Balancing Equations

Reactions rearrange atoms — they never create or destroy them. So if you count two oxygen atoms before the reaction, you must find exactly two afterward. This is the law of conservation of mass.

An unbalanced equation breaks that law. H₂ + O₂ → H₂O has two oxygens on the left but only one on the right. Where did the second oxygen go? Nowhere — the equation just isn't balanced yet.