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5.7 Neutralisation and the pH Scale

GCSE Chemistry · AQA · Chapter 5: Chemical Changes

A free GCSE Chemistry lesson for AQA (8462) from OBA School: what makes a solution acidic or alkaline, how we measure pH, and what happens in neutralisation. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Recall that acids produce H⁺ ions and alkalis produce OH− ions in aqueous solution
  • Describe the pH scale and how to measure pH with universal indicator or a pH probe
  • Write the ionic equation for neutralisation and equations for acid–alkali reactions
  • Interpret pH and temperature data from a neutralisation, and (Higher tier) calculate reacting volumes

Key words

  • Acid: A substance that produces hydrogen ions, H⁺(aq), in aqueous solution. pH below 7.
  • Alkali: A soluble base that produces hydroxide ions, OH−(aq), in aqueous solution. pH above 7.
  • pH scale: A scale from 0 to 14 that measures how acidic or alkaline a solution is.
  • Neutral: pH 7: neither acidic nor alkaline, e.g. pure water.
  • Indicator: A substance that changes colour depending on pH.
  • Neutralisation: The reaction of an acid with a base. With an alkali: H⁺(aq) + OH−(aq) → H₂O(l).

Acids and alkalis

When an acid dissolves in water it releases hydrogen ions, H⁺(aq). These make the solution acidic. Common lab acids are hydrochloric acid (HCl), sulfuric acid (H₂SO₄) and nitric acid (HNO₃).

Bases neutralise acids. Bases that dissolve in water are alkalis. They release hydroxide ions, OH−(aq). Examples: sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonia solution.

  • Acid; Ion released in water H⁺(aq); pH below 7; Examples HCl(aq), H₂SO₄(aq), lemon juice, vinegar
  • Alkali; Ion released in water OH−(aq); pH above 7; Examples NaOH(aq), KOH(aq), limewater, ammonia solution

nitric acid dissolving: HNO₃(l) → H⁺(aq) + NO₃−(aq) (water written over the arrow)

sodium hydroxide dissolving: NaOH(s) → Na⁺(aq) + OH−(aq)

It is the H⁺(aq) ions that make a solution acidic and the OH−(aq) ions that make it alkaline. Insoluble metal oxides and hydroxides, such as copper(II) oxide, are bases but not alkalis. Pure water is neutral.

The pH scale and measuring pH

Key fact: Use the symbols < (“is less than”) and > (“is greater than”): pH < 7 means acidic, pH = 7 means neutral, pH > 7 means alkaline.

Universal indicator is a mixture of indicators. It gives a different colour for each pH, so you can estimate the pH by comparing with a colour chart. A pH probe and meter gives a reading to 0.1 or 0.01 pH units. It is more precise, does not depend on judging a colour, and works with coloured solutions.

  • litmus; Colour in acid red; Colour in alkali blue
  • phenolphthalein; Colour in acid colourless; Colour in alkali pink
  • methyl orange; Colour in acid red; Colour in alkali yellow

Single indicators such as phenolphthalein change colour suddenly at one pH, so they are used to spot the end point in a titration. Universal indicator changes gradually, so it is not used for titrations.

Neutralisation

When an acid reacts with an alkali, the H⁺ ions and OH− ions join to make water. The other ions stay in solution and form the salt.

ionic equation: H⁺(aq) + OH−(aq) → H₂O(l)

sodium hydroxide + hydrochloric acid → sodium chloride + water NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

potassium hydroxide + sulfuric acid → potassium sulfate + water 2KOH(aq) + H₂SO₄(aq) → K₂SO₄(aq) + 2H₂O(l)

Key fact: Neutralisation is exothermic: the temperature rises as acid is added to an alkali, and is highest at the neutral point.

Following a neutralisation

A pH probe was placed in 25.0 cm³ of sodium hydroxide solution. Hydrochloric acid of the same concentration was added from a burette and the pH recorded.

Worked example: (Higher tier) reacting volumes

What volume of 0.20 mol/dm³ hydrochloric acid neutralises 20.0 cm³ of 0.10 mol/dm³ sodium hydroxide?
moles of NaOH = 0.10 × (20.0 ÷ 1000) = 0.0020 mol
NaOH + HCl → NaCl + H₂O, ratio 1 : 1, so 0.0020 mol HCl is needed
volume of HCl = moles ÷ concentration = 0.0020 ÷ 0.20 = 0.010 dm³

Answer: 10.0 cm³

Common mistakes

  • Thinking a higher pH means more acidic. The lower the pH, the more acidic the solution, because there is a higher concentration of H⁺ ions.
  • Writing that alkalis contain H⁺ or that neutralisation makes “salt and hydrogen”. Acid + alkali gives salt + water.
  • Using universal indicator to find the end point of a titration. Use a single indicator such as phenolphthalein or methyl orange.
  • Forgetting to convert cm³ to dm³ (÷ 1000) in concentration calculations.

Revision questions

Try these, then check your answers below each one.

Question 1. Name the ion that alkalis produce in aqueous solution.

Answer: hydroxide

Working: Alkalis produce hydroxide ions, OH−(aq). Acids produce hydrogen ions, H⁺(aq).

Question 2. What colour is universal indicator in a solution of pH 2? (Choose from: red, green, blue, purple.)

Answer: red

Working: pH 0–3 is strongly acidic: universal indicator is red. Green = pH 7; blue and purple are alkaline.

Question 3. Which is the ionic equation for neutralisation of an acid by an alkali? (Choose from: H⁺(aq) + OH−(aq) → H₂O(l), H⁺(aq) + Cl−(aq) → HCl(aq), Na⁺(aq) + OH−(aq) → NaOH(aq), H₂(g) + O²−(aq) → H₂O(l).)

Answer: H⁺(aq) + OH−(aq) → H₂O(l)

Working: Hydrogen ions from the acid react with hydroxide ions from the alkali to make water.

Related lessons

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