Move the lamp, read the colour. Control everything else.
Planning an investigation into light intensity and photosynthesis comes up again and again in 9700 Paper 5. Here algae trapped in alginate beads sit in red hydrogencarbonate indicator while an LED lamp is moved to measured distances. 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: 9700/53 May/June 2023, Question 1 (light intensity, algal beads and hydrogencarbonate indicator) · official past papers
Practise next: 9700/51 October/November 2024, Question 1 (colour of light, algal beads and a colorimeter) · official past papers
Practise next: 9700/52 October/November 2024, Question 1 (DCPIP and a chloroplast suspension) · official past papers
Examiner reports used: 9700 June 2023 (Paper 53) and November 2024 (Papers 51, 52 and 53).
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What a full-mark diagram shows
The independent variable
Light intensity, set by moving an LED lamp to at least five distances measured with a metre rule (intensity ∝ 1/d²), checked with a light meter.
The dependent variable
The pH of the indicator after a stated time, read by matching against colour standards of known pH. Rate = change in pH ÷ time.
Standardised variables
Same number of beads, same volume of red (pH 8.4) indicator, a heat filter between lamp and tubes, a darkened room.
Control and repeats
A tube of indicator with beads that hold no algae; at least three tubes at each distance and a mean for each.
Habits the examiner reports keep asking for
The dependent variable is what you measure: the pH (or absorbance), not “colour” or “rate of photosynthesis”.
Use a heat filter, not a water bath, to stop the lamp warming the tubes.
Start every tube red, at pH 8.4, and never add a buffer: the change in pH is the result.
Give a stated time, not “about 30 minutes”, and do not time the colour change itself.
Safety needs a hazard, its risk and a precaution, not “low risk”. The indicator is an irritant, not corrosive.