AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
This means that chemical reactions can be represented by symbol equations…
Full specification wording
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.
Because atoms are conserved, a symbol equation must show the same number of atoms of each element on both sides.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 3
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
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 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.
What earns the marks
- 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.
Common mistakes and how to put them right
Changing a subscript in a formula to make the counts match.
Put it right: Change only the coefficient in front of the formula.
Forgetting to multiply every atom in a formula by its coefficient.
Put it right: Count each element after applying the coefficient to the whole formula.
Leaving the equation with fractional coefficients.
Put it right: Multiply all coefficients by the same factor to obtain the simplest whole-number ratio.
Exam 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.
Try the lesson
The start of the 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. lesson, made from checked teaching notes for this specification point.
Have a go first
Magnesium reacts with oxygen to form magnesium oxide, MgO. Which balanced symbol equation represents this reaction?
Listen
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 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. A strong answer earns marks for these 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. A common slip: Changing a subscript in a formula to make the counts match. Instead, change only the coefficient in front of the formula. Exam tip: Balance one element at a time and tick it off once its count matches on both sides.
Key words
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Balanced symbol equations
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Coefficients
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See every definition
- Balanced symbol equations
- Symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.
- Coefficients
- Whole-number multipliers placed in front of chemical formulae to balance an equation, never changing the formulae themselves.
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