Investigating SA with Agar
- The effect of changing surface area to volume ratio on diffusion can be investigated by timing the diffusion of ions through cubes of agar of different sizes
Method
- Coloured agar is made up and cut into cubes of the required dimensions (eg. 0.5cm x 0.5cm x 0.5cm, 1cm x 1cm x 1cm and 2cm x 2cm x 2cm)
- Purple agar can be created if it is made up with very dilute sodium hydroxide solution and Universal Indicator
- Alternatively, the agar can be made up with Universal Indicator only
- The surface area, volume and surface area to volume ratio of these cubes is calculated and recorded
- The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
- The same volume of dilute hydrochloric acid should be carefully measured out into each boiling tube
- The acid should have higher molarity than the sodium hydroxide so that its diffusion can be monitored by a change in colour of the indicator in the agar blocks
- Measurements can be taken of either:
- The time taken for the acid to completely change the colour of the indicator in the agar blocks
- The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time (eg. 5 minutes)
The steps used to investigate the effect of changing the surface area to volume ratio on diffusion
Analysis
- If the time taken for the acid to completely change the colour of the indicator in the agar blocks is recorded, these times can be converted to rates
- A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area : volume ratio of the agar cubes
To analyse the results of the investigation, calculate the rates of diffusion before drawing a graph for rate of diffusion against surface area : volume ratio
