Moving Molecules

INTRODUCTION

The purpose of this experiment is to observe molecular motion within liquids and the effect of temperature change on that motion will also be noted.

MATERIALS

• Clear plastic cups
• Red, blue, and green food coloring
• Hot water
• Room temperature water
• Ice water

WHAT TO DO

1. Place three plastic cups on a counter. Place hot water in one, room temperature water in another and ice water in the third.
2. Have students measure the temperature in each cup. After measuring the temperature, leave the cups undisturbed for a couple of minutes.
3. Add a drop of food coloring to the center of each cup and time how long it takes the dye to spread evenly throughout the cup. The children identify best with red food coloring in the hot water, blue in the cold water, and green in the room temperature water.
4. Compare the diffusion of the colors in each of the three cups.

QUESTIONS

1. Did you observe a difference in the time it takes for the dye to spread throughout the cups? If there are observed differences, propose possible reasons for the differences.
2. Ask the students to make a statement about what changes in thermal motion to expect as the temperature is varied.

SUMMARY

Diffusion is the random thermal motion of molecules in all phases of matter. The moving water molecules in the cup push drops of food coloring. The molecules in the hotter solution move faster than in the cooler solution. The observed faster dispersion of the food coloring in the hotter solution is the result of the faster molecular motion in the warmer solution.

SOURCE

"Investigating Solids, Liquids and Gases with Toys: States of Matter and Changes of State." Jerry Sarquis, Lynn Hogue, Mickey Sarquis, Linda Woodward, McGraw-Hill, 1997, ISBN 0-07-048-235-7.
"Unseen Movement in Chemistry for Every Kid." Janice Van Cleave, John Wiley, 1989, ISBN: 0-471-62085-8.

© S. Olesik, WOW Project, Ohio State University, 2000.