Year 7 Science · Unit 2 · Lesson 11
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Learning Goals
Order the steps of a fair test
Jess is investigating whether water temperature affects how quickly sugar dissolves. Number her steps 1 to 7. Use the lesson’s five-step plan (question, hypothesis, variables, method, results table), then carry out the test and analyse it.
| Order | Step |
|---|---|
| List the variables: temperature (change), time to dissolve (measure), and mass of sugar, volume of water and stirring (keep the same). | |
| Carry out the test three times at each temperature and record every time in the table. | |
| Write a hypothesis: “If the water is hotter, then the sugar will dissolve faster, because faster-moving water particles knock sugar particles loose more quickly.” | |
| Calculate the average time at each temperature and decide whether the results support the hypothesis. | |
| Draw a blank results table with a column for temperature, three trial columns and an average column. | |
| Write the research question: “Does water temperature affect how quickly sugar dissolves?” | |
| Write a step-by-step method with the mass of sugar and volume of water to use each time. |
1. Jess keeps the mass of sugar, the volume of water and the stirring the same in every trial. Explain why she must do this.
2. A student tests whether the type of salt affects how fast it dissolves, but uses a different volume of water each time. Which is reduced, reliability or validity? Explain why.
Plan the investigation: how much salt?
Tariq’s question: Does the volume of water change how much salt it can hold? His equipment: table salt, a balance, beakers, a glass rod, a measuring cylinder and a thermometer. Plan the variables and method for his fair test.
Variables
Method
Say how Tariq adds the salt, how he knows when to stop, how he finds the mass dissolved, and how many trials he does.
Read and graph the data: Tariq’s results
Tariq’s results
Tariq added salt one gram at a time and stirred after each gram. He stopped when grains remained after stirring. Every test used water at 20 °C.
| Water volume (mL) | Salt dissolved, trial 1 (g) | Trial 2 (g) | Trial 3 (g) | Average (g) |
|---|---|---|---|---|
| 25 | 9 | 8 | 10 | 9 |
| 50 | 17 | 19 | 18 | 18 |
| 75 | 27 | 26 | 28 | 27 |
| 100 | 35 | 37 | 36 |
Source: sample class results for this worksheet.
3. Complete Table 1 with the missing 100 mL average. Then graph Tariq’s four averages, with water volume on the horizontal axis, units on both axes, a title and a best-fit line.
4. Describe the pattern in the averages. Then use it to predict the mass of salt that would dissolve in 150 mL of water at 20 °C, and explain how you worked it out.
5. Tariq writes: “My results prove that doubling the volume of water doubles the mass of salt that dissolves, at any temperature.” Evaluate Tariq’s conclusion. Use Table 1 and the ideas of controlled variables, reliability and validity.
Order the steps of a fair test
Reading down the table as printed, the correct step numbers are 3, 6, 2, 7, 5, 1, 4. In order: question; hypothesis; variables; method; blank results table; carry out three trials at each temperature; average and compare with the hypothesis.
Question 1
If one also changed, she could not tell whether it or temperature caused a difference (1); kept the same, any difference must be due to temperature (1). [2 marks]
Question 2
Validity (1): volume changes along with salt type, so the student cannot tell which caused any difference (1). [2 marks]
Plan the investigation: how much salt?
Independent: volume of water (mL); dependent: mass of salt dissolved (g) (1). Controlled, any three: water temperature; type of salt; stirring; size of each addition; beaker type (1). Method: e.g. 1. Measure the water into a beaker; check its temperature. 2. Add salt 1 g at a time, stirring after each. 3. Stop when grains remain after stirring; record the mass dissolved. 4. Repeat three times per volume and average (1 for adding, stopping rule and three trials). [3 marks]
Read and graph the data: Tariq’s results
3. 108 ÷ 3 = 36 g (1). Axes labelled “Volume of water (mL)” (x) and “Average mass of salt dissolved (g)” (y), even scales (1); points at (25, 9), (50, 18), (75, 27), (100, 36) (1); straight line of best fit and a title (1). [4 marks]
4. More water, more salt, at a steady 9 g per 25 mL (1). Prediction: about 54 g (1), by extending the line or 1.5 × 36 g (1). [3 marks]
Question 5
Marking guide: 1 position (supported at 20 °C only); 1 data: 25 to 50 mL doubles 9 g to 18 g, 50 to 100 mL doubles 18 g to 36 g; 1 trials agree within 2 g, so reliable; 1 temperature was controlled at 20 °C, so results only show 20 °C; 1 temperature changes how much solid dissolves; 1 repeat the method at several temperatures, all else the same. [6 marks]
Model answer: Supported at 20 °C, not “at any temperature”. Doubling 25 to 50 mL doubled the salt from 9 g to 18 g, and 50 to 100 mL doubled it to 36 g; trials agree within 2 g, so it is reliable. But temperature was kept at 20 °C, and temperature changes how much solid dissolves, so he must repeat the method at other temperatures.
Common error: stretching a conclusion to untested conditions.