Summary

In this demonstration students will experience diffusion, and then model the process of diffusion of microwave popcorn “flavor particles” in a room filled with still air.

Grade Level

High School

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative positions of particles (objects).
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Constructing Explanations and Designing Solutions

Objectives

By the end of this demonstration, students should be able to

• Explain how the “flavor particles” moved from one place to another.
• Communicate how the particles moved from one place to another using a storyboard.
• Explain that particles in gases are in constant random motion.

Chemistry Topics

This demonstration supports students’ understanding of

• Gases
• Kinetic Molecular Theory
• Particle Model of Matter
• Diffusion of Gases
• Particle Motion

Time

Teacher Preparation: 5 minutes

Lesson: 45 minutes

Materials

• Aerosol can of room freshener, or;
• Cologne or perfume, or;
• Microwave and microwave popcorn (this is my favorite, but the microwave should be in a different room so that the students don’t “pre-smell” the popcorn)
• Whiteboards and Dry Erase Markers; or large sheets of paper and markers or colored pencils

Safety

• Always wear safety goggles when handling chemicals in the lab.

Teacher Notes

• Students should already be able to describe matter of any state (solid, liquid, or gas) as having a particulate nature.
• For this explanation, I’m going to assume that you are using microwave popcorn, but spray air freshener like Febreze or perfume works as well.If I use an aerosol spray, I usually just spray the room freshener into a box or a sink or something. Do not spray it directly into the room!
• When I do this demonstration, I come out of my stockroom (where there is a microwave) with a freshly popped bag of microwave popcorn.I stop in a specific location in my classroom – I don’t move around.
• The next thing I do is start telling the students a story that doesn’t have anything to do with the popcorn, while I nonchalantly eat the popcorn, shaking the bag every thirty seconds or so.This usually raises the interest level of every student.
• After two or three minutes, I ask the students, “If you can smell the popcorn, raise your hand.”Depending on how long it has been, usually only the closest students will raise their hand (sometimes a student from far away will raise their hand as well.I don’t dismiss this, but I think that the smell of popcorn is so familiar that sometimes students may say that they can smell the popcorn, even though they may not.)
• After a little more of my irrelevant story, I ask the students again if they can smell the popcorn. By now most, if not all, of the class has their hands raised, although there may be a hold out or two.
• This may lead to the discussion of how we are able to smell the popcorn in the first place.Students may or may not know that the flavoring particles need to engage with their olfactory system. The idea that the smell of the popcorn is actually particles getting into the nose should come out in the discussion. The question for the class becomes: “How did the flavoring particles get from the bag to your nose?”

• Ask students to make a storyboard of the situation.Tell the students that you would like them to divide their whiteboard (or large sheet of paper) up into 6 equal squares Students should make a storyboard, describing how the flavor particles got from the bag to their noses. The first square represents when the popcorn was first brought into the room. The last square represents the time when everyone can smell the popcorn. The four squares in between represent four times in between the first and the last.
• Most students will be very good at representing what happened, but they’re not so good at explaining how it happened.This is why is important to monitor the students work as they are making their storyboards, and have conversations with students while they are working. This is not to tell the students what you think, but to ask them what they think.
• After a reasonable amount of time, students are asked to display their work.I use large 24” x 36” whiteboards in my classroom, so students display their work by putting their whiteboard in a stand that holds the board up. Whiteboards can be displayed in a circular array, they can be all arranged together on one side of the classroom, or they can be displayed “gallery walk” style.Posters can be taped to the walls. Two example boards are shown.
• Note that in the first example the students did represent air particles as being present (although not necessarily playing a role in the motion of the flavor particles, because collisions or interactions between the two types of particles are not indicated) but in the second example students did not.
• When students get a chance to look at all of the boards, a whole class discussion should take place. Questions can be asked such as:
  • How did we represent the flavor particles?
  • How did the flavor particles get to your nose?
  • What other particles may have been involved?
  • What role did the other particles play?
• Collisions between particles of air and the flavor particles should come out of the discussion, and that the air particles must be in motion in order for the particles to cover that much distance in a relatively short period of time.
• As a whole class discussion, the goal of the students is to come up with a mechanism for how the flavor particles moved from one location to everywhere in the classroom, and a consensus should be reached. The goal of the discussion should be to get to the fact that the flavor particles leave the bag and collide with air particles in a random way, until the particles are evenly distributed around the room.
• Tip #1: A clear explanation of how to create the storyboard is sufficient - no student handout is necessary.This saves on paper; some classrooms are going paperless, such as schools that are 1:1 with Chromebooks, for example.
• Tip #2: Let students know that their storyboard can include the popcorn bag, and the desks, and the students, and whatever else, but it is the particles that are the star.The flavor particles, and any other relevant particles should be the main focus of the story.
• Tip #3: Students will often suggest that there is a breeze in the classroom, and that it is a mass of air (a breeze) that carries the particles around.This is why I take great pains to close all doors, windows, and disable any other draft causing agents.Even though students will suggest a breeze as the agent, often times their story will not reflect that air particles are involved.
• Tip #4: Don’t rush the discussion.Students should be allowed to develop their ideas about how the particles spread out.The whiteboards or the discussion should not be used as a graded task, but used to assess how the students think about particles and how they behave.