Case study·Standard multi-step (3/5)·No calculator
Difficulty guide
The label describes the problem, not your ability.
1/5 - Foundational
Warm-up setup; checks one basic idea.
2/5 - Routine exam skill
Normal syllabus skill; usually one main rule or interpretation.
3/5 - Standard multi-step
Exam-style problem with multiple steps or a common trap.
4/5 - Hard exam skill
High-end exam problem; combines methods carefully.
5/5 - Challenge
Stretch problem beyond normal exam pressure.
Question
A water-treatment demonstration uses a colloidal impurity. Alum is added, the mixture is stirred, and larger flocs settle. In a second beaker, soap stabilises tiny oil droplets for a longer time.
Total: 4 marks
(a) What process is alum causing in the first beaker? (1 mark)
(b) Why do the particles settle after alum is added? (1 mark)
(c) What is soap acting as in the oil-water mixture? (1 mark)
(d) Name the type of colloid formed by oil droplets dispersed in water. (1 mark)
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