Investigating Surface Area (Cambridge (CIE) AS Biology): Revision Note

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. 0.5 cm × 0.5 cm × 0.5 cm, 1 cm × 1 cm × 1 cm and 2 cm × 2 cm × 2 cm)

• 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 measured carefully into each boiling tube

  • The acid should have higher concentration 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 (e.g., 5 minutes)

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 plotted showing how the rate of diffusion (rate of colour change) changes with the surface area : volume ratio of the agar cubes