Secondary 3 Combined Science Scientific Inquiry Quiz

Secondary 3 Combined Science Quiz - Scientific Inquiry — ANSWER KEY

Total Marks: 40

Section A: Experimental Design and Variables (Questions 1–5)

1. (a) Identify the independent variable. [1]

• Answer: Temperature (of the reaction mixture / at which the enzyme and starch are mixed).

1. (b) Identify the dependent variable. [1]

• Answer: Time taken for starch to be completely broken down / Rate of enzyme activity (as measured by time for starch breakdown).

1. (c) State two variables that must be kept constant. [2]

• Answer (any two, 1 mark each):
  • Volume / concentration of amylase solution
  • Volume / concentration of starch solution
  • pH of the solution
  • Volume of the reaction mixture
  • Same method of determining when starch is completely broken down (e.g., iodine test timing)

2. (a) Suggest a suitable hypothesis. [1]

• Answer: As light intensity increases, the rate of photosynthesis (number of oxygen bubbles produced per minute) increases / Light intensity affects the rate of photosynthesis.
• Accept any reasonable hypothesis linking light intensity to rate of photosynthesis.

2. (b) Explain why the plant should adjust for 5 minutes before counting. [2]

• Answer:
  • To allow the plant to acclimatise / adjust to the new light intensity [1]
  • So that the rate of photosynthesis stabilises / to ensure the readings taken are reliable and reflect the true rate at that light intensity [1]

3. (a) Purpose of Plate 4. [1]

• Answer: It acts as a control / control experiment / to show that any bacterial death is due to the disinfectant and not other factors / to compare the effect of disinfectants against a plate with no disinfectant.

3. (b) Why measure diameter of clear zone rather than just observe growth. [2]

• Answer:
  • Measuring the diameter provides quantitative / numerical data [1]
  • This allows for more precise comparison of the effectiveness of the three disinfectants / allows a more objective conclusion to be drawn rather than a subjective visual observation [1]

4. (a) State the independent variable. [1]

• Answer: Type of surface (material at the bottom of the ramp).

4. (b) State one controlled variable and explain how to control it. [2]

• Answer (any one, 1 mark for variable, 1 mark for explanation):
  • Height of the ramp: Measure and fix the height of the ramp for each trial.
  • Starting position of the toy car: Mark a starting line and always release the car from the same point.
  • Same toy car: Use the identical toy car for all trials.

5. (a) Purpose of starch solution. [1]

• Answer: It acts as an indicator / to show when all the vitamin C has reacted (the iodine then reacts with starch to form a blue-black colour).

5. (b) Why use the same concentration of iodine solution. [1]

• Answer: To ensure a fair test / so that the number of drops needed depends only on the amount of vitamin C present, not on the concentration of iodine.

Section B: Data Interpretation and Analysis (Questions 6–10)

6. (a) Plot a graph of the results. [3]

• Marking:
  • [1] Axes correctly labelled with quantity and unit: x-axis "Concentration of sugar solution (mol/dm³)", y-axis "Change in mass of potato cylinder (g)"
  • [1] Appropriate linear scales chosen, points plotted correctly (± half a small square)
  • [1] Smooth best-fit line or curve drawn through the points
• Note: A line of best fit should pass through or near the points. The relationship is approximately linear.

6. (b) Estimate concentration for no change in mass. [1]

• Answer: Approximately 0.42–0.45 mol/dm³ (accept 0.4–0.5 mol/dm³ based on graph reading).
• Award mark for correct reading from the student's own graph where the line crosses zero change in mass.

6. (c) Explain why the potato cylinder lost mass in 1.0 mol/dm³ solution. [2]

• Answer:
  • The sugar solution has a lower water potential than the potato cells / the potato cells have a higher water potential than the surrounding solution [1]
  • Water moves out of the potato cells by osmosis into the sugar solution, causing a decrease in mass [1]

7. (a) Describe the trend shown by the data. [2]

• Answer:
  • The volume of carbon dioxide produced increases rapidly at first (from 0 to about 120 seconds) [1]
  • Then the rate of gas production slows down and eventually stops / the volume levels off at 84 cm³ after 240 seconds [1]

7. (b) Explain why the volume of gas stops increasing after 240 seconds. [2]

• Answer:
  • The reaction is complete / all of the limiting reactant (marble chips or hydrochloric acid) has been used up [1]
  • No more reactants are available to produce carbon dioxide gas [1]

8. (a) Describe the relationship between time and temperature. [1]

• Answer: As time increases, the temperature increases (at a constant rate / linearly).

8. (b) Predict the temperature at 12 minutes. [1]

• Answer: 85 °C (temperature increases by 10 °C every 2 minutes, so 75 + 10 = 85 °C).

9. (a) Calculate the mean heart rate after exercise. [1]

• Answer: (120 + 118 + 122) ÷ 3 = 360 ÷ 3 = 120 bpm.

9. (b) Suggest why repeating the experiment is better. [1]

• Answer: To improve reliability / to identify anomalous results / to calculate a mean and reduce the effect of random errors.

10. (a) Describe the trend shown by the data. [1]

• Answer: As the length of the wire increases, the resistance increases (proportionally / approximately linearly).

10. (b) State one source of error and how to reduce it. [2]

• Answer (any one, 1 mark for error, 1 mark for reduction):
  • Error: The wire may heat up during the experiment, affecting resistance. Reduction: Switch off the current between readings / use a low current.
  • Error: Zero error on the ammeter or voltmeter. Reduction: Check for zero error before starting and adjust readings accordingly.
  • Error: Poor connections / loose contacts. Reduction: Ensure all connections are tight and secure.

Section C: Practical Techniques and Safety (Questions 11–15)

11. (a) Describe how to add copper(II) oxide to ensure reaction is complete. [2]

• Answer:
  • Add copper(II) oxide in small amounts / a little at a time / in excess [1]
  • Stir the mixture and continue adding until no more copper(II) oxide dissolves / until some solid remains at the bottom of the beaker [1]

11. (b) State what is collected in filter paper and filtrate. [2]

• Answer:
  • Filter paper: Unreacted / excess copper(II) oxide (solid residue) [1]
  • Filtrate: Copper(II) sulfate solution [1]

11. (c) Name the technique to obtain crystals from filtrate. [1]

• Answer: Crystallisation / evaporation (followed by cooling) [1]

12. (a) Purpose of adding iodine solution. [1]

• Answer: To stain the cells / to make the structures (nucleus, cell wall) more visible / to provide contrast for viewing under the microscope.

12. (b) Calculate total magnification. [1]

• Answer: 10 × 10 = ×100 [1]

12. (c) Structure in onion cell not found in human cheek cell. [1]

• Answer: Cell wall (accept: large central vacuole, chloroplasts — though typically not present in onion epidermis, accept cell wall as the definitive answer).

13. (a) Identify the nutrient present. [1]

• Answer: Reducing sugar / glucose (accept: simple sugar, monosaccharide).

13. (b) State one safety precaution. [1]

• Answer (any one):
  • Use a test tube holder to handle hot test tubes
  • Point the test tube away from yourself and others while heating
  • Wear safety goggles
  • Use a water bath instead of direct flame for even heating
  • Do not touch the hot water bath with bare hands

14. (a) Name the technique to separate sand from salt solution. [1]

• Answer: Filtration.

14. (b) How to obtain solid salt from filtrate. [1]

• Answer: Evaporation / heating the filtrate to evaporate the water (leaving salt crystals behind).

15. (a) Explain why a polystyrene cup is used. [1]

• Answer: Polystyrene is a good heat insulator / reduces heat loss to the surroundings, so the temperature change measured is more accurate.

15. (b) State one safety precaution for handling sodium hydroxide. [1]

• Answer (any one):
  • Wear gloves
  • Wear safety goggles
  • Wash hands after handling
  • Avoid contact with skin as it is corrosive

Section D: Scientific Reasoning and Evaluation (Questions 16–20)

16. (a) State the optimum pH for catalase activity. [1]

• Answer: pH 7 [1]

16. (b) Explain why very little foam is produced at pH 3 and pH 11. [3]

• Answer:
  • At pH 3 (very acidic) and pH 11 (very alkaline), the pH is far from the optimum pH of catalase (pH 7) [1]
  • The shape of the active site of the enzyme is altered / the enzyme is denatured [1]
  • The substrate (hydrogen peroxide) can no longer fit into the active site / the enzyme-substrate complex cannot form, so very little reaction occurs and little foam is produced [1]

17. (a) Calculate the mean length for each student. [2]

• Answer:
  • Ali's mean: (15.2 + 15.3 + 15.2 + 15.1 + 15.2) ÷ 5 = 76.0 ÷ 5 = 15.2 cm [1]
  • Bella's mean: (15.0 + 15.5 + 14.8 + 15.3 + 14.9) ÷ 5 = 75.5 ÷ 5 = 15.1 cm [1]

17. (b) Compare accuracy and precision of Ali's and Bella's measurements. [3]

• Answer:
  • Accuracy: Ali's mean (15.2 cm) equals the actual length (15.2 cm), so Ali's measurements are more accurate. Bella's mean (15.1 cm) is slightly lower than the actual value, so Bella's measurements are less accurate. [1]
  • Precision: Ali's measurements are all very close to each other (range: 15.1–15.3 cm), showing high precision / good repeatability. Bella's measurements are more spread out (range: 14.8–15.5 cm), showing lower precision. [1]
  • Conclusion: Ali's measurements are both accurate and precise. Bella's measurements are reasonably accurate (mean close to true value) but less precise (greater variation in readings). [1]

18. (a) State the independent variable. [1]

• Answer: Brand of battery (Brand X or Brand Y).

18. (b) Suggest one way to improve reliability. [1]

• Answer: Repeat the experiment several times for each brand and calculate the mean time / use more than one battery of each brand.

19. (a) Write a suitable hypothesis. [1]

• Answer: Drinking cold water after exercise helps you recover faster (than drinking water at room temperature / not drinking water).
• Accept any reasonable hypothesis that can be tested.

19. (b) Identify one variable to control. [1]

• Answer (any one):
  • Type and intensity of exercise
  • Amount of water drunk
  • Temperature of the cold water
  • Same person doing the exercise
  • Time after exercise when recovery is measured

20. (a) Calculate the percentage error. [1]

• Answer: Percentage error = |(25.0 – 23.5) / 25.0| × 100% = (1.5 / 25.0) × 100% = 6% [1]

20. (b) Suggest one reason for this error and how to avoid it. [2]

• Answer (any one, 1 mark for reason, 1 mark for avoidance):
  • Reason: Parallax error / reading the measuring cylinder from an angle. Avoidance: Read the bottom of the meniscus at eye level.
  • Reason: Using a measuring cylinder that is too large for the volume being measured. Avoidance: Use a more appropriately sized measuring cylinder (e.g., 25 cm³ or 50 cm³).

END OF ANSWER KEY