Chemical equations

Home learning focus

Learn about word equations and symbol equations, including how to balance them.

This lesson includes:

  • one video explaining how to balance symbol equations
  • one activity

Learn

In order to study reactions more clearly, chemists use chemical equations. Equations allow us to see:

  • the reactants - the chemicals reacting together
  • the products - the chemicals being produced in the reaction

Reactants always go on the left of an equation and the products go on the right. If you have more than one reactant or product, then they are separated by a + symbol.

The reactants and products are separated by an arrow pointing to the right which shows the direction of the reaction.

Word equations

reactants → products

This is a word equation.

For example, the word equation for the reaction of oxygen and methane would be:

oxygen + methane → carbon dioxide + water

A model of the reaction of methane and oxygen.

Symbol equations

Word equations only tell us which chemicals are involved in a reaction.

Chemists will instead use symbol equations as they show exactly which atoms are present in each of the reactants and products.

Here is the equation again for the combustion of oxygen and methane but in symbol form:

O₂ + CH₄ → CO₂ + H₂O

However, this is not a full picture of the reaction. You can see that the number of atoms on each side of the reaction are unbalanced. For example; there are two atoms of oxygen on the left and three on the right.

We therefore need to balance the equation.

Balancing equations

To balance an equation we need to change the amounts of each chemical in the reaction, without changing the formula. We do this by adding a number in front of each chemical so that the number of atoms on each side are equal.

Here is the balanced combustion reaction of oxygen and methane:

2 O₂ + CH₄ → CO₂ + 2 H₂O

Both sides of the equation now have:

  • 4 oxygen atoms
  • 4 hydrogen atoms
  • 1 carbon atom

In other words, two oxygen molecules will react with one methane molecule to form one carbon dioxide molecule and two water molecules.

Watch this video which shows the process of balancing equations in more detail.

How to balance chemical equations.

When balancing an equation, follow these basic rules:

  1. check that all the formulae in the equation are correct
  2. deal with only one element at a time
  3. balancing is adding BIG numbers. You cannot change any of the small numbers in a chemical formula. If balancing is required, put the number in front of the substance
  4. check each element again and repeat step 3 if needed

State symbols

State symbols are often put in brackets after each substance in a reaction. These symbols show the physical state of the substance.

state symbolmeaning
(s)solid
(l)liquid
(g)gas
(aq)aqueous solution

So, using the example of the reaction of oxygen with methane, the equation would look like this:

2 O₂(g) + CH₄(g) → CO₂(g) + 2 H₂O(l)

Practise

There are lots of ways to try out your science skills.

Activity 1

Balancing chemical equations

Practise balancing chemical equations with this downloadable worksheet from teachit science.

You will need a piece of paper and something to write with.

Balancing chemical equations

There's more to learn

Have a look at these other resources around the BBC and the web.

KS3 Chemistry
11-14 Chemistry
Bitesize Daily lessons