Why must chemical equations be balanced?

Chemical equations must be balanced to satisfy the law of conservation of mass, which dictates that matter—specifically atoms—cannot be created or destroyed during a chemical reaction.12

Because chemical reactions simply rearrange existing atoms into new substances, the total number of atoms for each element must remain constant throughout the process. Balancing an equation ensures that the number of atoms of each kind is identical on both the reactant and product sides, which in turn confirms that the total mass of the reactants equals the total mass of the products.132

This principle is foundational to stoichiometry, allowing chemists to accurately calculate the mass relationships and quantities of substances involved in a reaction. By adjusting the coefficients—the numbers placed before molecules—chemists can represent these reactions accurately without altering the chemical identity of the substances involved.21