Conservation of mass and balanced chemical equations — AQA GCSE Chemistry
Test yourself on Conservation of mass and balanced chemical equations with AQA GCSE practice questions.
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Conservation of mass and balanced chemical equations explained
Because atoms are conserved, a symbol equation must show the same number of atoms of each element on both sides.
Read the full explanation
You balance an equation by placing whole-number multipliers, called coefficients, in front of chemical formulae; you never change the formulae themselves, because that would change the substances involved. For example, hydrogen burning in oxygen is written as 2H₂ + O₂ → 2H₂O. Counting shows four hydrogen atoms and two oxygen atoms on each side. To balance an equation, start with the formula containing the most elements, adjust one element at a time using coefficients, and finish by checking every element. State symbols may be added to show solid, liquid, gas or aqueous conditions. A balanced equation is the quantitative link between the reaction and the masses of substances involved.
Your focus
- Write a balanced symbol equation with the same number of atoms of each element on both sides.
- Explain why coefficients, not subscripts, are used to balance an equation.
- Check a completed equation by counting the atoms of every element on each side.
Conservation of mass and balanced chemical equations exam tips
Marking Points
- Write correct chemical formulae for all reactants and products before attempting to balance.
- Use whole-number coefficients in front of formulae so that the number of atoms of each element is the same on both sides.
- Check each element in turn after balancing and adjust coefficients if any count is unequal.
- Explain that coefficients multiply the whole formula, so 2H₂O contains four hydrogen atoms and two oxygen atoms.
- Recognise that changing a subscript alters the substance and is not a valid way to balance an equation.
Examiner Tips
- 💡Balance one element at a time and tick it off once its count matches on both sides.
- 💡Do a final count of every element, including any you balanced early, because changing one coefficient can affect another element.
- 💡Write the simplest whole-number ratio of coefficients in your final answer.
Common Mistakes
- Changing a subscript in a formula to make the counts match; the correction is to change only the coefficient in front of the formula.
- Forgetting to multiply every atom in a formula by its coefficient; the correction is to count each element after applying the coefficient to the whole formula.
- Leaving the equation with fractional coefficients; the correction is to multiply all coefficients by the same factor to obtain the simplest whole-number ratio.