Chemical equations are illustrations of the reactants and products in a chemical reaction. Chemical equations are expressed as A + B -> C, where A and B are the reactants, and C is the product.
When speaking of a specific reaction, a chemical equation must be balanced. That means that the amount of reactants must equal the amount of products. The conversion of hydrogen and oxygen into water is a common chemical reaction. Without balancing the equation, this is expressed as H2 + O2 -> H20. However, there is one more oxygen on the left side than on the right.
A balanced form of the equation is 2H2 + O2 -> 2H20. This shows that there are a total of four hydrogen atoms and two oxygen atoms on each side of the equation.