A student carries out an experiment to investigate the factors affecting the rate of diffusion.
In her experiment:
- She prepares three agar jelly cubes containing pink phenolphthalein indicator: one with sides of 6.0 cm6.0\text{ cm}6.0 cm, one with sides of 4.0 cm4.0\text{ cm}4.0 cm, and one with sides of 2.0 cm2.0\text{ cm}2.0 cm.
- She places each cube into a beaker of dilute hydrochloric acid for 8 minutes.
- She then removes the cubes and cuts them in half to observe how far the acid has diffused (as the acid neutralizes the alkali, it turns the pink agar colorless).
The diagram shows a cross-section through the largest cube after 8 minutes.
Describe what is meant by the term diffusion.
Using the dimensions shown in the diagram, calculate the depth of acid penetration ddd into the large cube in millimeters (mm\text{mm}mm).
Describe the expected cross-section of the smallest cube (2.0 cm2.0\text{ cm}2.0 cm) after 8 minutes, including the dimensions of any decolorized and remaining pink regions.
The student then carries out a second experiment, but this time she leaves the cubes in the acid for 16 minutes. Describe the results you would expect to see in the cross-section of the largest cube after 16 minutes.
Other factors may affect the rate of diffusion of the acid. Identify two factors that the student should keep constant in her investigation, and explain how an increase in each factor would affect the rate of diffusion.
Explain how the results of the student's investigation support the idea that large organisms need a circulation system.