Unit #1

Activity 10
On Your Marks. Get Set. Go! ... A Molecular Race

Activity Overview

If mixed together, molecules with various masses will move at different speeds related to their mass.

Part A: Students observe a demonstration in which two gasses placed in opposite ends of a sealed glass tube, encounter one another and form a white powder or a cloud.

Part B: Students use a computer model to represent the same behavior of various gasses in a glass tube.

Learning Objectives

Students will:

• Interpret the results of actual and simulated experiments and describe why those results occurred.
• Predict the approximate location of a precipitate formed between gasses diffusing from opposite ends of a tube.

Conceptual Prologue

Macro-Micro Connection

This activity just extends the idea of diffusion of gasses to bring in the idea that not all substances will diffuse at an equal rate. Heavier molecules diffuse more slowly. The air inside the hot air balloon is comprised of several molecules with different masses. Even though the different molecules have the same kinetic energy they will move and diffuse at different speeds.*

*While this behavior of atoms is certainly true, it doesn't have much of an effect on why the balloon flies. This activity is done mainly to reinforce previous ideas of diffusion and kinetic energy.

Science Concepts

When molecules are mixed together the kinetic energy [energy of motion] of the the molecules will be transferred via collisions until they all have the same average kinetic energy.

If you mix two different gasses together, then the mixture contains molecules with two different masses. The kinetic energy each molecule has is affected by both its mass and velocity.

Because two different molecules end up with the same average kinetic energy but have different masses, they must necessarily have different velocities. For example, molecules with less mass must move faster than molecules with more mass in order for them to both have the same kinetic energy. If two different gasses are mixed together, then the lighter molecules will have higher average velocities.

If you place two gasses at either end of a glass tube and allow them to diffuse toward each other, they will always* encounter each other at a spot in the tube closer to the molecules with more mass. This is due to the fact that these molecules are moving more slowly with the same average kinetic energy.

*Assuming a uniform temperature for both gasses.

Naive Conceptions

• Kinetic energy is only related to velocity.

Kinetic energy is related to both mass and velocity. Often students forget about the mass component of kinetic energy.

Activity Design and Execution

Major Science Concepts:	• kinetic energy • diffusion
Assumed Previous Knowledge:	• That molecules have kinetic energy. • That molecules are in continual motion. • That kinetic energy is based on both mass and velocity. • That molecules which are all mixed together have the same average kinetic energy.
Time:	• Part A: approximately 30 minutes (depends on the size of the tube used in the demonstration) • Part B: approximately 30 minutes
Materials:	For the demonstration: • A glass tube that can be opened and sealed on either end. • A small amount of concentrated HCl. • A small amount of concentrated NH3. For each pair of students: • Computers with Workbench software.
Advanced Preparation: (if any)	• Prepare the glass tube apparatus.

Investigative Question: Why do the gasses react where they do in the tube?

Part A:

1. Place a drop of concentrated HCl (or attach a Q-tip to the inside of the cork used to seal the end of the glass tube and dip it into the HCl) and a drop of concentrated NH₃ (or similar Q-tip setup) in the glass tube at the same time and seal the ends.
2. Set the tube aside for 20-30 minutes. You should see a white precipitate form indicating where the two gasses meet in the tube. You should see this white precipitate form closer to the HCl side.
3. Ask students to discuss what happened and why the gasses didn’t react in the center of the tube. Encourage them to use explanations of what is happening at a molecular level.

Part B:

1. Using Workbench software, have students open the "Gas Diffusion" activity.
2. This software will ask students to:

   - Experiment with using different gasses of various masses at either end.
   - Then the software will bring up a set of mystery gasses with unknown mass and the students will be asked to determine which molecules have more mass.
   - Then the software will present a situation with molecules of known mass and ask students to predict where the molecules will first encounter each other.
   - Students will then be asked to explain their reasoning before running the model.
   - Depending on what their original answer was the computer may ask them why their original answer was incorrect or reward them for choosing correctly.

3. Discuss with students their observations.

Assessment

Have students write in their notebooks:

Imagine this experiment: You are standing at the center of a room. On one end of the room a vial of Helium gas which has atoms with a mass of 4 units is released. At the same time on the other side of the room a vial of radioactive Radon gas which has atoms with a mass of 222 units is released. If the Helium reaches you first it will make your voice sound very high pitched. If the radioactive Radon reaches you first it will expose your lungs to serious radiation. Which will occur first? Will you start talking funny first or will the radiation reach your lungs first?

Internal Notes:
• See Computer lab N for mock up of computer activity