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Word equations are useful to show which chemicals react together (reactants) and which chemicals are produced (products). Symbol equations allow chemists to work out the masses that will react or be produced.

Copper and oxygen reaction – getting a balanced equation

Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper(II) oxide.

For the reaction, copper + oxygen → copper(II) oxide:

• copper and oxygen are the reactants because they are on the left-hand side of the arrow
• copper(II) oxide is the product because it is on the right-hand side of the arrow

If the words shown above are replaced by the correct chemical formulae, it will result in an unbalanced equation, as shown here.

Cu + O₂ → CuO

Notice that there are unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides.

Here is the balanced symbol equation.

2Cu + O₂ → 2CuO

You can see that now there are two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.

State symbols

Sometimes it is useful to know whether the reactants and products in a chemical reaction are solids, gases, liquids or aqueous (dissolved in water). State symbols can be added to a symbol/ionic equation to show this.

State symbol and meaning

For example, for the reaction between sodium and water, this is the symbol equation with state symbols. Note that the state symbols are written in brackets after the chemical formula.

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)