A learner half-filled a beaker with cold, still water and used a dropper to release a single drop of blue dye just below the surface, without stirring. Fig....
Question 1 Report
A learner half-filled a beaker with cold, still water and used a dropper to release a single drop of blue dye just below the surface, without stirring. Fig. 4.1 shows the beaker from the side after 4 minutes, when the dye had spread downwards and outwards to form a coloured plume. It is drawn beside a millimetre scale with zero at the water surface. Every minute the learner measured how far the dye had reached below the surface. Her readings are shown in Table 4.1, together with a column for the distance the dye moved during each one-minute interval.
| Time / min | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Depth of dye below surface / mm | 6 | 11 | 15 | 18 |
| Distance moved in each interval / mm | - | ? | ? | ? |
(a) Measure, using the scale in Fig. 4.1, the distance the dye has spread down from the surface. [1]
(b) Measure the greatest width of the coloured plume. [1]
(c) Complete Table 4.1 by calculating the distance the dye moved down during each one-minute interval. [2]
(d) State the dependent variable in this investigation. [1]
(e) Describe how the dye spreads through the water during the four minutes. [2]
(f) Explain why the learner did not stir the water. [2]
Answer Details
This tests measurement from a scale, calculating interval distances, and describing/explaining diffusion in still water.
(a) The distance the dye has spread down from the surface, read against the mm scale, is about 44 mm (accept 40 to 48) [1].
(b) The greatest width of the coloured plume is about 28 mm (accept 24 to 32) [1].
(c) Each interval distance is the difference between successive depths: \( 11-6=5 \), \( 15-11=4 \), \( 18-15=3 \) mm. All three correct [1], with the first interval left blank [1] (max 2). Completed table:
| Time / min | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Depth of dye below surface / mm | 6 | 11 | 15 | 18 |
| Distance moved in each interval / mm | - | 5 | 4 | 3 |
(d) The dependent variable is the depth the dye has spread below the surface (in mm) [1].
(e) The dye spreads out from the drop, moving down and sideways through the water [1]; it moves fastest at first and then more slowly as the interval distances get smaller (5, 4, 3 mm) [1] (max 2).
(f) The learner did not stir because stirring would mix the dye by creating currents rather than by diffusion [1], so the effect of diffusion alone could not be seen and the test would be unfair [1] (max 2).
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