Summary

In this demonstration, students will investigate what happens to air when it is heated.

Grade Level

Middle and Elementary School

Objectives

By the end of this demonstration, students should be able to

• Explain why warm air will rise.
• Discuss how the change in the temperature of the air relates to wind.

Chemistry Topics

This demonstration supports students’ understanding of

• Gases
• Temperature
• Energy
• Thermodynamics
• Heat
• Weather

Time:

Teacher Preparation: 10-15 minutes

Lesson: 30 minutes

Materials

• String
• Yardstick
• Pencil
• Two paper bags of equal size and weight
• Candle
• Matches/lighter
• Scotch tape

Safety

• Always use caution around open flames. Keep flames away from flammable substances.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
• Open flames can cause burns.
• Exercise caution when using a heat source.
• When lighting the match be cautious with the flame.
• An operational fire extinguisher should be in the classroom.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

Background information:

• • When air rises, its temperature decreases. When air subsides, its temperature increases. When the temperature of a parcel of air decreases, its relative humidity increases. When the temperature of a parcel of air increases, its relative humidity decreases.
  • The normal environmental lapse rate applies to still air. The dry adiabatic lapse rate applies to rising air, when the relative humidity is below 100%. The dry adiabatic lapse rate also applies to air that is subsiding, if there is no moisture present, and no evaporation is taking place.
• Key Points accessed Information from this website.
• Teachers should practice this demonstration prior to completing in a classroom.

Demonstration Procedure:

1. Make a loop out of a foot long piece of string and use it to support the yardstick at the half-way mark.
2. Tie the other end of the string in a loop and attach over a pencil. Secure string with tape to meter stick and to pencil.
3. Using a pencil, poke two small-size holes in the bottom of each of the bags, run a three-foot long piece of string through the holes and tie the string into a loop.
   
4. Make sure that the bags hang at the same length. Attach each string and bag to the yard stick one inch from each end. You may need to tape the string onto the yard stick to secure it and finalize length.
5. Both bags will hang from the yardstick as shown in the photo. If the bags are exactly the same weight, they should make the stick balance/level when they are placed one inch from each end. You may have to adjust one bag slightly until you get them balanced. Having an additional set of hands to help at this step would be beneficial!
   
6. Optional: Use a stack of books to secure the set-up, placing it on top of the pencil, allowing the set-up to dangle freely from the edge of a table or desk. This will allow the teacher to be hands-free.
7. Hold a lighted candle a foot or more below the mouth of one of the bags until the air inside is warmed.
8. Remove the candle.