Cut the cube, time the pink. Smaller cubes clear faster.
Agar cubes as model cells come up again and again in 0610 Paper 6. Here pink agar is cut into cubes of different sizes, and each cube goes into the same volume of hydrochloric acid until no pink is left. 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 IGCSE Biology 0610 Paper 6 (Alternative to Practical). 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: 0610/62 May/June 2024, (acid concentration and agar with bromothymol blue; time against cube size) · official past papers
Practise next: 0610/61 May/June 2025, (acid concentration and agar with universal indicator) · official past papers
Practise next: 0610/62 February/March 2023, (surface area and diffusion of dye from potato cubes) · official past papers
Examiner reports used: 0610 June 2024 (Paper 62), June 2025 (Paper 61), March 2023 (Paper 62) and November 2023 (Paper 61).
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What a full-mark diagram shows
The independent variable
Surface area : volume ratio, changed by cutting cubes with sides of 0.5 to 2.5 cm on a white tile with a scalpel, every side measured with a ruler.
The dependent variable
The time taken for the whole cube to lose its colour, from the moment it goes into the acid, judged against a white background.
Control variables
The concentration and volume of the acid, the temperature (thermostatic water bath, checked with a thermometer), and agar and indicator from one block.
Repeats and safety
Three cubes of each size and a mean time. Hazards: the acid and the scalpel; wear eye protection and cut away from your fingers.
Habits the examiner reports keep asking for
The dependent variable is the time taken, not “the rate of diffusion”.
Name the quantity in a control variable: “the concentration of the acid”, not “the acid”.
Table headings carry the units: time taken / s, not “sec”, and convert minutes:seconds to seconds.
A surface area counts all six faces: a 1 cm cube has 6 cm² and 1 cm³, a ratio of 6 : 1.
A conclusion answers the aim: as the surface area : volume ratio increases, the rate of diffusion increases.