Elena investigated the effect of initial concentration on the rate of diffusion of ammonia gas. Inside a fume cupboard, she set up a horizontal glass tube with small squares of damp red litmus paper spaced at 6 cm intervals along its length. She added 3 drops of concentrated ammonia solution to a piece of cotton wool and placed it at one end of the tube, sealing it immediately with a rubber stopper. She recorded the time taken for each square of litmus paper to turn blue.
She then repeated the experiment under identical conditions using a second tube but with 8 drops of concentrated ammonia solution.
Her results are shown in the table below:
| Number of drops of ammonia | Time at 6 cm (s) | Time at 12 cm (s) | Time at 18 cm (s) | Time at 24 cm (s) | Time at 30 cm (s) | Time at 36 cm (s) |
|---|---|---|---|---|---|---|
| 3 drops | 10 | 21 | 32 | 43 | 54 | 65 |
| 8 drops | 4 | 8 | 12 | 16 | 20 | 24 |
Describe what is meant by the term diffusion.
Calculate the average rate of diffusion, in centimeters per second (cm/s\text{cm/s}cm/s), of ammonia gas from the 8 drops experiment between the litmus papers at 12 cm12\text{ cm}12 cm and 36 cm36\text{ cm}36 cm. Show your working.
Explain, in terms of particles, why the rate of diffusion is faster when 8 drops of ammonia are used compared to 3 drops.
23 exam-style questions on Edexcel IGCSE Biology Movement of substances into and out of cells. Each one has a worked solution and a mark scheme showing where the marks go.