Unit #1



 Activity 9
Wow! How did that smell get over here?



Activity Overview

Gas will diffuse through a room at an equal rate in all directions from the point where it is released.

A bottle full of a pungent gas, either perfume or ammonia, is opened, and students consider how to explain the diffusion of the odor throughout the room. They use both writing and kinesthetic modeling to express their ideas.

Learning Objectives

Students will:

Conceptual Prologue

Macro-Micro Connection

Gases inside the hot air balloon are spreading out in all directions and filling every part of the balloon. By studying how an odor (which is just a gas) spreads out, students can understand how the gases inside a hot air balloon behave.

Science Concepts

Gas molecules are in continual motion. They move in a straight line until they collide with another molecule or with the walls of their container (also made of atoms and molecules). When molecules collide, they rebound with 100% elasticity [bounciness].

Air is made of colliding gas molecules and odors are also made of gas molecules. If some odor is released from a container then the molecules will mix with molecules in the air. The multiple random collisions will cause the odor molecules to diffuse [spread randomly in all directions] into the surroundings.

For simplicity all gasses will be modeled as if they are made of single atoms. While true of some gasses (the ones in the Noble gas column of the periodic table), this is not true of other gasses at room temperature. For now, use the term atoms to refer to the particles of a gas.

Naive Conceptions

• Odors are not made of atoms.
Odors are made of atoms (or more correctly molecules, tightly bound groups of atoms). The only way you can smell something is if some molecules of that substance or the part of that substance that forms a gas, float up and into your nose.
• Odors travel in blobs.
Odors disperse at an equal rate in all directions. There is no interaction between the molecules of the odor substance keeping them close together in groups or large blobs of odor.

Activity Design and Execution

Major Science Concepts: • diffusion
Assumed Previous Knowledge: • That substances are made of atoms.
• That atoms are in continual motion.
• That atoms collide with 100% elasticity [bounciness].
• Experience with kinesthetic modeling.
Time: • Approximately 30 minutes
Materials:

• T-shirts or hats of two colors to represent two different kinds of molecules or fabric strips to be tied around students arms or heads.
• Some ammonia or perfume.
Advanced Preparation: (if any) • None

Investigative Question: How do odors spread out?

  1. Have students spread out throughout the room.
  2. Explain to students that when they smell something, some of the atoms* of that thing come in contact with receptors in their nose, so in order to smell something, small pieces of that thing must travel through the air to their nose.
  3. Ask them to predict what will happen if you release an odor in the middle of the room. Who will smell it first?
  4. Release the odor and have students raise their hand when they smell it. Be sure to prevent air movement as much as possible during this experiment, close windows, vents, etc.
  5. After the experiment is finished, have them break into small groups, and ask them to come up with a kinesthetic model to explain how the pattern of odor permeating the room can be explained.
  6. Have students perform the kinesthetic model and discuss what happened. Ask for modifications people would make to the model and why. If they don't include air in their model then you might prompt them in this direction.
  7. Tell them that they just modeled diffusion. Ask them to come up with a definition of diffusion.

Assessment

Have students write several things in their notebooks:

  1. Define diffusion.
  2. Talk about the ways in which the experiment you did with ammonia (or perfume) in the class is, or is not, like how a drop of ink spreads out in water.
Extensions
• None
Additional Resources
• None