# Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. The movement is due to molecular collisions, which occur more frequently in areas of higher concentration.

## Materials Needed

• Two or three glasses that are approximately 6.5 cm (approximately 2.5 in) in diameter and at least10 cm (4 in) tall.
• Plastic wrap
• Rubber band
• Lemon or other light colored gelatin desert (example: Jell-O)
• Ice water. This can be prepared by cooling water in a refrigerator or by putting ice cubes in water. The ice must be removed before using the water in the experiment below.
• Potato
• Salt

## Effect of Temperature

1. Before you begin, read the experiment below and create a hypothesis regarding the rate of diffusion of food coloring in hot and cold water.

Hypothesis:

2. Add approximately 9 cm of very cold water (3.5 in) to one glass and 9 cm of very hot water to another.

3. Place the glasses in a location where they can remain undisturbed. Allow the glasses to remain undisturbed for at least 2 minutes before proceeding to the next step.

4. Carefully place one small drop of food coloring on the surface of the water in the center of each glass. Care should be taken not to touch the glasses or disturb the water when adding the food coloring. The water should be as still as possible.

5. Observe the pattern of diffusion from the side. Do the glasses differ?

6. Estimate the percent of water in the glass that is covered every 30 seconds for 5 minutes. Initially (time = 0 sec.) there will be 0% colored. Round your answers to the nearest 10 percent (10%, 20%, 30%, etc.). The results section of your report should contain a graph of these results.

7. Record the amount of time that it takes for the food coloring to spread throughout the water in each glass. Depending on conditions, this may occur in a few minutes or it may take several hours.

In which glass does diffusion occur the fastest? How does temperature affect the rate of diffusion? Why?

## Diffusion in a Gelatin

Cells are composed mostly of water but there are many membranous compartments and dissolved molecules that create a gel-like consistency. In this experiment, you will explore the rate of diffusion in a gelatin.

1. Before you begin, read the experiment below and create a hypothesis regarding the rate of diffusion of food coloring in a gelatin compared to that in water.

Hypothesis:

2. Prepare a gelatin desert mix and add approximately 9 cm (3.5 in) of the liquid to a glass and allow it to harden in a refrigerator.

3. After the gelatin hardens, remove it from the refrigerator and place it in an area where it will not be disturbed for several hours.

4. Add approximately 9 cm of water to a second glass and place it next to the glass containing the gelatin.

5. Both glasses should remain undisturbed for approximately 2 hours so that they are at room temperature when the experiment begins.

6. When the water and gelatin are at room temperature, carefully place one drop of food coloring on the surface of the water and another drop on the surface of the gelatin. The drops should be located in the center of the glasses.

7. Cover the glass containing the gelatin with plastic wrap to prevent it from drying and secure the plastic wrap with a rubber band. This is necessary because the experiment may take more than one day.

8. Record the amount of time that it takes for the color to diffuse throughout the water.

9. Allow the gelatin to remain undisturbed for 3 days; then observe the pattern of diffusion. Record the diameter and depth of the diffused spot and estimate the percentage of gelatin that contained diffused food coloring.

How does the rate of diffusion in a gelatin at room temperature compare with the rate of diffusion in water at the same temperature?

# Osmosis

Unlike animal cells, plant cells are surrounded by a cell wall. The cell wall protects the cell and it acts much like a tire, enabling the cell to become rigid (turgid) when there is pressure inside the cell. When the pressure inside plant cells drops, the cells becomes flaccid; the plant wilts. The pressure, called turgor pressure, is due to osmosis. In the experiment below, a potato strip will become soft and limp when water leaves the cells and it will be come firm and rigid when water enters.

1. Before you begin, read the experiment below and create a hypothesis regarding what will happen to the firmness of the potato placed in freshwater and the potato placed in salt water.

Hypothesis:

3. Mix one tbsp (15 ml) of salt with 2 cm of water in a glass. A second glass should contain an equal amount of pure water.

4. Cut two strips of potato about the size of a French fry. They should be no thicker than 0.5 cm.

5. Place one of the strips in the salt water and the other strip in the pure water. Be sure that the water covers the potato strips.

6. Leave the strips in the water for 60 minutes. After 60 minutes, examine each strip and record your observations of the firmness of the strips.

# Lab Report

A t-test is not appropriate for the type of data collected in this exercise. It is not necessary to perform a statistical test for this lab report.

All of the experiments should be included in one lab report. The report should have only one Introduction section, one Methods section, one Results section, one Discussion section and one Literature Cited section.

See the Document titled Instructions for Preparing a Lab Report for details on preparing your report.

After your report is prepared, go through the Grading Checklist before submitting the report.