Conservation of mass and balanced chemical equations: This means that chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.

The idea

Because atoms are conserved, a symbol equation must show the same number of atoms of each element on both sides. You balance an equation by writing correct formulae first, then placing whole-number multipliers in front of formulae until every element matches. For example, H₂ + O₂ → H₂O is unbalanced because there are two oxygen atoms on the left but only one on the right. Writing 2H₂ + O₂ → 2H₂O gives four hydrogen atoms and two oxygen atoms on each side. Never change a subscript inside a formula, because that changes the substance. Check each element in turn and recount after every change. Balanced equations are used to calculate reacting masses and to show the ratio in which substances react.

What earns the marks

• Writes correct chemical formulae before attempting to balance an equation.
• Uses whole-number multipliers in front of formulae to balance atoms of each element.
• Counts atoms of every element on both sides and confirms they are equal.
• Recognises that changing a subscript alters the substance and is not allowed.
• Uses a balanced equation to identify the ratio of substances reacting.

Common mistakes and how to put them right

• Changing a subscript, such as turning H₂O into H₂O₂, to balance atoms.

  Put it right: Only place multipliers in front of formulae.

• Balancing only one element and ignoring the others.

  Put it right: Check every element in turn.

• Forgetting to multiply through when a formula contains brackets.

  Put it right: Treat the bracket as a group and multiply all atoms inside it.

Exam tips

• Write a tally of atoms for each element before and after balancing.
• Balance the element that appears in the fewest formulae first, then leave hydrogen and oxygen until last.
• Recheck the final equation by counting all atoms once more.