A student poured pink agar jelly containing an indicator into a shallow dish and let it set. She then flooded the surface with dilute acid. As the acid diff...
Question 1 Report
A student poured pink agar jelly containing an indicator into a shallow dish and let it set. She then flooded the surface with dilute acid. As the acid diffused down into the jelly the pink colour was replaced by a colourless band that grew deeper with time. Fig. 6.1 shows the dish from the side after several minutes, beside a millimetre scale. Every two minutes the student measured the depth of the colourless band from the surface. Her readings are shown in Table 6.1. A printed grid is provided for plotting the results.
| Time / min | 0 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| Depth of colourless band / mm | 0 | 6 | 11 | 15 | 18 | 21 |
(a) Measure, using the scale in Fig. 6.1, the depth of the colourless band shown. [1]
(b) Measure the depth of the whole layer of agar in the dish. [1]
(c) Name the apparatus the student used to measure the depth of the band. [1]
(d) On the grid provided, plot the readings from Table 6.1 as a line graph. [3]
(e) Describe the shape of the graph you have drawn. [2]
(f) Explain why the band deepened more slowly in the last few minutes than at the start. [2]
(g) State two variables that must be kept constant. [2]
Answer Details
Acid diffuses down into the pink indicator agar, bleaching a colourless band that deepens with time. This tests measuring, plotting and interpreting the rate of diffusion.
(a) Depth of the colourless band [1] Reading the mm scale, the band is about \(14\ \text{mm}\) deep (accept 11 to 17 mm). [1]
(b) Depth of the whole agar layer [1] About \(50\ \text{mm}\) (accept 46 to 54 mm). [1]
(c) Apparatus used [1] A millimetre ruler. [1]
(d) Line graph of the readings [3]
Marks: axes the correct way round with labels and units [1]; a suitable scale using more than half the grid [1]; all six points plotted accurately and joined with a smooth line [1].
(e) Shape of the graph [2] The depth increases with time [1], but the increase slows down so the line curves and begins to flatten [1].
(f) Why it slows near the end [2] As the acid spreads deeper the concentration gradient becomes smaller and the acid has further to travel [1], so the net rate of diffusion is slower [1].
(g) Two controlled variables [2] Any two of: temperature; concentration of the acid; concentration of the agar; surface area of jelly exposed. [2]
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