AQA GCSE Combined Science (8464), Higher tier · Chemistry › Chemical changes › Reactions of acids
Practise Strong and weak acids (HT only). 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Higher tier only. The difference between strong and weak acids (how much they ionise) and between concentrated and dilute acids (how much acid is in a given volume), and how pH is linked to hydrogen ion concentration by factors of 10.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
5
Name examples of strong and weak acidsStrong: hydrochloric, nitric and sulfuric acids. Weak: ethanoic, citric and carbonic acids.
6
Define strong and weak acidsA strong acid is completely ionised in aqueous solution; a weak acid is only partially ionised.
6
Explain the terms dilute and concentratedThey describe the amount of acid in a given volume of solution, not how much it ionises.
7
Explain why a stronger acid has lower pHAt the same concentration, a strong acid releases more H+ ions, so its pH is lower.
8
Relate pH changes to H+ concentrationEach decrease of 1 pH unit means the H+ concentration is 10 times greater.
9
Separate strength from concentration in unfamiliar dataFor example, explain why a concentrated weak acid can have a lower pH than a very dilute strong acid.
Notes
Strong and weak acids
Acids ionise in water to release hydrogen ions, H+.
A strong acid is completely ionised in aqueous solution. Examples: hydrochloric, nitric and sulfuric acids. HCl(aq) → H+(aq) + Cl−(aq)
A weak acid is only partially ionised in aqueous solution: only a small fraction of its molecules release H+ ions at any time, and the ionisation is reversible (⇌). Examples: ethanoic, citric and carbonic acids.
Strength is not concentration
Strength (strong or weak) is about the proportion of the acid that ionises.
Concentration (concentrated or dilute) is about the amount of acid dissolved in a given volume of solution. A concentrated acid has a large amount of acid per dm3; a dilute acid has a small amount per dm3.
So you can have a dilute strong acid or a concentrated weak acid.
At the same concentration, a strong acid has a higher concentration of H+ ions than a weak acid, so it has a lower pH. It also reacts faster, e.g. with magnesium or a carbonate.
pH and hydrogen ion concentration
The lower the pH, the higher the concentration of H+ ions.
When the pH decreases by 1, the H+ ion concentration increases by a factor of 10.
pH 3 → pH 1 is a decrease of 2 units, so the H+ concentration is 10 × 10 = 100 times greater.
pH 4 → pH 7 is an increase of 3 units, so the H+ concentration is 1000 times smaller.
Diluting a strong acid 10 times makes its H+ concentration 10 times smaller, so its pH goes up by 1.
Cheatsheet
Strong acid: completely ionised in water (hydrochloric, nitric, sulfuric)
Weak acid: partially ionised in water (ethanoic, citric, carbonic)
Concentrated / dilute = amount of acid in a given volume
Same concentration: stronger acid → more H+ ions → lower pH
pH down by 1 → H+ concentration × 10
pH down by 2 → × 100; pH down by 3 → × 1000
HCl(aq) → H+(aq) + Cl−(aq)
How to answer each type of question
Explain the difference between strong and weak (or dilute and concentrated)
2 to 4 marks6
Strong or weak: use 'completely ionised' and 'partially ionised' in aqueous solution.
Concentrated or dilute: talk about the amount of acid in a given volume.
Never use 'strong' to mean concentrated.
Example. Hydrochloric acid is a strong acid. Ethanoic acid is a weak acid. (a) Explain the difference between a strong acid and a weak acid. (b) A student says 'a dilute acid is the same as a weak acid'. Explain why the student is wrong.
Show the model answer
(a) A strong acid is completely ionised in aqueous solution (1) a weak acid is only partially ionised (1) (b) Dilute means there is a small amount of acid in a given volume of solution (1) weak describes how much the acid ionises, so a dilute acid can still be strong (1)
Explain pH differences between acids of the same concentration
3 to 4 marks7
Say which acid is strong (completely ionised) and which is weak (partially ionised).
Say which has the higher concentration of H+ ions.
Link a higher H+ concentration to a lower pH.
Example. Solutions of nitric acid and citric acid have the same concentration. The nitric acid has pH 1 and the citric acid has pH 2. (a) Explain why the pH values are different. (b) How many times greater is the hydrogen ion concentration in the nitric acid?
Show the model answer
(a) Nitric acid is a strong acid, so it is completely ionised (1) Citric acid is a weak acid, so it is only partially ionised (1) So the nitric acid has a higher concentration of hydrogen ions, giving a lower pH (1) (b) 10 times (1)
Work out the change in H+ concentration from a pH change
1 to 2 marks8
Find the change in pH (always whole numbers at GCSE).
Each unit is a factor of 10: 1 unit = 10, 2 units = 100, 3 units = 1000.
Say whether the H+ concentration goes up (pH falls) or down (pH rises).
Example. The pH of a solution changes from 5 to 2. Describe how the hydrogen ion concentration changes.
Show the model answer
The pH decreases by 3 units (1) so the hydrogen ion concentration increases by a factor of 1000 (1)
Shortcuts and memory tricks
Strong or weak = how much it splits up into ions. Concentrated or dilute = how much acid is in the water.
Count the pH steps and write that many zeros after a 1: 3 steps → 1000.
pH and H+ concentration go in opposite directions: lower pH, more H+.
Where marks are lost
Using 'strong' to mean concentrated or 'weak' to mean dilute.
Saying a pH change of 2 means 2 times (or 20 times) more H+: it is 100 times.
Saying weak acids do not ionise: they do, but only partially.
Saying strong acids always have a lower pH without adding 'at the same concentration'.
Getting the direction wrong: a higher pH means fewer H+ ions.
Exam technique
Use the exact phrases: 'completely ionised', 'partially ionised', 'in aqueous solution', 'hydrogen ions'.
Only whole-number pH changes are asked, so answers are always 10, 100, 1000 and so on.
When comparing two acids, say which has the higher H+ concentration and link it to pH.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Acids can be described as strong or weak. Give one example of a weak acid.
ethanoic acid (or citric acid or carbonic acid)
Complete the sentence. A weak acid is only .................... ionised in aqueous solution.
partially
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Acids can be strong or weak.
(a) Which acid is a weak acid? Tick (✓) one box.[1]
Hydrochloric acid
Nitric acid
Citric acid
Sulfuric acid
(b) Complete the sentence. A weak acid is only .................... ionised in aqueous solution.[1]
(c) A strong acid and a weak acid have the same concentration. Which acid has the lower pH?[1]
(d) Solution X has a pH of 3. Solution Y has a pH of 4. Which solution has the higher concentration of hydrogen ions?[1]
Show the answer and mark scheme
(a)Answer: Citric acid
(b)Answer: partially
partially / partly
(c)Answer: the strong acid
the strong acid
(d)Answer: X
X
Question 2Medium5 marks
Acids can be described as strong or weak.
(a) What is meant by a strong acid?[1]
(b) Give one example of a weak acid.[1]
(c) Which statement about a weak acid is correct? Tick (✓) one box.[1]
It is always a dilute solution.
It is only partially ionised in aqueous solution.
It has a pH greater than 7.
It contains no hydrogen ions.
(d) The ionisation of hydrochloric acid and ethanoic acid in water can be shown as: HCl(aq) → H+(aq) + Cl−(aq) CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq) Explain what the different arrows show about the two acids.[2]
Show the answer and mark scheme
(a)
an acid that is completely ionised in aqueous solution
(b)Answer: ethanoic acid (or citric acid or carbonic acid)
ethanoic acid / citric acid / carbonic acid
(c)Answer: It is only partially ionised in aqueous solution.
(d)
→ shows that hydrochloric acid ionises completely (strong acid)
⇌ shows that ethanoic acid only partially ionises / the ionisation is reversible (weak acid)
Question 3Hard5 marks
As the pH of a solution decreases by one unit, the hydrogen ion concentration increases by a factor of 10.
(a) Solution A has a pH of 2. Solution B has a pH of 4. How many times greater is the hydrogen ion concentration in solution A than in solution B?[1]
(b) The hydrogen ion concentration of a solution with a pH of 3 is decreased by a factor of 10. Give the new pH.[1]
(c) Lemon juice has a pH of 2. Rainwater has a pH of 5. How many times greater is the hydrogen ion concentration in lemon juice than in rainwater?[1]
(d) A student added 10 cm3 of hydrochloric acid with a pH of 1 to 990 cm3 of distilled water and mixed it. Predict the pH of the new solution. Explain your answer.[2]
Show the answer and mark scheme
(a)Answer: 100 times
100
(b)Answer: 4
4
(c)Answer: 1000 times
1000
(d)Answer: pH 3
the volume increases 100 times, so the hydrogen ion concentration becomes 100 times smaller