Evaporation

INTRODUCTION

Evaporation is the change of a liquid to a gas. This experiment allows students to monitor the evaporation of liquids and compare the relative rates of evaporation of two different liquids.

MATERIALS

• 4 plastic cups
• 1 straight pin
• 1 drinking straw
• 1 paper towel
• Water
• Safety goggles
• Isopropyl alcohol

CAUTION: Isopropyl alcohol is poisonous and should not be swallowed. It is also flammable.

WHAT TO DO

1. Push a straight pin through the center of a plastic drinking straw. Suspend the pin across the two large cups to make a balance. The plastic straw should balance, if not, try adjusting the position of the pin in the straw until it balances.
2. Cut two strips of paper towels 1" wide by 6" long.
3. Put on safety goggles for work with isopropyl alcohol.
4. Put enough water in one cup and enough isopropyl alcohol in another cup to be able to soak the paper towel strips, one strip in each.
5. Soak one paper towel strip in the water and the other in the alcohol until they are completely wet.
6. Hang the two strips on the opposite ends of the straw balancing on the other two cups, making note of which side the water soaked strip is hanging on and which side has the alcohol soaked strip.
7. Let the excess drip off the paper strips.
8. Rebalance the plastic straw across the cups, adjusting the positions of the towels so the straw will balance.
9. Does the straw stay balanced?

QUESTIONS

1. Which side seems to be losing more weight?
2. What is causing this difference?

SUMMARY

The attraction between isopropyl alcohol molecules is not as strong as that between water molecules. Therefore, isopropyl alcohol molecules evaporate faster than water molecules. The paper towel strip that was soaked in alcohol will dry faster than the strip soaked in water. A dry strip weighs less than a wet strip, so the straw will tilt down on the side with the water soaked strip since it will take longer to dry.

EXTENSION

Evaporation and Condensation are opposite processes. In the winter when it is cold outside, water forms on the inside of windows of a house. In the summer when it is cooler inside the house, water forms on the outside of the windows. Explain this.

SOURCE

"The Best of Wonder Science: Elementary Science Activities." American Chemical Society and the American Institute of Physics, Delmar Publishers, 1997, ISBN 0-8273-8094-1.

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