Balancing chemical equations

If the names of substances are simply replaced by their symbols and formulae, most chemical equations are not correct. This is because the numbers of atoms of each element in the reactants may differ from those in the products.

To make the chemical equation correct it must be balanced.

1Write a word equation for the reaction.
2Write a simple equation by writing the name of each substance with its symbol or formula.
3Check whether the equation is balanced by counting the atoms of each element.
4If necessary, change the number of one of the molecules to make the atoms of at least one element balance. You may need to multiply both sides if the number of an atom on the right is not a multiple of the atoms on the left.
5Repeat steps 3 and 4 until the chemical equation is balanced.


Magnesium reacts with oxygen to produce magnesium oxide. Write the balanced chemical equation for this reaction.

1magnesium + oxygen → magnesium oxide
2Mg + O2 → MgO
3reactants: 1 × Mg, 2 × O products: 1 × Mg, 1 × O Not balanced.
4Mg + O2 → 2MgO
Step 3 (repeat)reactants: 1 × Mg, 2 × O products: 2 × Mg, 2 × O Not balanced.
Step 4 (repeat)2Mg + O2 → 2MgO
Step 3 (repeat)reactants: 2 × Mg, 2 × O products: 2 × Mg, 2 × O This chemical equation is now balanced.

Write a balanced chemical equation for the reaction of calcium with oxygen.

2Ca + O2 → 2CaO