Topic 6 of 8
Plan an investigation
Specify a fair comparison and how it will answer the question.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Plan a comparison that can answer the question
Specify variables, measurements, analysis and relevant precautions.
A plan should allow another student to carry out the comparison. Define the independent variable and a sensible range; define the dependent measurement; name important controls and how you will maintain them. Then describe apparatus, quantities, the order of operations, repeat measurements and the intended graph or calculation. A diagram helps where arrangement matters, but does not replace operational detail.
| 1.00 mol dm-3 HCl / cm3 | Water / cm3 | HCl concentration / mol dm-3 |
|---|---|---|
| 4.0 | 16.0 | 0.20 |
| 8.0 | 12.0 | 0.40 |
| 12.0 | 8.0 | 0.60 |
| 16.0 | 4.0 | 0.80 |
| 20.0 | 0.0 | 1.00 |
Worked example
Turn a rate question into a usable plan
Compare the initial rate of Mg + 2HCl -> MgCl2 + H2 as acid concentration changes. Use the dilution series above and pieces cut from the same cleaned magnesium ribbon, each of mass about 0.024 g and the same dimensions.
- Check feasibility first. n(Mg) = 0.024 / 24.3 = 9.88 × 10-4 mol. It needs about 0.00198 mol HCl, while even the most dilute run contains 0.20 × 0.0200 = 0.00400 mol. Acid is in excess in every run. At room conditions the expected final hydrogen volume is about 23.7 cm3, which fits a 50 cm3 gas syringe.
- Measure stock acid and water with separate graduated apparatus into the same reaction flask. Use a water bath at 25 °C, allowing each solution to reach that temperature. Keep the magnesium surface preparation, exposed area, mixing and collection apparatus the same.
- Put the ribbon in a small dry inner tube so it is initially separate from the acid. Connect the stoppered flask to a freely moving gas syringe, check for leaks and note its initial reading. Tilt to mix without removing the stopper and start the timer at mixing.
- In a preliminary run, check that readings every 2 s resolve the early curve. If reaction is too fast, choose a consistently lower concentration range or use a suitable data logger, then keep the chosen method for all runs. Record gas volume against time through the initial region and towards a plateau.
- Repeat each concentration three times with fresh reactants. Draw volume-time curves and compare initial tangent gradients in cm3 s-1; plot mean initial rate against initial HCl concentration and show the spread. Do not use the final gas volume as the rate.
- Wear eye protection, keep hydrogen away from ignition sources and ensure the syringe can move freely. An airtight connection prevents gas loss; a blocked outlet creates pressure and invalidates the experiment.
The plan states quantities, controls, how the reaction starts, what is measured and how the evidence answers the question. The preliminary run tests whether the proposed range and recording interval are practical.
For a fixed-endpoint timing method, 1/t can compare rates only when each experiment reaches the same small extent of reaction and the other relevant conditions are controlled. Doubling the endpoint amount while also changing concentration would confound the comparison.