Experimental Design Practice Questions

“Plan an investigation to find out…” is one of the most common and most feared exam question types. It isn’t a test of memory; it tests whether you can think like a scientist: decide what to change, what to measure, what to keep the same, and how to make the results trustworthy. These fourteen questions build that skill step by step, with model answers.

Background reading: independent, dependent and control variables, fair tests, experimental errors and graphs in chemistry.

Part A: Variables

1. A student investigates how temperature affects the time taken for magnesium ribbon to react completely with hydrochloric acid. Identify the independent, dependent and two control variables.

Answer:

• Independent: temperature of the acid (the variable deliberately changed).
• Dependent: time for the magnesium to disappear (the variable measured).
• Control: concentration of acid; volume of acid; length (or mass) of magnesium ribbon; surface condition of the magnesium (e.g. cleaned the same way).

2. In question 1, why must the length of magnesium ribbon be kept the same?

Answer: A longer piece has more magnesium to react and a larger surface area, so it would take a different time regardless of temperature. If it varied, you couldn’t tell whether a change in time was caused by the temperature or by the magnesium. This is what makes it a fair test.

3. A student says: “My independent variable is the acid.” Why is this not precise enough?

Answer: A variable must be a measurable quantity. “The acid” could mean its concentration, volume, temperature or type. A precise statement would be, for example, “the concentration of hydrochloric acid, in mol/dm³”.

Part B: Hypotheses and predictions

4. Write a testable hypothesis, with a scientific reason, for question 1.

Answer: “As the temperature of the acid increases, the time for the magnesium to react will decrease, because particles have more kinetic energy, so they collide more often and a greater proportion of collisions have energy equal to or greater than the activation energy.” See reaction rates and catalysts.

5. What is the difference between a hypothesis and a prediction?

Answer: A hypothesis is a proposed explanation that can be tested (why something happens). A prediction states what you expect to observe in your specific experiment if the hypothesis is correct (what will happen).

Part C: Choosing equipment, range and repeats

6. In an investigation of how concentration affects the rate of reaction between marble chips and acid, the volume of carbon dioxide is measured every 30 seconds. Suggest suitable equipment, and explain one reason for each choice.

Answer:

• Gas syringe to collect and measure CO₂; it’s more precise than collecting over water, and CO₂ is slightly soluble in water.
• Stopclock to time the 30-second intervals.
• Balance (0.01 g) to use the same mass of marble chips each time.
• Measuring cylinder (or better, a burette or pipette) for the acid volume.
• Sieve or selected chips of similar size, to control surface area.

7. A student uses only two acid concentrations, 0.5 and 1.0 mol/dm³. Why is this a weak design, and what would be better?

Answer: Two points can’t reveal the shape of the relationship (is it linear? does it level off?). Use at least five concentrations across a wide range, for example 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³, evenly spaced, so a graph can show the trend clearly.

8. Why should each measurement be repeated, and what do you do with repeat results?

Answer: Repeats let you check repeatability, spot anomalies, and reduce the effect of random errors by calculating a mean. Anomalous results (clearly out of line with the others) are identified, investigated if possible, and excluded from the mean.

Part D: Controls and blanks

9. In an investigation of whether a new catalyst speeds up the decomposition of hydrogen peroxide, what is the control experiment, and why is it needed?

Answer: An identical experiment without the catalyst (same volume, concentration and temperature of hydrogen peroxide). It shows how fast the reaction goes on its own, so any increase can be attributed to the catalyst.

10. In a colorimetry experiment, what is a “blank”, and why is it used?

Answer: A blank is a cuvette containing everything except the substance being measured (usually the solvent plus any reagents). It’s used to set the absorbance to zero, so the reading reflects only the analyte. See colorimetry.

Part E: Spotting flaws and improving methods

11. A student investigates the effect of temperature on solubility. They add potassium nitrate to 50 cm³ of water at each temperature, stirring “until no more dissolves”, measuring the water volume with a beaker. Identify two weaknesses and suggest improvements.

Answer:

• A beaker isn’t accurate for measuring volume. Use a measuring cylinder, or better, weigh the water on a balance.
• The temperature may change while dissolving, especially as dissolving potassium nitrate is endothermic. Use a thermostatically controlled water bath and measure the temperature of the solution itself.
• (Also valid) “Until no more dissolves” is subjective; instead, find the mass of solid that dissolves by weighing the solid before and after, or find the temperature at which crystals first appear on cooling a known mixture.

12. A student’s results for the time taken for a cross to disappear under a flask (sodium thiosulfate and acid) are very inconsistent. Suggest two causes and how to reduce them.

Answer:

• Judging when the cross disappears is subjective. The same person should judge each time, looking from the same height; better still, use a light sensor or colorimeter to detect the cloudiness.
• Reaction timing starts inconsistently. Start the stopclock at the moment the acid is added, and swirl each flask the same way.
• Temperature varies between runs. Use a water bath or record the temperature each time.

13. Rewrite this vague improvement as a specific one: “Be more accurate.”

Answer: For example: “Measure the 25.0 cm³ of acid with a 25.0 cm³ pipette instead of a 100 cm³ measuring cylinder, which reduces the percentage uncertainty in the volume from about ±2% to about ±0.2%.” Specific improvements name the change and explain how it reduces error. See calculating uncertainty.

Part F: A full planning question

14. Plan an investigation to find how the concentration of copper(II) sulfate solution affects the temperature rise when zinc powder is added in excess.

Model answer:

• Independent variable: concentration of CuSO₄ (e.g. 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³), made by diluting a stock solution with a burette or pipette.
• Dependent variable: maximum temperature rise (°C).
• Control variables: volume of solution (25.0 cm³ by pipette); mass of zinc (in excess, e.g. 1.0 g weighed on a 0.01 g balance); particle size of zinc; starting temperature; type of container.
• Method:
  1. Pipette 25.0 cm³ of solution into a polystyrene cup with a lid, in a beaker for stability.
  2. Record the starting temperature every 30 s for 2 minutes to get a steady value.
  3. Add the zinc, stir, and record the temperature every 30 s until it has clearly fallen from its maximum.
  4. Repeat three times for each concentration and calculate mean temperature rises.
• Analysis: plot mean temperature rise against concentration. The prediction is a straight line through the origin, because the heat released is proportional to the moles of copper(II) sulfate reacting (zinc is in excess).
• Safety: eye protection; copper(II) sulfate is harmful and toxic to aquatic life, so dispose of it in heavy metal waste. See disposing of chemical waste.
• See also exothermic vs endothermic reactions.

A planning checklist

1. State the independent, dependent and control variables precisely.
2. Give a hypothesis with a scientific reason.
3. Choose equipment with suitable precision, and say why.
4. Choose at least five values over a sensible range.
5. Include repeats and a control experiment if relevant.
6. Describe the method in numbered steps, with quantities.
7. Say how you’ll process results (means, graph).
8. Include specific safety precautions.

Key takeaways

• Change one independent variable, measure one dependent variable, and control everything else.
• Hypotheses explain; predictions describe what you expect to see.
• Use five or more values across a wide range, with repeats.
• Controls and blanks show what happens without the factor being tested.
• Improvements must be specific and linked to reducing a particular error.