The pH scale and neutralisation - AQA GCSE Chemistry Exam Questions

Practise 25 exam-style questions on The pH scale and neutralisation for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

25 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

In every neutralisation reaction between an acid and an alkali, hydrogen ions react with hydroxide ions.

Write the ionic equation for this reaction. Include state symbols.

Model answer

H⁺(aq) + OH⁻(aq) → H₂O(l)

Explanation

Neutralisation between an acid and an alkali is always the reaction of hydrogen ions with hydroxide ions to make water. The ions are dissolved, so they are (aq), and the water formed is a liquid, so it is (l).

Mark scheme

1 mark
H⁺ + OH⁻ → H₂O
allow H+ + OH- → H2O typed without sub/superscripts; allow multiples; ignore state symbols for this mark
1 mark
state symbols: H⁺(aq) + OH⁻(aq) → H₂O(l)
all three state symbols must be correct

Question 2

2 marks

Explain what is meant by the term alkali.

Model answer

An alkali is a base that dissolves in water. Its aqueous solution contains hydroxide ions (OH⁻) and has a pH greater than 7.

Explanation

All alkalis are bases, but not all bases are alkalis: copper oxide is a base but it does not dissolve in water. When an alkali dissolves it releases OH⁻ ions, which give the solution a pH above 7.

Mark scheme

1 mark
a base that dissolves in water / a soluble base
allow a soluble metal hydroxide
1 mark
it produces hydroxide ions (OH⁻) in aqueous solution
allow it forms a solution with a pH greater than 7

Question 3

3 marks

Rubidium hydroxide, RbOH, is very soluble in water. Rubidium is in Group 1 of the periodic table.

Suggest a pH value for a solution of rubidium hydroxide, name the ion that gives the solution this pH, and write the ionic equation for the reaction that happens when the solution is neutralised by an acid.

Model answer

The pH would be about 14. The solution is alkaline because it contains hydroxide ions, OH⁻. When it is neutralised: H⁺(aq) + OH⁻(aq) → H₂O(l)

Explanation

Group 1 hydroxides such as sodium hydroxide dissolve to give alkaline solutions containing OH⁻ ions, so rubidium hydroxide will behave in the same way. Neutralisation by any acid is the reaction of H⁺ ions with these OH⁻ ions to form water.

Mark scheme

1 mark
any pH value greater than 7 up to 14
e.g. 13 or 14
1 mark
hydroxide ion / OH⁻
do not accept rubidium ion
1 mark
H⁺ + OH⁻ → H₂O
ignore state symbols; do not accept an equation including Rb⁺ unless it appears on both sides

Question 4

4 marks

A student adds a few drops of universal indicator to some dilute hydrochloric acid in a conical flask. The student then adds sodium hydroxide solution from a burette, a little at a time, until the sodium hydroxide is in excess.

Describe the colour changes the student would see. Explain the changes in terms of the ions present.

Model answer

At the start the indicator is red because the acid contains a high concentration of H⁺ ions. As sodium hydroxide is added, OH⁻ ions react with the H⁺ ions to form water, so the pH rises and the colour changes through orange and yellow to green at pH 7, when the acid is neutralised. Adding more sodium hydroxide gives excess OH⁻ ions, so the solution becomes alkaline and the indicator turns blue and then purple.

Explanation

Universal indicator shows the pH by its colour: red for strongly acidic, green for neutral and purple for strongly alkaline. The pH rises because the hydroxide ions remove hydrogen ions by forming water, and once all the H⁺ has gone the extra OH⁻ makes the solution alkaline.

Mark scheme

1 mark
the indicator is red at the start
max 4; allow orange
1 mark
it turns green when the solution is neutral
allow changes through orange / yellow to green
1 mark
it turns blue / purple when the alkali is in excess
allow violet
1 mark
hydroxide ions react with hydrogen ions to form water, so the concentration of H⁺ ions decreases / the pH increases
allow H⁺ + OH⁻ → H₂O
1 mark
when all the H⁺ ions have reacted, extra OH⁻ ions make the solution alkaline
allow OH⁻ ions are in excess

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