represents hydrogen peroxide. Both are entirely different and the meaning of the reaction has changed completely.

If prefix 2 is given to water, it will become ${\text{2.H}}_{\text{2}}\text{O}$. It represents two water molecules. The compound remains the same.

The balanced chemical equation is:

${\text{2H}}_{\text{2}}{\text{+O}}_{\text{2}}\to 2{\text{H}}_{\text{2}}\text{O}$

A chemical equation represents a chemical change that occurs when particular compounds are reacted. While balancing a chemical equation, it is required to change the number of molecules of the given substances, and not to change the substance itself.

Conclusion

While balancing a chemical equation, the coefficients should be used. If subscripts are changed, then the meaning of the chemical reaction changes.