A student adds a few drops of universal indicator to separate samples of dilute hydrochloric acid, sodium hydroxide solution, and sodium chloride solution. Describe and explain the colours observed, and explain what the colour of the sodium chloride solution tells you about the products formed when an acid and an alkali react together.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Hydrochloric acid turns the universal indicator red (or orange‑red) because it is acidic and its pH is below 7.
Sodium hydroxide turns the universal indicator purple (or violet/blue) because it is alkaline and its pH is above 7.
Sodium chloride solution turns the universal indicator green because it is neutral and its pH is about 7.
The green colour of the sodium chloride solution shows that the reaction between the acid and the alkali produced a salt (sodium chloride) and water.
This reaction is called a neutralisation reaction.
Sodium hydroxide turns the universal indicator purple (or violet/blue) because it is alkaline and its pH is above 7.
Sodium chloride solution turns the universal indicator green because it is neutral and its pH is about 7.
The green colour of the sodium chloride solution shows that the reaction between the acid and the alkali produced a salt (sodium chloride) and water.
This reaction is called a neutralisation reaction.
Examiner tips
- Use the exact colour terms (red, purple, green) and link them to pH ranges; mention the neutralisation reaction explicitly.
- Show the logical chain: colour → pH → type of solution → products of acid‑base reaction.
- Keep the answer concise and avoid extra wording such as "in this essay".
Common mistakes
- Confusing the colours of the indicator (e.g., saying blue for NaOH instead of purple/blue); "green" for NaCl instead of neutral colour.
- Failing to state that the reaction is a neutralisation or that the products are a salt and water.
Mark scheme (5 marks)
- Hydrochloric acid turns universal indicator red (or orange/red) because it is acidic / has a pH below 7
- Sodium hydroxide turns universal indicator purple (or violet/blue) because it is alkaline / has a pH above 7
- Sodium chloride solution turns universal indicator green because it is neutral / has a pH of 7
- The neutral pH of sodium chloride solution shows that an acid and an alkali produce a salt (sodium chloride) and water
- The reaction between an acid and an alkali is called neutralisation
Key terms in this question
universal indicator · acid · alkali
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
More Acid-base equilibria questions
- Explain why hydrochloric acid is described as a strong acid, and why ethanoic ac…
- A student dissolves the same number of moles of sulfuric acid and carbonic acid …
- A student measures the pH of two solutions: a 0.1 mol/dm³ solution of sodium hyd…
- A student adds sodium carbonate powder to two separate beakers, one containing d…