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    Comparing concentration with strength

    I can use the pH scale to work out concentration differences between solutions relating to their pH, and discuss the differences between strong and weak acids.

    New
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      Comparing concentration with strength

      I can use the pH scale to work out concentration differences between solutions relating to their pH, and discuss the differences between strong and weak acids.

      Lesson details

      Key learning points

      1. pH is a logarithmic scale (as hydrogen ion concentration increases by a factor of 10, the pH decreases by 1).
      2. For a given concentration, a stronger acid contains more H⁺ ions.
      3. Titration curves can be used to show the change in pH to allow chemists to pinpoint a more accurate end point.
      4. A strong acid is completely ionised in aqueous solution, a weak acid only partially ionises.
      5. Titration curves show how the pH changes throughout a neutralisation reaction.

      Keywords

      • Hydrogen ion - A positively charged ion formed when a hydrogen atom loses an electron (H⁺).

      • PH - The measure of the hydrogen ion (H⁺) concentration of a solution.

      • Concentration - The amount of solute dissolved in a certain volume of solution.

      • Strong acid - An acid which fully dissociates to form H⁺ ions in aqueous solution.

      • Weak acid - An acid which partially dissociates to form H⁺ ions in aqueous solution.

      Common misconception

      Pupils often confuse the chemistry meaning of strong/weak and concentrated/dilute due to the use of these words in everyday conversation.

      Clear definitions of the words are required with practice opportunities to recognise/describe whether the substances are strong or weak acids and if concentrated or dilute.

      pH provides a good opportunity to practise magnitude calculations where changes in pH are a reflection of changes in the magnitude of H⁺ ion concentration. Practise by giving either the pH value or concentration of a solution and then ask pupils to calculate the amended value when a change occurs.
      Teacher tip

      Licence

      This content is © Oak National Academy Limited (2026), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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      Prior knowledge starter quiz

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      6 Questions

      Q1.
      What is the pH scale used to measure?

      the concentration of salts in a solution
      the colour of a solution
      Correct answer: the acidity or alkalinity of a solution
      the temperature of a solution

      Q2.
      What is ionisation?

      the process of dissolving a substance in water
      the release of energy in a reaction
      the mixing of acids and bases
      Correct answer: the formation of ions by gaining or losing electrons

      Q3.
      What is the charge of a hydrogen ion?

      negative
      neutral
      Correct answer: positive

      Q4.
      What does it mean if an acid is completely ionised in an aqueous solution?

      the acid has fully reacted with a base
      Correct answer: all acid molecules dissociate into ions
      the acid becomes neutral
      the acid stops reacting

      Q5.
      What does a weak acid do in an aqueous solution?

      it completely dissociates into ions
      Correct answer: it partially dissociates into ions
      it does not ionise
      it becomes a strong base

      Q6.
      What is the significance of titration curves?

      they show the temperature change during a reaction
      they indicate changes in volume
      Correct answer: they graph pH changes to find neutralisation points
      they track time during a reaction

      Assessment exit quiz

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      6 Questions

      Q1.
      Match the following key terms to their definitions.

      Correct Answer:hydrogen ion (H⁺),a positive ion formed when a hydrogen atom loses an electron

      a positive ion formed when a hydrogen atom loses an electron

      Correct Answer:pH,the measure of hydrogen ion concentration in a solution

      the measure of hydrogen ion concentration in a solution

      Correct Answer:concentration,the amount of solute dissolved in a certain volume of solution

      the amount of solute dissolved in a certain volume of solution

      Correct Answer:strong acid,an acid that fully dissociates to form H⁺ ions in aqueous solution

      an acid that fully dissociates to form H⁺ ions in aqueous solution

      Correct Answer:weak acid,an acid that partially dissociates to form H⁺ ions in aqueous solution

      an acid that partially dissociates to form H⁺ ions in aqueous solution

      Q2.
      Which of the following statements about a strong acid in an aqueous solution is correct?

      It is partially ionised.
      It releases very few H⁺ ions.
      Correct answer: It is completely ionised.
      It has a pH above 7.

      Q3.
      Why do strong acids have a lower pH than weak acids of the same concentration?

      Correct answer: they contain more H⁺ ions
      they contain fewer H⁺ ions
      they ionise partially
      they absorb more water

      Q4.
      What happens to the pH of a solution if the hydrogen ion concentration increases by a factor of 10?

      the pH increases by 1
      Correct answer: the pH decreases by 1
      the pH increases by 10
      the pH decreases by 10

      Q5.
      How can titration curves help chemists?

      they measure the temperature of a reaction
      they show the colour change in the solution
      Correct answer: they allow chemists to pinpoint an accurate end point
      they help identify weak acids

      Q6.
      What happens during the neutralisation of a weak acid?

      the pH decreases rapidly at first
      the pH stays constant throughout
      Correct answer: the pH changes gradually at the start
      the pH changes quickly after the end point is reached

      Lesson appears in

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