AQA GCSE Combined Science (8464), Higher tier · Chemistry › 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.
What pH tells you, how to measure it with universal indicator or a pH probe, and which ions make solutions acidic or alkaline. Neutralisation between an acid and an alkali is summed up by the ionic equation H+ + OH− → H2O.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Use pH to classify solutionsBelow 7 is acidic, 7 is neutral and above 7 is alkaline.
4
Measure pH with universal indicatorAdd the indicator and match its colour to a pH colour chart.
5
Name the ions in acids and alkalisAcids produce H+ ions in water and alkalis produce OH− ions in water.
5
Compare universal indicator with a pH probeA pH probe gives a more precise numerical reading; universal indicator gives an approximate value from a colour.
5
Describe the pH change during neutralisationAs alkali is added to acid, the pH rises to 7 at neutralisation and above 7 when alkali is in excess.
6
Write the ionic equation for neutralisationH+(aq) + OH−(aq) → H2O(l).
Notes
The pH scale
The pH scale runs from 0 to 14. It measures how acidic or alkaline a solution is.
pH 7 is neutral, e.g. pure water. Acids have a pH below 7. Alkalis have a pH above 7.
The lower the pH, the more acidic the solution. The higher the pH, the more alkaline.
Measuring pH
Universal indicator is a wide range indicator: its colour changes across the whole pH scale.
Colours: red (strongly acidic), orange and yellow (weakly acidic), green (neutral), blue (weakly alkaline), purple (strongly alkaline).
Add a few drops to the solution, or dip universal indicator paper into it, and match the colour to a pH chart. This gives an approximate pH, as a whole number.
A pH probe (pH meter) gives a numerical reading, usually to one or two decimal places. It is more precise and does not depend on judging a colour.
Ions and neutralisation
Acids produce hydrogen ions, H+, in aqueous solution.
Alkalis produce hydroxide ions, OH−, in aqueous solution.
When an acid reacts with an alkali, hydrogen ions react with hydroxide ions to make water: H+(aq) + OH−(aq) → H2O(l).
The other ions, e.g. Na+ and Cl−, stay in the solution and make up the salt.
Adding alkali to an acid makes the pH rise: it is 7 when the acid is exactly neutralised and goes above 7 if more alkali is added.
Universal indicator: red → orange → yellow → green (pH 7) → blue → purple
pH probe: numerical reading, more precise than indicator
How to answer each type of question
Use pH values to identify solutions
1 mark each3
Below 7 is acidic, 7 is neutral, above 7 is alkaline.
The further from 7, the more strongly acidic or alkaline.
Example. The pH values of four solutions are: A = 1, B = 7, C = 9, D = 13. (a) Which solution is neutral? (b) Which solution is the most strongly alkaline? (c) What colour would universal indicator turn in solution A?
Show the model answer
(a) B (1) (b) D (1) (c) Red (1)
Describe how to measure pH
2 marks4
Say what you use: universal indicator (solution or paper) or a pH probe.
Say how you read the result: compare with a colour chart, or read the meter.
Example. Describe how a student could use universal indicator to find the pH of a sample of rainwater.
Show the model answer
Add a few drops of universal indicator to a sample of the rainwater (1) Match the colour to a pH colour chart to find the pH (1)
Give advantages of a pH probe
2 marks5
A probe gives a number, often to decimal places, so it is more precise.
It does not rely on someone judging a colour.
Example. Give two advantages of using a pH probe rather than universal indicator to measure the pH of a solution.
Show the model answer
Gives a more precise value / reads to decimal places (1) Does not depend on judging a colour / not affected by the colour of the solution (1)
Write the ionic equation and describe pH changes in neutralisation
2 to 5 marks6
The acid supplies H+ ions and the alkali supplies OH− ions.
Write H+ + OH− → H2O, with state symbols if asked.
Describe pH changes with direction and values: rises, reaches 7, goes above 7.
Example. Potassium hydroxide solution is added to nitric acid until the potassium hydroxide is in excess. (a) Write the ionic equation for the neutralisation reaction, including state symbols. (b) Describe how the pH of the solution in the flask changes.
Show the model answer
(a) H+(aq) + OH−(aq) → H2O(l): correct equation (1), correct state symbols (1) (b) Starts below 7 / low and increases (1) reaches 7 when the acid is neutralised (1) goes above 7 when the alkali is in excess (1)
Shortcuts and memory tricks
Think of the pH scale as a number line: acids at the low end, alkalis at the high end, neutral in the middle at 7.
Universal indicator colours run roughly like a rainbow: red for strong acid through green for neutral to purple for strong alkali.
Neutralisation always makes water: H+ + OH− → H2O.
Where marks are lost
Saying alkalis have a pH below 7, or that pH 7 is weakly acidic.
Mixing up the ions: acids produce H+, alkalis produce OH−.
Writing the ionic equation with H2 or O2− instead of H+ and OH−.
Leaving out state symbols when the question asks for them: (aq), (aq), (l).
Exam technique
Link pH to ions: a lower pH means a higher concentration of H+ ions.
When describing a pH change, give the direction and the values, e.g. 'rises from 1 to 7, then above 7'.
Learn the universal indicator colours for strong acid, neutral and strong alkali: red, green, purple.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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)