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Year 10
AQA
Higher

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.

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New
New
Year 10
AQA
Higher

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.

Download all resources
Share activities with pupils

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.

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.

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.

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.
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Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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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

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