Bring Science Home

Size-Changing Science: How Gases Contract and Expand

A chemistry challenge from Science Buddies

The amazing shrinking balloon: just add cold!
George Retseck

Key concepts
States of matter
Have you ever baked—or purchased—a loaf of bread, muffins or cupcakes and admired the fluffy final product? If so, you have appreciated the work of expanding gases! They are everywhere—from the kitchen to the cosmos. You’ve sampled their pleasures every time you’ve eaten a slice of bread, bitten into a cookie or sipped a soda. In this science activity you’ll capture a gas in a stretchy container you’re probably pretty familiar with—a balloon! This will let you to observe how gases expand and contract as the temperature changes.
Everything in the world around you is made up of matter, including an inflated balloon and what’s inside of it. Matter comes in four different forms, known as states, which go (generally) from lowest to highest energy. They are: solids, liquids, gases and plasmas. Gases, such as the air or helium inside a balloon, take the shape of the containers they’re in. They spread out so that the space is filled up evenly with gas molecules. The gas molecules are not connected. They move in a straight line until they bounce into another gas molecule or hit the container’s wall, and then they rebound and continue in another direction until they hit something else. The combined motion energy of all of the gas molecules in a container is called the average kinetic energy.
This average kinetic (motional) energy changes in response to temperature. When gas molecules are warmed, their average kinetic energy also increases. This means they move faster and have more frequent and harder collisions inside of the balloon. When cooled, the kinetic energy of the gas molecules decreases, meaning they move more slowly and have less frequent and weaker collisions.

  • Freezer with some empty space
  • Two latex balloons that will inflate to approximately nine to 12 inches
  • Piece of string, at least 20 inches long
  • Permanent marker
  • Cloth tape measure. (A regular tape measure or ruler can also work, but a cloth tape measure is preferable.)
  • Scrap piece of paper and a pen or pencil
  • Clock or timer
  • A helper
  • Make sure your freezer has enough space to easily fit an inflated balloon inside. The balloon should not be smushed or squeezed at all. If you need to move food to make space, be sure to get permission from anybody who stores food in the freezer. Also make sure to avoid any pointy objects or parts of the freezer.
  • Blow up a balloon until it is mostly—but not completely—full. Then carefully tie it off with a knot. With your helper assisting you, measure the circumference of the widest part of the balloon using a cloth tape measure or a piece of string (and then measure the string against a tape measure). What is the balloon’s circumference?
  • Inflate another balloon so it looks about the same size as the first balloon, but don’t tie it off yet. Pinch the opening closed between your thumb and finger so the air cannot escape. Have your helper measure the circumference of the balloon, then adjust the amount of air inside until it is within about half an inch or less (plus or minus) of the first balloon’s circumference (by blowing in more air, or letting a little escape). Then tie off the second balloon.
  • Turn one of the balloons so you can look at the top of it. At the very top it should have a slightly darker spot. Using the permanent marker, carefully make a small spot in the center of the darker spot.
  • Then take a cloth tape measure (or use a piece of string and a regular tape measure or ruler) and carefully make two small lines with the permanent marker at the top of the balloon that are two and one half inches away from one another, with the darker spot as the midpoint. To do this you can center the tape measure so that its one-and-one-quarter-inch mark is on the small spot you made and then make a line at the zero and two-and-one-half-inch points.
  • Repeat this with the other balloon so that it also has lines that are two and one half inches apart on its top.
  • Somewhere on one balloon write the number “1” and on the other balloon write the number “2.”
  • Because it can be difficult to draw exact lines on a balloon with a thick permanent marker, now measure the exact distance between the two lines you drew on each balloon, measuring from the outside of both lines. (For example, the distance might be two and three eighths inches or two and five eighths inches.) Write this down for each balloon (with the balloon’s number) on a scrap piece of paper. Why do you think it’s important to be so exact when measuring the distances?
  • Put balloon number 1 in the freezer in the area you cleared out for it. Leave it in the freezer for 45 minutes. Do not disturb it or open the freezer during this time. How do you think the size of the balloon will change from being in the freezer?
  • During this time, leave balloon number 2 somewhere out at room temperature (not in direct sunlight or near a hot lamp).
  • After balloon number 1 has been in the freezer for 45 minutes, bring your cloth tape measure (or piece of string and regular tape measure) to the freezer and, with the balloon still in the freezer (but with the freezer door open to let you access the balloon), quickly measure the distance between the two lines as you did before. Did the distance between the two lines change? If so, how did it change? What does this tell you about whether the size of the balloon changed? Why do you think this is?
  • Then measure the distance between the two lines on balloon number 2, which stayed at room temperature. Did the distance between the two lines change? If so, how did it change? How did the balloon’s size change? Why do you think this is?
  • Overall, how did the balloon change size when placed in the freezer? What do your results tell you about how gases expand and contract as temperature changes?
  • Extra: After taking balloon number 1 out of the freezer leave it at room temperature for at least 45 minutes to let it warm up. Then remeasure the distance between the lines. How has the balloon changed size after warming up, if it changed at all?
  • Extra: Try this activity again but instead of putting balloon number 1 in the freezer, put it in a hot place for 45 minutes, such as outdoors on a hot day or inside a car on a warm day. (Just make sure the balloon is not in direct sunlight or near a hot lamp, as this can deflate the balloon by letting the gas escape.) Does the balloon change size when put in a hot place? If so, how?
  • Extra: In this activity you used air from your lungs but other gases might behave differently. You could try this activity again but this time fill the balloons with helium. How does using helium affect how the balloon changes size when placed in a freezer?

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