3.1.2 Agar Blocks Practical

• 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