Balanced chemical equations

Word equations

A chemical reaction can be summarised using a word equation:

sulfur + oxygen → sulfur dioxide

This shows that:

Chemical equations

In a chemical equation, the names of substances are replaced by symbols and formulae:

S + O2 → SO2

Conservation of atoms

Atoms are not created or destroyed in chemical reactions. This means the numbers of atoms of each element in the reactants is the same as the numbers of atoms of each element in the products.

This law applies to ionic formulae too, even though there are large numbers of each type of ion present in an ionic compound.

The equation above (S + O2 → SO2) is correct:

Atoms in the reactantsAtoms in the products
1 × S1 × S
2 × O2 × O

This also means that the total mass of the reactants must equal the total mass of the products. This is known as the law of conservation of mass.