Unit #1

Activity 11
Molecular Rematch: Heavyweight vs. Lightweight Championship Race

Activity Overview

Atoms with various masses will move at different speeds related to their mass if mixed together.

Students put a small amount of lead nitrate solution on one side of a petri dish and potassium iodide solution on the other side. They then observe a reaction between the lead ion and the iodide ion, forming a yellow precipitate.

Learning Objectives

Conceptual Prologue

Macro-Micro Connection

This activity builds upon and reinforces their ideas of diffusion and their atomic level conceptions of different states of matter. Beyond the general idea of diffusion related to the motion of various molecules in the hot air balloon, there is little this activity adds to their understanding of the hot air balloon story.

Science Concepts

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

If you mix two different solutions together, then the liquid contains atoms and molecules with various masses. The kinetic energy each atom or molecule has is affected by both its mass and velocity.

Because two different atoms or 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 solutions are mixed together, the lighter molecules or atoms will have higher average velocities.

In the lab students will perform the reaction occurs between lead atoms and iodine atoms.* These atoms are mixed with the water and so are part of the liquid. When solids are dissolved in a liquid you have a solution. This experiment is just like the diffusion experiment in Activity 9 except that this time all the atoms involved are in a liquid (water) instead of a gas (air).

*Actually, the lead and iodine are in the form of ions. The only difference between a lead or iodine atom and ions of those elements is that the ions are charged atoms. This should have little effect on the results of the experiment.

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 atoms have kinetic energy. • That atoms are in continual motion. • That kinetic energy is based on both mass and velocity.
Time:	• Part A: approximately 25 minutes • Part B: approximately 25 minutes
Materials:	For each group: • A petri dishe • Water • Solutions of Lead(II) Nitrate and Potassium Iodide • An eyedropper dedicated to each solution.
Advanced Preparation: (if any)	• None

Investigative Question: Why does the reaction happen where it does and not in the center?

1. Handout and have the students perform the Petri Diffusion Lab.
   
2. Discuss their answers to the lab questions.

Assessment

Have students write in their notebooks:

The atoms you observed in the liquid were moving at about the same speed as the atoms in the gas diffusion experiment. Why did it take a longer for atoms to diffuse through a liquid than a gas?

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