Balanced chemical equations

A symbol equation shows the substances that react and the substances they form. The substances on the left are the reactants; those on the right are the products.

Atoms are rearranged in a chemical reaction, not created or destroyed. A balanced chemical equation therefore has the same number of atoms of each element on both sides.

Coefficients and subscripts

A coefficient is a whole-number multiplier written in normal script before a chemical formula. It changes the number of whole particles represented. For example, $$\ce{2H2O}$$ represents two water molecules, containing four hydrogen atoms and two oxygen atoms in total.

A subscript is the small number within a formula. It shows how many atoms of an element are in one particle. In $$\ce{H2O}$$, the subscript 2 means that each water molecule contains two hydrogen atoms; there is one oxygen atom because no subscript is written after O.

To balance an equation, change the coefficients, not the subscripts. Changing a subscript changes the formula and represents a different substance.

Balancing an equation for water

Consider the equation for hydrogen reacting with oxygen:

The hydrogen atoms balance: there are two on each side. The oxygen atoms do not: there are two on the left but only one on the right.

Place a coefficient of 2 before $$\ce{H2O}$$. The product side now has two oxygen atoms, but it also has four hydrogen atoms. Place a coefficient of 2 before $$\ce{H2}$$ to give four hydrogen atoms on the reactant side:

The equation is now balanced: each side has four hydrogen atoms and two oxygen atoms. The formulas have not changed. No coefficient is written before $$\ce{O2}$$ because its coefficient is 1.

Conservation of mass in chemical reactions

In a chemical reaction, the starting substances, called reactants, change into new substances called products. The atoms in the reactants are rearranged to form the products; they are not created or destroyed.

Because the same atoms are present before and after the reaction, the total mass stays the same. This is the law of conservation of mass: the total mass of the products equals the total mass of the reactants, provided all substances are accounted for.

For example, hydrogen reacts with oxygen to form water:

The reactants contain four hydrogen atoms and two oxygen atoms. The two water molecules contain the same four hydrogen atoms and two oxygen atoms, rearranged into a new substance. The total mass is therefore unchanged.