AQA GCSE Chemistry (8462), Higher tier · Chemical changes › Reactions of acids
Practise The pH scale and neutralisation. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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The pH scale is a measure of the acidity or alkalinity of a solution. What is the pH of a neutral solution?
7
Write the ionic equation, including state symbols, for the reaction between hydrogen ions and hydroxide ions.
H+(aq) + OH−(aq) → H2O(l)
Universal indicator changes colour depending on the pH of a solution. What colour is universal indicator in a neutral solution?
green
The pH scale measures how acidic or alkaline a solution is. State the range of values on the pH scale.
0 to 14
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
The pH scale is a measure of the acidity or alkalinity of a solution.
(a) What is the pH of a neutral solution?[1]
(b) Which pH value shows a strongly acidic solution? Tick (✓) one box.[1]
pH 1
pH 6
pH 8
pH 13
(c) Name the ions that acids produce in aqueous solution.[1]
(d) Name the ions present in all aqueous solutions of alkalis.[1]
Show the answer and mark scheme
(a)Answer: 7
7
(b)Answer: pH 1
(c)Answer: hydrogen ions, H+
hydrogen ions / H+
(d)Answer: hydroxide ions, OH−
hydroxide ions / OH−
Question 2Medium5 marks
In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions.
(a) Write the ionic equation, including state symbols, for the reaction between hydrogen ions and hydroxide ions.[2]
(b) Describe how a student could use universal indicator to find the approximate pH of a solution.[2]
(c) Give one advantage of using a pH probe instead of universal indicator.[1]
Show the answer and mark scheme
(a)Answer: H+(aq) + OH−(aq) → H2O(l)
H+ + OH− → H2O
state symbols (aq), (aq), (l)
(b)
add a few drops of universal indicator to a sample of the solution / dip universal indicator paper into it
compare the colour with a pH colour chart
(c)
gives a more precise / accurate (numerical) value / does not depend on judging a colour / works with coloured solutions / can take continuous readings
Question 3Hard7 marks
A student mixed solutions of acids and alkalis. All of the acids and alkalis are completely ionised in solution. Mixture A: 25.0 cm3 of 0.10 mol/dm3 hydrochloric acid + 20.0 cm3 of 0.10 mol/dm3 sodium hydroxide solution Mixture B: 25.0 cm3 of 0.10 mol/dm3 sulfuric acid + 50.0 cm3 of 0.10 mol/dm3 sodium hydroxide solution
(a) Is mixture A acidic, neutral or alkaline? Explain your answer using calculations.[3]
(b) Is mixture B acidic, neutral or alkaline? Explain your answer using calculations.[3]
(c) Suggest the colour of universal indicator in mixture A.[1]
Show the answer and mark scheme
(a)Answer: acidic — 0.0025 mol H+ but only 0.0020 mol OH−
moles of H+ (HCl) = 0.0025 and moles of OH− (NaOH) = 0.0020
hydrogen ions are in excess / not all the H+ ions react
so mixture A is acidic (pH less than 7)
(b)Answer: neutral — 0.0050 mol H+ and 0.0050 mol OH−
moles of H2SO4 = 0.0025, so moles of H+ = 0.0050
moles of OH− = 0.0050
all the H+ and OH− ions react (H+ + OH− → H2O), so mixture B is neutral (pH 7)