Return after a break
What can you recall?
describe / explain
Recall and explain
describe: give the features or what happens. explain: give reasons and connect them to evidence.
Keep the lesson closed. A drop of ink is put into a glass of still water. (a) Describe what diffusion is. (b) Use kinetic particle theory to explain why the ink slowly colours all the water. (c) Say what the ink particles are doing once the colour is even all through the glass. (d) Stirring would spread the ink in seconds. Explain why that is not diffusion.
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Scope and sources
Cambridge IGCSE Chemistry 0620 · 2026–2028 v2 · Original learning materials.
Diffusion explained with the random movement of particles (1.2.1) and the effect of relative molecular mass on the rate of diffusion of gases (Supplement 1.2.2). The experiment is a virtual version of a teacher demonstration with concentrated ammonia; time is schematic. Graham’s law as an equation is not part of this module.
Apparatus, amounts and times in the experiment are illustrative. Real practical work follows a teacher-approved safety method.
Official Cambridge syllabus