OCR AS Biology

Revision Notes

1.2.7 Practical: Investigating the Rate of Diffusion

Practical: Investigating the Rate of Diffusion

Recording a range of quantitative measurements

  • It is important to use appropriate apparatus when recording a range of quantitative measurements
  • Quantitative measurements include things such as mass, time, volume, temperature, length and pH
    • Mass should be recorded using a digital balance
    • Time should be recorded using a digital stopwatch
    • Volume should be recorded using a measuring cylinder
    • Temperature should be recorded using a digital thermometer (although water baths have one built-in)
    • Length should be recorded using a ruler
  • The practical below is an example of an experiment that has used the correct apparatus to measure the dependent variable

Practical: Investigating the rate of diffusion using agar

  • The effect of surface area to volume ratio on the rate of diffusion can be investigated by timing the diffusion of ions through different sized cubes of agar
  • Coloured agar is 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 cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
    • The acid should have a 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
      • A stopwatch is used
    • The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time period (eg. 5 minutes)
      • A microscope and a stage micrometer (essentially a mini ruler) are used
  • These times can be converted to rates (1 ÷ time taken)
  • A graph could be drawn showing how the rate of diffusion (rate of colour change) changes with the surface area to volume ratio of the agar cubes

Investigating diffusion using agar, downloadable AS & A Level Biology revision notes

An example of how to set up an experiment to investigate the effect of changing surface area to volume ratio on the rate of diffusion.

Exam Tip

When an agar cube (or for example a biological cell or organism) increases in size, the volume increases faster than the surface area, because the volume is cubed whereas the surface area is squared. When an agar cube (or biological cell / organism) has more volume but proportionately less surface area, diffusion takes longer and is less effective. In more precise scientific terms, the greater the surface area to volume ratio, the faster the rate of diffusion!

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