THE MAGIC BALLOON

Knight Foundation Summer Institute

Stephen Walker

Emily Dorean, Haverford College

Introduction:

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.

Objectives:

1. To understand how heat causes air to expand, and to relate that to how it may affect our lives everyday.
2. To thoughtfully set up the preliminary inquiry needed in order to test what causes air to expand.
3. To critically observe and record their observations, and to write a lab report that accurately conveys the results of their experiment.

Vocabulary:

air

contract

expand

heat

high pressure

low pressure

Materials:

• 1 small narrow-mouth bottle per cooperative group (a 16 oz. soda bottle works well)
• 1 balloon
• 1 pan or bowl of ice water, if possible.
• 1 pan or bowl of hot water (I found that covering the bottle in an ice bucket, and then submerging the bottle in near-boiling water gave the most drastic results. However, this does work using only hot and cold water from the tap. The response is just not quite as dramatic.)

Procedure:

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:

1. Stretch the end of the deflated balloon (leaving little air in it) over the top of the bottle.
2. Place the bottle in ice or ice water for 10 minutes. This time is approximate; feel free to leave the bottle in the ice as long as you want, after several minutes. This would be a good opportunity for a mini-lecture on air pressure, or for the students to prepare a data sheet. This cold bottle and balloon could also be prepared in advance.
3. Remove the bottle with the balloon from the ice and place it directly into the hot water.
4. Observe for a few minutes and record your observations. What do you see?
5. Return the bottle to the ice. Observe and record.

Assessments:

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.

Extensions:

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.