Syllabus Edition

First teaching 2023

First exams 2025

|

Investigating Surface Area (CIE AS Biology)

Revision Note

Test yourself
Phil

Author

Phil

Last updated

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)
    • 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 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 (eg. 5 minutes)

Investigating Surface Area with Agar - Method

investigating surface area with agar method

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

Investigating Surface Area with Agar - Analysis 

investigating surface area with agar analysis

To analyse the results of the investigation, calculate the rates of diffusion before plotting a graph for rate of diffusion against surface area : volume ratio

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.