Part 1 of 2

A smell you can't see

identify / state

Try it yourself

identify: name or select. state: give a concise answer.

A pad of concentrated ammonia solution is put near the left end of the closed tank, and the lid is closed. The table lists the six damp red litmus strips in the order they turned blue; strip 1 is nearest the pad. Choose the statement that describes how the ammonia moved. Then state whether the amount of ammonia in the tank changed as it spread.

Strip (in turning order)	Wait since the one before
1	—
2	short
3	longer
4	longer still
5	even longer
6	longest

Choose one answera.It reached all six strips at the same moment, because a gas fills its container at once.b.It moved along the tank at a steady speed, so the waits between the strips were equal.c.It spread out from the pad: the nearer strips turned first, and the waits got longer.d.It shot straight to the far end first, then filled the tank back towards the pad.

Did the amount of ammonia in the closed tank change as it spread? Say how you know.

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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