Sanjay investigated the effect of concentration of an acid on the rate of diffusion of gas. In a fume cupboard, he set up a horizontal glass tube with small squares of damp blue litmus paper spaced at 5 cm intervals along its length. He added 2 drops of concentrated hydrochloric acid to some cotton wool and placed it at one end of the tube, closing it with a stopper. He timed how long it took for each square of litmus paper to turn red.
He then repeated the experiment using an identical tube but with 5 drops of concentrated hydrochloric acid.
His results are shown in the table:
| Number of drops of acid | Time taken for litmus paper to change colour (seconds) | | :---: | :---: | :---: | :---: | :---: | :---: | :---: | | | 5 cm | 10 cm | 15 cm | 20 cm | 25 cm | 30 cm | | 2 drops | 8 | 15 | 23 | 30 | 38 | 45 | | 5 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 hydrogen chloride gas from the 5 drops experiment between the litmus papers at 5 cm5\text{ cm}5 cm and 30 cm30\text{ cm}30 cm. Show your working.
Explain, in terms of particles, why the rate of diffusion is faster when 5 drops of acid are used compared to 2 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.