Diffusion Lab

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Name:	________________________________	Date:	___________
Teacher:	________________________________	Period:	___________

Aim
To determine how surface area and volume relate to one another and how the rate of diffusion varies with the ratio of surface area to volume.

Materials
three cubes of 3% agar-phenolphthalein (1 cm, 2 cm, and 3 cm on a side), 4% NaOH solution, a ruler, a razor, a plastic spoon, a paper towel, and a beaker

Overview
The phenolphthalein in the agar cubes reacts with the NaOH, changing the color of the cube to pink. After the cubes are exposed to NaOH, you will be able to see how far the NaOH diffused based upon the change in color which it caused. This will allow you to determine the relationship between diffusion and the surface area and volume of the cubes.

Procedure

1. Calculate the surface area and volume of each agar cube and record these values in the table below.
2. Carefully fill the beaker with 4% NaOH so that the cubes will be completely submerged when placed in the beaker.
3. Place the three agar cubes in the beaker. After 15 minutes, remove them and place them on a paper towel.
4. Using the paper towel, blot the cubes dry.
5. Use the razor blade to carefully cut each cube in half.
6. Measure the distance across the portion of the cube which did not change color.
7. Calculate the surface area and volume of this portion of the cube and record these values in the table below.

Observations
Record the data you gathered in the table below.

Diffusion into the agar cubes
Cube size	Surface area	Volume	Surface area/volume ratio	Size of uncolored portion of cube	Surface area of uncolored portion	Volume of uncolored portion
1 cm/side
2 cm/side
3 cm/side

Questions

1. If the length of the side of a cube is increased, the surface area to volume ratio of the cube    ________________
   
   
2. The rate of diffusion into the cubes is the volume of the colored area divided by the time it took. Calculate the rate of diffusion for each of the cubes.
   
   Tube #1: _______________
   Tube #2: _______________
   Tube #3: _______________
   
   
3. In which of the cubes was the rate of diffusion greatest?    ________________
   
   

Conclusions
In the space below, write down any conclusions you can draw from this experiment. For example, you may wish to address the question as to why cells are so small (hint: a large cell would have a large surface area for diffusion, but what about the volume?). You can also discuss why many cell organelles have folded membranes as opposed to flat membranes.

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Sources of Error
Identify any possible sources of error which may have affected the results of this experiment.

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