Diffusion

Activity 1: Pre-Assessment

1. What happens when an air freshener is sprayed in a corner? What is the name of the process that causes the molecules to move?
2. Do you think that the rate of the air freshener molecules moving would change if the room temperature was warmer or colder? Why or why not?
3. Discuss the answers to questions 1 and 2 with the class.

Activity 1: Diffusion

The movement of molecules from a higher concentrated area to a wider and less concentrated area is referred to as diffusion. For example, you can smell the aroma of food flowing through the atmosphere as you walk towards a cafeteria. Molecules collide with each other and are in constant motion because of their kinetic energy. This activity propels molecules to move where there is a less concentrated area. Therefore, the net movement of molecules is always from a tightly concentrated area to a less tightly packed area. Osmosis is the process of water diffusion through a selectively permeable membrane. In body systems, various constituents such as gases, liquids, and solids are dissolved in water when they flow through the cell membrane from a highly concentrated place to a less concentrated area in bodily systems. In a solution, the dissolved substance is called the solute and the substance in which the solute is dissolved is called the solvent.

Diffusion is the movement of molecules from an area where the molecule is highly concentrated to an area of low concentration, as illustrated in Figure 6.1. The rate of diffusion is dependent upon the temperature of a system, molecular size, and the medium through which diffusion is occurring (i.e., semi-solid, liquid, air). In this activity, we will be observing the diffusion of a dye through a beaker of water and through agar (a gelatinous substance), diffusion as a function of temperature, and diffusion as a function of molecular weight.

Safety Precautions

• Inform your teacher immediately of any broken glassware, as it could cause injuries.
• Clean up any spilled water or other fluids to prevent other people from slipping.
• Be careful with the dye as it can stain your clothes, and it should not be ingested.
• Wash your hands with soap and water after completion of the activity.

For this activity, you will need the following:

• Three 250 mL beakers
• Water
• Food coloring
• Agar plates
• Potassium permanganate
• Congo red
• Methylene blue
• Thermometer
• Incubator
• Refrigerator
• Clock or timer

For this activity, you will work in groups of four.

Structured Inquiry

Step 1: Measure 200 mL of room temperature water in a beaker. Put three drops of food coloring into the water. Time how long it takes for the dye to completely diffuse throughout the water. Record the time and describe in your notebook what you observe. Create a data table for your observations.

Step 2: Hypothesize/Predict: Predict what would happen to the rate of diffusion if you had beakers with both very hot and very cold water in them. Add your predictions to the data table you created in step 1.

Step 3: Student-led Planning: Determine how diffusion of the food color would be affected when the water is either very hot or very cold. Use a thermometer and record the temperature for each. Use a timer to measure how long it takes for complete diffusion to occur in all scenarios.

Step 4: Critical Analysis: Create a graph that shows how the diffusion rate is affected because of temperature change. Are the predictions you made in step 2 supported by your data? Why or why not? What methods could you use to improve your results? Discuss with your group and then write your answers in your notebook.