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IGCSE · Practice questions · pH, indicators and neutralisation

Read the colour.

Six original Cambridge-style questions on pH: the scale, universal indicator, H+ concentration and neutralisation in practice.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Lower number, more acid.

01
Recall
[3 marks]

Give the approximate pH and universal indicator colour of (a) pure water, (b) dilute hydrochloric acid, (c) aqueous sodium hydroxide.

  • (a) pH 7, green. ✓
  • (b) pH 1 to 2, red. ✓
  • (c) pH 13 to 14, purple. ✓
02
Analysis
[2 marks]

Four solutions have pH values of 1, 6, 9 and 13. Which is (a) a weak alkali and (b) the solution with the highest concentration of hydrogen ions?

  • (a) pH 9. ✓
  • (b) pH 1. ✓
03
Explain
[3 marks]

Describe and explain the changes in pH as dilute nitric acid is added a little at a time to aqueous potassium hydroxide until the acid is in excess.

  • The pH starts high (about 13 to 14) because OH− ions are present. ✓
  • It falls as H+ ions from the acid react with OH− ions to form water, reaching 7 when neutralisation is complete. ✓
  • It then falls below 7 as excess H+ ions are added. ✓
04
Recall
[2 marks]

Describe how you would find the pH of a sample of soil water using universal indicator paper.

  • Dip (or spot) the soil water onto universal indicator paper. ✓
  • Compare the colour with the pH colour chart and read off the pH. ✓
05
Explain
[2 marks]

A lake has become acidic because of acid rain. Suggest how the pH could be raised and explain why it works.

  • Add powdered limestone or lime (calcium carbonate or calcium hydroxide). ✓
  • It neutralises the acid, removing H+ ions, so the pH rises towards 7. ✓
06
Recall
[1 marks]

State the name of the process in which an acid reacts with a base to form a salt and water.

  • Neutralisation. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.