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Biology · Diffusion · Surface-area-to-volume
Diffusion into agar cubes
Three agar cubes of side 1 cm, 2 cm and 3 cm have been made purple with phenolphthalein indicator and dilute sodium hydroxide. They are placed in dilute hydrochloric acid. As acid diffuses in from every face, the indicator fades to colourless. Time how long each cube takes to decolourise completely.
t = 0 s
Surface area / volume table
| Side | SA | V | SA:V | t (s) |
|---|---|---|---|---|
| 1 cm | 6 cm² | 1 cm³ | 6.0 : 1 | — |
| 2 cm | 24 cm² | 8 cm³ | 3.0 : 1 | — |
| 3 cm | 54 cm² | 27 cm³ | 2.0 : 1 | — |
Exam-style questions
1. Which cube decolourised first?
2. Calculate the surface-area-to-volume ratio of the 1 cm cube and the 3 cm cube.
3. Why does the SMALLEST cube decolourise FASTEST?
4. This experiment models which biological limit?