Free Lesson

Balancing Symbol Equations

GCSE 30 min

Video Lesson

Introduction

In chemistry, reactions are represented by symbol equations. The reactants and products are different substances, but the total number of atoms on both sides of the equation must always be equal. This follows the Law of Conservation of Mass: matter cannot be created or destroyed in a chemical reaction. It can only be rearranged into new substances.

Imagine you have a bag of Lego bricks. You can rearrange the bricks to build new things, but you cannot create new bricks or throw any away. The total number of bricks stays the same before and after, even though you have built something different:

Lego bricks used to demonstrate conservation of mass: the same number of hydrogen and oxygen bricks are present on both sides of the equation 2H2 + O2 to 2H2O

Balancing a chemical equation ensures that you always have the same number of atoms on both sides of the equation, without changing the individual chemical formulae of the substances reacting or of the new products forming. There are some important balancing rules to follow:

Balancing Rules

1. You cannot change the element symbols in any formula (for example, you cannot swap an O for an N).

2. You cannot change, add, or remove the small (subscript) numbers inside a formula, because that would turn it into a different substance.

3. The only thing you can do is place a large number (called a coefficient) in front of a whole formula to increase how many of that substance you have.

An equation showing N2 + 3H2 to 2NH3, with annotations: big numbers CAN be added but only at the beginnings of formula; little numbers already in formula CANNOT be changed, added in or removed

Reading Chemical Formulae

Before we can successfully learn the skill of balancing, we have to be able to read and break down chemical formulae. We need to be able to count the number of atoms and work out the different types of atoms as shown below:

Reading the formula CO2: C represents 1 carbon atom and O2 represents 2 oxygen atoms, giving 2 elements and 3 atoms in total Reading the formula CaCO3: Ca is 1 calcium atom, C is 1 carbon atom, and O3 is 3 oxygen atoms, giving 3 elements and 5 atoms in total Reading the formula Al2(SO4)3: Al2 is 2 aluminium atoms, S is 1 sulfur atom multiplied by 3 from the bracket giving 3 sulfur atoms, and O4 multiplied by 3 gives 12 oxygen atoms, giving 3 elements and 17 atoms in total

Counting Atoms Carefully

Once we can count atoms in formulae, we are ready to balance chemical equations. There are a few common traps to watch out for. The symbol for aluminium is Al (one capital letter followed by one lowercase letter). Do not mistake the lowercase "l" for the element iodine (I); the whole symbol Al represents a single element.

Another example is CO compared with Co. CO contains two capital letters, so it represents two separate elements: carbon (C) and oxygen (O). Co has only one capital letter followed by a lowercase letter, so it is a single element: cobalt. The rule is that every element symbol starts with exactly one capital letter, sometimes followed by one lowercase letter.

Balancing Methods

Example 1: Magnesium and Oxygen

This first example shows how magnesium reacts with oxygen to form magnesium oxide. The first image shows how this equation can be balanced with the Symbols Method. The second image shows the same reaction being balanced using the Shapes Method:

Balancing with the Symbols Method: Magnesium + Oxygen to Magnesium Oxide. The balanced equation is 2Mg(s) + O2(g) to 2MgO(s). Each element is listed below the equation to verify equal atoms on both sides. Balancing with the Shapes Method: the same reaction 2Mg(s) + O2(g) to 2MgO(s), using coloured shapes to represent different atoms and confirm the count matches on both sides

Example 2: Sodium Hydroxide and Sulfuric Acid

This second example shows how sodium hydroxide (NaOH) reacts with sulfuric acid (H2SO4) to form sodium sulfate and water. The first image shows how this equation can be balanced with the Symbols Method. The second image shows the same reaction being balanced using the Shapes Method:

Balancing with the Symbols Method: Sodium Hydroxide + Sulfuric Acid to Sodium Sulfate + Water. The balanced equation is 2NaOH(aq) + H2SO4(aq) to Na2SO4(aq) + 2H2O(l). Each element is listed to verify equal atoms on both sides. Balancing with the Shapes Method: the same reaction 2NaOH + H2SO4 to Na2SO4 + 2H2O, using coloured shapes to represent different atoms and confirm the count matches on both sides
Key Concept

Whichever method you use, the principle is the same: count the atoms of each element on both sides. If they do not match, add big numbers in front of the formulae until they do. Never change the small (subscript) numbers, as that would change the substance itself.

Summary

  • Chemical equations must be balanced so that the number of atoms of each element is the same on both sides. This follows the Law of Conservation of Mass.
  • Only big coefficients in front of formulae can be changed. Symbols and subscript numbers must stay as they are.
  • Before balancing, practise reading formulae and counting the total number of each type of atom.
  • Watch out for elements with two-letter symbols (e.g. Al, Co, Na) and do not confuse them with two separate elements.
  • You can balance using the Symbols Method or the Shapes Method. Try both and use whichever you find easiest.