THE MAGIC BALLOON
Knight Foundation Summer Institute
Emily Dorean, Haverford College
Air is made up of millions of atoms and molecules bouncing around pushing each other. There are so many of these particles bumping into each other that they create a pressure called air pressure. This force affects our everyday lives in many ways, even though we can't see air, and even have a hard time feeling it. We can test to see that air takes up space (see below), but in this experiment we are going to investigate what causes air to expand, or how air can take up an increasing or decreasing amount of space.
As an introduction to this experiment, have the students brainstorm about some preliminary tests to determine that air takes up space. For example, have them fill their cheeks with air like a chipmunk, and then push on one side or the other. They will see that pushing in one side pushes out the other, which indicates that air takes up space in their mouths. You could also blow up a balloon to demonstrate that air takes up space within the confines of the balloon. Talk about what causes increases or decreases in air pressure, which can be related to how the air pressure drops after a summer storm (This is due to a change in pressure in the atmosphere). This can lead into inquiry and determination of a problem to solve in the lab; to find what it is that causes air to expand. The students will be able to observe and test that heat causes air to expand or to increase pressure, filling a balloon with air, and that when the air is cooled it contracts, causing the balloon to deflate.
Divide the class into small cooperative groups, or set this up as a station, and give each group a bottle and a balloon. They can then:
Have each cooperative group turn in a formal lab report complete with pictures and suggestions for improving the results of the experiment. It is important that they state their hypothesis and how the results fit or did not fit with that hypothesis.
Lead the students in a class discussion about why the balloon was blown up after the jar was placed in the hot water. Ask them what caused the air to expand, and how they came to that conclusion. You could talk about why helium balloons float at room temperature, what might cause helium balloons to deflate, what effect the air pressure is having in that case, and what the difference is when a student blows up a balloon.
Help the students to brainstorm different ways in which they observe the effects of air pressure in their daily lives.
Another experiment that is very similar, although it doesn't use ice. is to put approximately 100ml of boiling water in a bottle, and quickly stretch a balloon over the top. This will "magically" create a balloon inside the bottle, due to the decrease in pressure in the bottle as the hot air vapor cools and partially recondenses into water. Another good experiment about air pressure can be found at the web site http://ww.miamisci.org/hurricane/weatherstation.html, which presents a view of the effects of air pressure in natural disasters. Reference the lab in this booklet "Tornado in a Bottle" for several similar concepts about air pressure.
Philadelphia Science Content Standards:
SCIENCE CONTENT STANDARD 1: NATURE OF SCIENCE
This experiment satisfies Benchmark 3 for grades 5-8: "collect and summarize data from an experiment and interpret the results in terms of the data."
SCIENCE CONTENT STANDARD 2: PHYSICAL SETTING
This experiment satisfies Benchmark 4 for grades 5-8 "investigate the relationship between force and motion."
This is great as a component of a unit on storms and weather patterns, as a conceptual understanding of air pressure and the change in temperature associated with this. See the lab in this packet entitled Tornado in a Bottle for further suggestions. The lab report can also include mathematical skills, such as graphing.