Key concepts
Physics
Heat
Molecules
Forces
 
Introduction
Have you ever noticed that some objects tend to expand when they get hot and contract when they cool down? For example, you might run hot water over the lid of a jar that's stuck—this causes the lid to expand, making it easier to twist off. Does this effect work the same way for all materials? Try this fun activity to find out!
 
Background
Materials are made up of atoms and molecules. (Molecules are groups of multiple atoms that are bonded together.) Even though you can't see it with the naked eye, these molecules are constantly vibrating around and bumping into one another. Normally when a material gets hotter, the molecules vibrate more intensely. Because they bump into one another harder this causes the material to expand a bit. This phenomenon is called thermal expansion, which is responsible for many of the cracks you see in roadways and sidewalks. As the temperature changes from very hot to very cold throughout the summer and winter, the materials expand and contract over and over again, eventually leading to cracks.
 
Rubber bands are different than many other materials because their molecules are actually very long chains, kind of like a tangled pile of spaghetti. These long chains are called polymers, which can behave in surprising ways when you heat or cool them. In this activity you will change the temperature of a rubber band using hot water and ice water. Do you think a rubber band will expand or contract when it heats up?
 
Materials

  • Two-liter soda bottle
  • Permanent marker
  • Scissors
  • Pencil
  • Small, thin rubber band
  • About 20 to 25 quarters
  • Tape (any kind will work)
  • Paper clip
  • Hot tap water
  • Ice cubes
  • Large shallow bowl or pan
  • Work area where it's easy to clean spills
  • Dish towel or paper towels
 
Preparation
  • Make sure you have a work area set up where you can easily clean spilled water, such as a kitchen counter.
  • Use adult assistance for cutting the bottle and when using very hot tap water.
 
Procedure
  • Have an adult use scissors to cut the top off a two-liter soda bottle, turning it into a cylinder shape.
  • Use tape to cover any sharp edges around the top rim of the bottle.
  • Use adult assistance to poke small holes (just large enough for a pencil to pass through) in opposite sides of the bottle using scissors, about one inch below the top edge.
  • Poke the pencil through one of the holes from the outside of the bottle.
  • Loop your small rubber band around the pencil.
  • Continue pushing the pencil through the hole on the other side of the bottle.
  • Tape together a stack of about 20 quarters, with a paper clip as a "hook" that you can use to hang them from the rubber band. How many quarters do you think your rubber band will be able to comfortably support? (The exact number of quarters you need will depend on the size and strength of your rubber band. You want the rubber band to visibly stretch out when you hang the quarters from it but not so much that it hits the bottom of the bottle. A strong rubber band may require more quarters, a weaker rubber band may require fewer.) You can try spreading the quarters out to make them easier to hang—for example, two stacks of 10 taped next to one another or four stacks of five taped next to one another—instead of one big stack of 20 quarters.
  • Place your two-liter bottle into a shallow bowl or pan to catch any water that spills.
  • Fill the bottle up to the pencil with hot tap water so the rubber band is completely submerged. Wait a minute or two for the rubber band to come to the same temperature as the water. What do you think might happen to the length of the rubber band?
  • Look at the bottle from the side so that your eyes are level with the bottom of the rubber band. Use a permanent marker to draw a line on the side of the bottle that is even with the bottom of the rubber band, and label it "hot."
  • Now, add ice cubes to the bottle and stir gently. This will cause the water level to rise, so some water will spill out of the holes you poked for the pencil.
  • If the water is still very hot, the ice cubes may melt quickly. Continue adding ice cubes and stirring gently until the bottle and water feels very cold. What do you think will happen to the length of the rubber band at this new temperature?
  • Look at the side of the bottle again, so your eyes are level with the bottom of the rubber band. Draw a new line that is even with the bottom of the rubber band and label it "cold."
  • Is your "cold" line above or below your "hot" line? Is this what you expected to happen? Why or why not?
  • Extra: You can quantify the activity by using a thermometer to measure the different water temperatures and a ruler to measure the lengths of the rubber band. If you take measurements at multiple points (for example, hot water, room-temperature water, ice water), then you can make a graph of your results.
  • Extra: Try other methods of heating and cooling instead of using hot and cold water. For example, pour the water out of the bottle, and heat the rubber band with a hair dryer. Then to cool it, try leaving the bottle in your refrigerator for about half an hour. Do you get the same results when you measure the rubber band?
  • Extra: Try the activity again with different rubber bands. Do some shrink or lengthen more or less than others? Why do you think that is?


Observations and results
Your results might have surprised you! The rubber band actually expands when it gets colder! This seems counterintuitive because most materials expand when they are heated and contract when they get cold. This occurs because of the unusual polymer structure of rubber. When the long chains get hotter and vibrate, they actually shorten, causing the material to contract. When the chains cool down, they relax and stretch out, causing the material to expand.
 
To help visualize this take any rope, string or cable you can find around the house that is a few feet long (short enough that you can hold one end of it in the air without the other end touching the floor). Hold the rope out so it hangs down straight (it is not vibrating) and observe how close the bottom end is to the floor. Now, shake your hand back and forth rapidly (to make the rope vibrate). Does the end of the rope get closer to the floor or farther away? It should get farther away as the rope "bunches up" and vibrates. The same thing happens when the polymer chains in rubber heat up and vibrate—they actually get shorter.
 
Cleanup
Pour the water down the drain (or use it—with permission—to water plants) and clean up any spilled water. Remove the pencil and tape from your two-liter bottle and recycle it—or save it for a future science activity!
 
More to explore
Heat Shrink! Why rubber bands get shorter when you heat them, from The Naked Scientist
Rubber Band Elasticity and Temperature, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies