Year 7 Science · Unit 1 · Lesson 11
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Learning Goals
Order the stages of Sam’s investigation
Sam investigated slaters (woodlice). These six notes come from his report, but they are mixed up. Number them 1 to 6 in Q-H-M-R-D-C order.
| Order | Sam’s note |
|---|---|
| More slaters gathered on the damp side, so slaters choose damp places. | |
| If slaters can choose, then more will gather on the damp side, because they dry out quickly. | |
| Damp side: 8, 9 and 7 slaters; dry side: 2, 1 and 3 slaters. | |
| Do slaters spend more time on the damp side or the dry side of a tray? | |
| Most were on the damp side in all three trays; one tray sat nearer the window, which may have affected it. | |
| Put 10 slaters in a tray with damp paper on one half and dry paper on the other; set up three trays and count each side after 10 minutes. |
1. Students often mix up the last two steps. What is the job of the Discussion, and what is the job of the Conclusion?
Plan the investigation
Ella’s question: Does the temperature of the water affect how many radish seeds germinate (sprout) in 5 days? She has 12 identical dishes, paper towel, radish seeds and four places kept at different temperatures. Plan her investigation.
Hypothesis
Ella first wrote “Seeds like warm water better.” That cannot be tested, because “like” cannot be measured. Write a testable version: If … then … because …
Variables
Method
Say how many seeds go in each dish, how many dishes at each temperature, and what you count and when.
Read and graph the data
Ella’s results
Ella put 20 seeds in each dish and kept three dishes at each temperature. Table 1 shows how many seeds had germinated after 5 days.
| Temperature (°C) | Dish 1 | Dish 2 | Dish 3 | Average |
|---|---|---|---|---|
| 5 | 2 | 4 | 3 | 3 |
| 15 | 13 | 15 | 14 | 14 |
| 25 | 19 | 18 | 17 | |
| 35 | 11 | 12 | 13 | 12 |
Source: made-up sample class results for this worksheet.
2. Work out the missing average for 25 °C and write it in Table 1. Then plot the four averages as a line graph, with temperature on the x-axis, both axes labelled with units, and a title.
3. Ella’s hypothesis was: “If the water is warmer, then more radish seeds will germinate.” Describe the trend in the averages and say whether the results fully support her hypothesis. Use numbers from Table 1.
4. Noah says: “Replicates are a waste of time. If I measure one dish really carefully, I will get the right answer.” Evaluate Noah’s claim. Use Ella’s results in your answer.
Order the stages of Sam’s investigation
Reading down the table as printed, the correct step numbers are 6, 2, 4, 1, 5, 3. In order: Question (damp or dry side?); Hypothesis (if slaters can choose…); Method (10 slaters, damp and dry paper, three trays, count after 10 minutes); Results (damp side 8, 9 and 7; dry side 2, 1 and 3); Discussion (pattern in all three trays, the tray near the window); Conclusion (slaters choose damp places).
Question 1
Discussion: explains what the results show, points out possible errors and compares the results with the hypothesis (1). Conclusion: one or two sentences that answer the original question using the results (1). [2 marks]
Plan the investigation
Hypothesis with a measurable change, result and reason, e.g. “If the temperature is raised from 5 °C to 25 °C, then more radish seeds will germinate in 5 days, because seeds need warmth to start growing” (1). Independent: water (or air) temperature, °C. Dependent: number of seeds germinated after 5 days. Controlled, any three: number of seeds per dish; volume of water; type of paper towel; dish size; seed type and age; light; time before counting (1 for IV and DV, 1 for three controls). Method: e.g. 1. Put 20 seeds on damp paper towel in each of 12 dishes. 2. Add the same volume of water to each. 3. Keep three dishes at each of four temperatures. 4. Count the germinated seeds in each dish after 5 days and average each set of three (1 for replicates, 1 for a clear count and time). [5 marks]
Question 2
(19 + 18 + 17) ÷ 3 = 18 seeds (1). Axes labelled “Temperature (°C)” on x and “Average seeds germinated” on y, with even scales (1); points plotted at (5, 3), (15, 14), (25, 18), (35, 12) (1); points joined and a title such as “Effect of temperature on radish germination” (1). [4 marks]
Question 3
Germination rises from 3 at 5 °C to a peak of 18 at 25 °C (1), then falls to 12 at 35 °C (1). The results support the hypothesis only up to 25 °C; it does not hold at 35 °C, so it needs a limit (the response levels off and falls, an upside-down U) (1). [3 marks]
Question 4
Marking guide: 1 clear position (Noah is wrong); 1 no two dishes or seeds are identical, so each dish includes chance differences as well as the effect of temperature; 1 averaging cancels those chance differences (they push results up as often as down) but does not make the measuring more accurate; 1 data, e.g. at 5 °C the dishes gave 2, 3 and 4 seeds, so one careful count could be 2 or 4; 1 a single dish could be a fluke; 1 a difference only counts as a result when it is bigger than the spread within each group (e.g. 14 vs 18 is bigger than the 2-seed spread). [6 marks]
Model answer: Noah is wrong. Careful measuring only makes each count accurate; it cannot fix the fact that no two dishes of seeds are the same. At 5 °C Ella’s dishes gave 2, 4 and 3 seeds, all counted correctly, so a single dish might have given 2 or 4. Averaging three dishes cancels out those chance differences, while the effect of temperature stays. Because the averages at 15 °C and 25 °C differ by 4 seeds, which is more than the 2-seed spread inside each group, Ella can trust that difference. With one dish she could not tell.
Common error: saying replicates make the measurement “more accurate”. They make the result more trustworthy by cancelling chance differences between individuals.