Air Density: Does warm air take up less room than cool air?
Purpose/Hypothesis
Density is the mass of anything divided by the volume it occupies. As the temperature of a given mass of air increases, its volume expands and the air gets less dense as a result--same mass, but larger volume, means less dense. As the temperature of a given mass of air decreases, its volume contracts and the air gets more dense. In this experiment, you will examine the density of air by causing a mass of air in a closed container to become both more and less dense by changing the temperature. To see these changes you will place a balloon over the open end of a bottle. When the trapped air expands, the balloon should get bigger; when the air contracts, the balloon should get smaller.
Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of air density. This educated guess, or prediction, is your hypothesis.
A hypothesis should explain these things:
- the topic of the experiment
- the variable you will change
- the variable you will measure
- what you expect to happen
A hypothesis should be brief, specific, and measurable. It must be something you can test through further investigation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "As the air gets warmer and less dense it will cause the the balloon to get larger; as the air gets cooler and less dense it will cause the balloon to get smaller."
Level of Difficulty
- Easy.
Materials Needed
- two rubber balloons
- ice
- hot water
- two plastic bottles, such as plastic soda bottles
- two containers that go at least midway up the sides of the bottles (one should be heatproof)
Approximate Budget
- $2
Timetable
- 15 minutes.
How to Experiment Safely
- Have an adult present when working with the hot water.
Step-by-Step Instructions
- Place a balloon over the mouth of each plastic bottle. Leave one bottle out as your control.
- Fill up one container with very hot water. Fill up the other container with a little ice and some cold water.
- Place the experimental bottle in the container of cold water and hold it there for roughly 1 minute. (Another option is to place the bottle in a freezer for 1 minute.) Note the size of the balloon compared to the control balloon.
- Place the experimental bottle in the container of hot water for 1 minute. (Another option is to carefully hold the bottle under running hot tap water.) Note the size of the balloon compared to the control balloon.
- Again, place the experimental bottle in the pan of cold water and hold for 30 seconds.
Summary of Results
Examine how much the balloon grew or shrunk in your experiment. Was your hypothesis correct? How did the size of the experimental balloon compare to the control balloon?
Did the experimental balloon shrink more or at a different rate the second time you placed it in the cold water? Draw a picture of the results of your experiment and write a brief summary.
Troubleshooter's Guide
Below is a problem that may arise during this experiment, some possible causes, and some ways to remedy the problems.
Problem: Nothing happened to the balloon.
Possible causes:
- Your water may not have been hot or cold enough. You may also not given enough time to allow the air temperature to change. Try the experiment again, placing your bottles deeper into the hot and cold water.
- Your balloon may have a slight leak. Try the experiment again with a new balloon.