Why do chemical equations have to be balanced?
Why do I have to pay my mortgage off at the end of the month? Like the principal and interest in mortgages, mass is balanced in a chemical equation.
In every chemical equation that has ever been performed, mass is conserved. If you start with
If the equation is not balanced, you know that it is not valid, and does not reflect chemical reality.
It is essentially a consequence of the Conservation of Mass-Energy rule (matter cannot be destroyed nor created).
The law of Conservation of Mass (which Lavoisier discovered in 1785) basically says that matter cannot be created nor destroyed, it can only be changed. (We also know now that matter can be converted to energy, but lets leave that issue for the moment as it is more applicable to radioactive decay and so on).
If your equation is not balanced it means that there will be a different total number of atoms on one side of the equation compared to the other side. For example:
this is balanced. If you write it like this:
then you have 2 oxygen atoms on the left side, but 3 on the right - you have "created" an extra oxygen atom, that is not possible.
Similarly, you have 4 hydrogen atoms on the left, but only 2 on the right, you have "destroyed" two hydrogen atoms, which is also not possible.
Therefore, balancing corrects this.