2 balloons of equal size
3 pieces of string at least 6 inches long
a wooden ruler
a small needle
Inflate the two balloons until they are equal in size and tie them off. Attach a piece of string to each balloon. Then, attach the other end of each of the strings to the opposite ends of ruler. Keep the balloons the same distance from the end of the ruler. The balloons will now be able to dangle below the ruler.
Tie the third string to the middle of the ruler and hang it from the edge of a table or support rod. Adjust the middle string until you find the balance point where the ruler is parallel to the floor. Once the apparatus is completed, the experiment can begin.
Puncture one of the balloons with the needle (or other sharp object) and observe the results. Students can write their observations in a science notebook or simply discuss the results in a lab group.
To make the experiment a true inquiry experiment, the objective of the demonstration should not be revealed until after students have had a chance to observe and comment on what they have seen. If the purpose of the experiment is revealed too soon, students will not have the chance to figure out what happened and why.
- Why It Works
The balloon that remains full of air will cause the ruler to tip showing that the air has weight. The empty balloon’s air escapes into the surrounding room and is no longer contained within the balloon. The compressed air in the balloon has a greater weight than the surrounding air. While the weight itself cannot be measured in this way, the experiment gives indirect evidence that air has mass.
- In the inquiry process, I often will not reveal the objective of an experiment or demonstration. Many teachers I know will actually cut off the title, objective, and opening questions for lab activities so that students observe the experiments knowing the outcome will help them to write their own title and objectives. Instead of standard after-lab-questions, ask students to complete the missing title and objectives. It is a fun twist and makes the lab more creative. Teachers of very young students can even play this up creating a scenario in which the teacher accidentally lost the rest!
- I suggest goggles for young students, especially if performing the experiment in groups or individually. When the balloons are blown up to a large size, small pieces of latex could injure the eye. I also suggest using something other than needles to bust the balloon. I always liked to go around the classroom and check on the apparatus set-up. Then, once the apparatus meets the standards, the teacher can bust the balloon.