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!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
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?
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.
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Ben Finio, PhD, Science Buddies
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