Diffusion Using Agar Blocks and Bromothymol Blue
Diffusion — the movement of particles from a region of high concentration to one of low concentration — is a fundamental concept in biology and chemistry. Demonstrating diffusion visually helps students understand how size, surface area, and concentration affect the rate of movement. A simple but effective classroom experiment uses agar blocks and bromothymol blue to make diffusion visible and measurable.
The Experiment
Equipment:
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Agar mixed with bromothymol blue indicator (slightly alkaline)
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Dilute hydrochloric acid
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Scalpel or knife
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Ruler
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Timer
Method:
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Prepare a block of indicator agar by mixing bromothymol blue into an alkaline agar solution and allowing it to set.
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Cut cubes of different sizes — e.g. 1 cm³, 2 cm³, and 3 cm³.
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Place each cube into a beaker of dilute hydrochloric acid.
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As acid diffuses into the cube, it neutralises the indicator, changing its colour from blue to yellow.
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Time how long it takes for the colour to change completely in each cube.
The Science
The smaller the cube, the faster it changes colour because it has a larger surface area-to-volume ratio.
Diffusion occurs across the agar surface, so smaller blocks allow molecules to reach the centre more quickly.
This simple model mirrors what happens in biological systems: cells rely on diffusion to absorb nutrients and release waste, which limits how large a single cell can grow.
Typical Results
| Cube Size (cm) | Surface Area (cm²) | Volume (cm³) | SA:V Ratio | Time to Fully Change (min) |
|---|---|---|---|---|
| 1 | 6 | 1 | 6.0 | 3 |
| 2 | 24 | 8 | 3.0 | 7 |
| 3 | 54 | 27 | 2.0 | 12 |
Smaller cubes diffuse faster — a clear demonstration that surface area to volume ratio directly affects diffusion rate.
Skills Highlight
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Designing fair tests and measuring reaction times
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Observing and recording qualitative colour changes
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Calculating and comparing SA:V ratios
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Linking diffusion principles to cell biology and transport systems
Why It Works in Teaching
This experiment turns an invisible process into something colourful and measurable. Students can easily see how size influences diffusion and understand why biological structures — from cells to alveoli — are adapted for maximum surface area.
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