Stretch It! How does temperature affect a rubber band?
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!
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 each other. Normally, when a material gets hotter, the molecules vibrate more intensely. Since they bump into each other harder, this causes the material to expand a bit. This phenomenon is called thermal expansion. Thermal expansion 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, and eventually this leads to cracks.
Rubber bands are unique because their molecules are actually very long chains, kind of like a tangled pile of spaghetti. These long chains are called polymers. Polymers can have some surprising behavior when you heat them up or cool them down. 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?
- 2 liter soda bottle
- Permanent marker
- Small, thin rubber band
- About 20-25 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 up spills
- Dish towel or paper towels
- Make sure you have a work area set up where you can easily clean up spilled water, like a kitchen counter.
- Cutting a 2 liter soda bottle can create sharp edges. Adult supervision is required for cutting the bottle.
- Hot tap water can burn you. Adult supervision is required for filling the bottle with hot water.
- Have an adult use scissors to cut the top off a 2 liter soda bottle, turning it into a cylinder shape.
- Use tape to cover up any sharp edges around the top rim of the bottle, to avoid cutting yourself.
- Use scissors to poke small holes in opposite sides of the bottle, about 1 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.
- 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 quarters.
- Try spreading the quarters out to make them easier to hang - for example, 2 stacks of 10 taped next to each other, or 4 stacks of 5 taped next to each other - instead of one big stack of 20 quarters.
- Place your 2 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.
- Look at the bottle from the side, so 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 feel ice-cold.
- 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? Keep reading below for an explanation.
Extra: you can use a thermometer to measure the temperature of the water, and a ruler to measure the length 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 try leaving the bottle in your refrigerator for about half an hour. Do you get the same results?
Observations and Results
Your results might have surprised you - the rubber band actually expands when it gets colder! This seems counterintuitive, since most materials expand when they heat up, and contract when they get cold. This occurs because of the unique 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 yourself 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 (this makes 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.
Ask an Expert
- Pour the water down the drain and clean up any spilled water. Remember to recycle your 2 liter bottle, or save it for a future science experiment!
- Science Activities for All Ages!, from Science Buddies
- Rubber Band Elasticity and Temperature, from Science Buddies
- Heat Shrink! Why rubber bands get shorter when you heat them, from The Naked Scientist