University of Virginia
Physics Department

Air and Density

A Physical Science Activity

2003 Virginia SOLs



Students will

Motivation for Learning

Show the students a balloon that is filled with helium. Have a string tied to it and note that it floats in air. Ask students, "What type gas is in this balloon? Why does it float in the air?" Students should know that the balloon is filled with helium. Ask them what other gas might it be filled with? (hydrogen). Ask students to predict what will happen when you gently warm balloon containing air with a hair dryer.  

Background Information

Density is the amount of matter per volume of D = M/V. Another way this can be explained is density is the amount of matter per space or how much stuff is in one area. Matter is constantly moving. Even molecules in the solid phase vibrate a little bit. Matter, or molecules, in a gas moves much faster and are further apart. There are fewer molecules per space in the gas phase than for liquids and solids. When heat is applied to any matter, the molecules speed up, start bouncing off each other, and tend to spread apart. Consider a balloon filled with air that we heat with a hair dryer. The balloon will expand, occupying more space. The number of air molecules is the same, but the volume has increased, causing the density to decrease. The balloon's volume is less dense than the space around it. Same amount of matter, different amount of space. As the balloon cools, the molecules will slow down, move closer together, and the balloon will sink to the floor.


Class Demonstration




  1. Obtain a Mylar balloon, partially filled (not rubber).
  2. Tie a piece of ribbon to the balloon.
  3. Keep trimming off the ribbon or adding weight to the ribbon until balloon achieves neutral buoyancy. The balloon should float about waist high on its own.
  4. Apply heat to the balloon. The sides of the balloon will expand, causing it to rise. As it cools, it will float back down to neutral buoyancy.


  1. Students can take the temperature of the air at the ceiling of the room and at the floor.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.