Dilute, sow, incubate, count. Every step needs a number.
Making a range of concentrations from one stock solution and testing them on living material comes up again and again in 9700 Paper 5. Here you make a ten-fold serial dilution of caffeine and test it on radish seeds, counting the percentage that germinate after three days. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Biology 9700 Paper 5, Question 1. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9700/51 October/November 2025, Question 1 (diluting juglone and planning a seed germination and growth investigation) · official past papers
Practise next: 9700/53 October/November 2025, Question 1 (the same juglone and germination question) · official past papers
Practise next: 9700/52 February/March 2023, Question 1 (proportional dilution of a glucose stock for a calibration curve) · official past papers
Examiner reports used: 9700 March 2023, June 2023, November 2023, March 2024, June 2025 and November 2025.
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What a full-mark diagram shows
The dilution series
At least five concentrations from the stock, each made with stated volumes of solution and of the same solvent (distilled water here), measured with graduated syringes; a table of volumes with units.
Exposing the seeds
The same number of seeds of one variety on filter paper in each dish, the same volume of solution in each, topped up daily, with lids on.
Control and replicates
A dish with the solvent and no chemical, to show the effect is caused by the chemical; at least 10 seeds per treatment, replicate dishes and a mean.
Measuring and standardising
Count seeds sown and seeds germinated after a set number of days; incubator at a stated temperature; a specific hazard with its precaution.
Habits the examiner reports keep asking for
The independent variable is the concentration, including the control. “Presence of” the chemical alone was not credited.
Dilute with the solvent the stock was made in, and call it distilled water, not just water.
Say “graduated syringe” or “graduated pipette”. “Pipette” alone is not precise enough.
Count after a set time. Timing germination, or counting daily with no end point, scores nothing.
“Low risk” is not a risk assessment: name the hazard, the risk and the precaution.