Molecular Motion Facilitator Guide Diffusion


Molecular Motion
Physical Science
Grade Level
Activity Name(s)
Being Prepared
This activity uses computer modeling software that allows students to test and retest as they work.  Working in small groups of 2 and no more than 3 encourages students to discuss and refine their ideas as they work.
Getting Started
Students will observe the path of the atoms to see how they react to each other.
Suggested Timeline
Each activity should take 45-50 minutes to complete.
Thinking about the Discovery Questions
The activities in this unit allow students to investigate motion at the molecular level.  Diffusion builds on the understanding of the random movement of molecules to explore the process of diffusion.  Students will model allows them to see differences in movement in hotter and colder rooms.

There are number of misconceptions that students may come to the activities with.  One of these is that atoms or molecules of a liquid, solid  or, gas are not moving when the the substance itself is still.  The assumption is that if movement can't be seen then the molecules making up the substance are not moving.  Another misconception may be that the average distance between the atoms or molecules of a substance remains the same when the temperature of the substance changes.
Learning Objectives
MS-PS1-4.	Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

MS-PS2-5.	Conduct an investigation and evaluate the experimental design to provide evidence that fields exist between objects exerting forces on each other even though the objects are not in contact.

MS-PS3-4.	Plan an investigation to determine the relationships among the energy transferred, the type of matter, the mass, and the change in the average kinetic energy of the particles as measured by the temperature of the sample.

Physical Science – Motion and Forces
If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object's motion.

Physical Science – Transfer of Energy
Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei and the nature of a chemical. Energy is transferred in many ways.
Discussion: Setting the Stage
Q: What makes up the air around you?
A: The air consists of gas molecules, water vapor, dust, and other small particles.

Q: Do gas molecules move in the air?  What is a common example that tells you that they move?
A: Yes.  One example is when you feel the wind blowing across your face.

Q: Imagine that a drop of peppermint oil was placed in one corner of the room.  After a few minutes you can smell it on the other side of the room.  Explain what you think is happening.
A: As gas molecules in the air bounce around in the gaseous state. The gas molecules move some of the peppermint oil molecules around by colliding with them and spreading out across the room.
Discussion: Formative Questions
Q: How would you describe the movement of the different molecules as you ran the models?
A: The movement appears to be random.  This is because they aren’t moving in an empty space. They bounce off of each other, constantly changing the direction of the movement.

Q: Besides temperature what else might affect the rate of diffusion?
A: The amount of different types of molecules is one factor.  The more molecules (concentration) in the air, the more collisions and bouncing would occur. Another would be resistance to movement, though we would only find this when a substance is highly pressurized.
Discussion: Wrapping Up
Q: How is air like a liquid?
A: Both take the shape of the form they are placed in both are made of atoms and molecules.

Q: If all things were kept equal, except the number of perfume molecules in the air what differences might you expect to see in how fast diffusion takes place?
A: As the number of perfume molecules in the air increases so does the rate of diffusion.

Q: Does diffusion happen in other substances?
A: We can see diffusion in water.  If you have ever place hot water in a clear mug and then place a tea bag in the mug you can watch the gradual movement of the tea through the mug.  You can speed the process of diffusion by stirring the water.

Diffusion uses two different models.
1) In Data Collection I students model what happens when a drop of dye is added to water.  To add the dye simply click on the model once it starts running.

2) In Data Collection II and III students will look at the movement of different size molecules as the temperature is raised or lowered.
Additional Background
Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles (
Q: How does the smell of perfume make its way throughout an entire room? Explain the molecular motion of the perfume.
A: The molecules that make up the perfume are in motion.  As they move that come into contact with other molecules that are also in motion. This changes the direction they are moving slightly allowing the perfume to make its way across the room.

Q: Once the perfume molecules reach the other side of the room, what do the perfume molecules do?
A: The molecules continue to move around the room.

Q: If there were an L shaped room and you were standing around the corner from where a bottle of perfume spilled, would you smell the perfume? Why or why not?
A: You would eventually smell the perfume because there is nothing blocking the molecules from moving around the corner of the L.

Q: How does a change in concentration influence the diffusion of perfume?
A: The the greater the concentration the quicker the diffusion across the room.
Further Investigation
Place vanilla extract in a balloon and tie it off.  Discuss why you can smell the vanilla.  Try other scents and see if some diffuse better than others.