3.1 practical: the effect of changing surface area on the rate of diffusion

Method:

1. Create an agar block containing 0.5M Na₂CO₃ and phenolphthalein indicator.

2. Place the agar block on a white tile, and use a scalpel to cut up the agar block into cubes with dimensions 0.5 cm, 1.0 cm, 1.5 cm and 2.0 cm.

3. Label boiling tubes with each of the agar cube volumes using a marker pen.

4. Add 10 cm³ of 1.0 M HCl into each boiling tube.

5. Place an agar cube into its corresponding boiling tube containing the acid, and use a stopwatch to measure the time taken for the pink colour of the agar cube to disappear.

6. Repeat for the other agar cubes and record the results.

Independant variable:

• the size/surface area of the agar cubes

Dependent variable:

• the time taken for the pink colour of the agar cubes to disappear

Control variables:

• the temperature of the acid

• the volume and concentration of the acid

Risk assessment:

• using a scalpel:

  • cut away from the body

  • cut onto a hard surface (e.g. white tile)

  • carry with blade protected

• handling HCl:

  • wear googles

  • wear gloves

• handling phenolphthalein indicator:

  • wear gloves

  • wash hands after use

  • do not consume the agar

Data presentation and conclusion:

• higher SA:V ratio = higher rate of diffusion

Analysis of the results of the practical:

• When an agar cube (or biological cell/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.

• Therefore, the greater the surface area to volume ratio, the faster the rate of diffusion.