AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The pH scale and neutralisation: This reaction can be represented by the equation:
In neutralisation, an acid reacts with an alkali or base to form a salt and water.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In neutralisation, an acid reacts with an alkali or base to form a salt and water. The general word equation is: acid + alkali → salt + water. For example, hydrochloric acid reacting with sodium hydroxide solution gives sodium chloride and water: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). The H⁺ ions from the acid combine with the OH⁻ ions from the alkali to form water: H⁺(aq) + OH⁻(aq) → H₂O(l). The remaining ions form the dissolved salt. When writing the equation, you must balance atoms and include state symbols where required. The equation shows the reaction quantitatively, so the number of each type of atom is the same on both sides. This links pH, neutralisation and ionic equations.
What earns the marks
- Write the correct word equation: acid + alkali → salt + water, naming the salt from the acid and the metal in the alkali.
- Write the balanced symbol equation, for example HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l), ensuring all atoms balance.
- Include state symbols correctly: (aq) for dissolved acid, alkali and salt, and (l) for water.
- Identify the ionic equation for neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l), showing that spectator ions are unchanged.
- Explain that the reaction occurs because H⁺ ions from the acid react with OH⁻ ions from the alkali to form water.
Common mistakes and how to put them right
Writing the salt as sodium chlorate or sodium hydroxide instead of sodium chloride.
Put it right by naming the salt from the acid's negative ion and the metal ion from the alkali.
Omitting state symbols or using (s) for the salt when it is dissolved.
Put it right by using (aq) for solutions and (l) for liquid water.
Forgetting to balance the equation, for example writing H₂SO₄ + NaOH → Na₂SO₄ + H₂O without checking atoms.
Put it right by counting atoms on each side and adjusting coefficients to H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.
Exam tips
- Read the question carefully to identify the acid and alkali before writing the equation.
- Check that the equation is balanced by counting each type of atom on both sides.
- If asked for an ionic equation, cancel spectator ions and write only the reacting ions and the product.
Try the lesson
The start of the The pH scale and neutralisation: This reaction can be represented by the equation: lesson, made from checked teaching notes for this specification point.
Have a go first
Which ionic equation correctly represents the neutralisation reaction between an acid and an alkali in aqueous solution?
Listen
In neutralisation, an acid reacts with an alkali or base to form a salt and water. The general word equation is: acid + alkali → salt + water. For example, hydrochloric acid reacting with sodium hydroxide solution gives sodium chloride and water: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). The H⁺ ions from the acid combine with the OH⁻ ions from the alkali to form water: H⁺(aq) + OH⁻(aq) → H₂O(l). The remaining ions form the dissolved salt. When writing the equation, you must balance atoms and include state symbols where required. The equation shows the reaction quantitatively, so the number of each type of atom is the same on both sides. This links pH, neutralisation and ionic equations. A strong answer earns marks for these points. Write the correct word equation: acid + alkali → salt + water, naming the salt from the acid and the metal in the alkali. Write the balanced symbol equation, for example HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l), ensuring all atoms balance. Include state symbols correctly: (aq) for dissolved acid, alkali and salt, and (l) for water. Identify the ionic equation for neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l), showing that spectator ions are unchanged. A common slip: Writing the salt as sodium chlorate or sodium hydroxide instead of sodium chloride. Instead, naming the salt from the acid's negative ion and the metal ion from the alkali. Exam tip: Read the question carefully to identify the acid and alkali before writing the equation.
See it
Key words
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Acid–base neutralisation
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Conservation of mass in equations
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Ionic reactions in solution
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Representing reactions with word and symbol equations
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See every definition
- Acid–base neutralisation
- An acid reacts with an alkali or base to form a salt and water.
- Conservation of mass in equations
- The equation shows the reaction quantitatively, so the number of each type of atom is the same on both sides.
- Ionic reactions in solution
- The H⁺ ions from the acid combine with the OH⁻ ions from the alkali to form water, and the remaining ions form the dissolved salt.
- Representing reactions with word and symbol equations
- The reaction can be represented by a word equation and by a symbol equation, balancing atoms and including state symbols where required.
Label the diagram
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