Agar Blocks Practical (AQA A Level Biology): Revision Note

Investigating diffusion rate

• It is possible to use agar blocks containing indicator to investigate the effect of different factors on the rate of diffusion of an acid or alkali, e.g.

  • the effect of surface area to volume ratio on diffusion can be investigated using agar cubes of different sizes

  • the effect of concentration gradient on diffusion can be investigated by placing agar cubes into solutions at a range of concentrations

Investigating the effect of surface area to volume ratio on diffusion rate

Method

1. Cut agar that contains indicator into cubes of different dimensions, e.g. with sides of length 0.5 cm, 1 cm and 2 cm

   • 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

2. Calculate and record the SA:V ratio of each agar block

3. Place the cubes into boiling tubes containing a solution that will affect the pH of the agar, e.g. dilute hydrochloric acid

   • The same volume and same concentration of dilute hydrochloric acid should be measured out into each boiling tube

4. Measure either:

   • the time taken for the agar blocks to entirely change colour

   • the distance travelled into the block by the colour change in a given time

Analysis

• The measurements taken in step 4 above can be converted to diffusion rates

• A graph can then be drawn that plots surface area to volume ratio against diffusion rate