Moving molecules: Rubber bands don't always behave the way you might expect when heat is involved. Their particular molecular structure helps to explain why. You'll learn more about it in this simple and snappy activity. Image: George Retseck
Ever wonder why rubber bands so easily snap back into place after being stretched? When stretched out, rubber pulls back hard to return to its original shape. It’s pretty resilient stuff! Of course, you’ll find that if you leave a rubber band wrapped around something long enough—say, a year or two—it will eventually stretch out permanently and may even snap. But what is it about rubber that makes it so stubbornly resist being permanently deformed? The answer has a lot to do with its unusual molecular structure and how thermal energy, or heat, constantly acts on its molecules. In this activity you’ll grab a hair dryer and a weight to explore just how rubber molecules go to work.
Matter usually takes up more space when it’s warmer and less space when it’s cooler. Scientists refer to this tendency as thermal expansion, and it’s most obvious in gases. If you’ve ever left a water bottle in a hot car, you’ll be familiar with the hiss of hot air and water vapor that escapes when you remove the cap. When it comes to liquids, thermal expansion helps explain how a thermometer works. Heated by warm temperatures, the expanding red liquid has nowhere to go except up the thermometer’s glass tube. It’s usually toughest to recognize thermal expansion in solid objects, but during a hot summer long sections of train tracks can actually expand and buckle due to the stress of thermal expansion compressing the rails lengthwise.
• Thick, broad rubber band
• Ice cube
• Graph paper
• Hair blow dryer
• Something relatively heavy to act as a weight (a hammer, a stapler or bicycle u-lock all work perfectly well)
• Graph paper
• Take a thick, broad rubber band and pinch one end with each hand. Gently pull it taught without stretching it out.
• Carefully put the rubber band to your face. Note the temperature of the rubber band. What do you notice?
• Quickly stretch the rubber band out, and keep it stretched. Put your face to the rubber band again. How did the temperature change? Why do you think this is?
• Don’t let the rubber band return to its original shape just yet—our goal is to let it return to room temperature first! Move the rubber band away from your face, and wait about 15 seconds.
• After 15 seconds have elapsed quickly release the tension on the rubber band without letting go, allowing it to contract and return to its original shape. Put it to your face again. What do you notice about the rubber band’s temperature this time? How do you think we can explain this?
• Tape a piece of graph paper to the wall at about shoulder height.
• Loop your rubber band around your weight so that the weight can hang from the rubber band.
• Pinch the other end of the rubber band between the thumb and forefinger of your nondominant hand (your left hand if you are right-handed, for example). Put the side of this hand against the wall at a point slightly above the graph paper so that the weight hangs freely from the rubber band. Make sure the weight isn’t in contact with the wall.
• With your other hand use a pen to mark the position of the bottom of the weight on the graph paper.
• Take your blow dryer and make sure it’s set to hot. Turn it on and direct the hot air toward the rubber band for about 15 seconds. Angle it downward so that you don’t burn your hand!
• After heating the rubber band, determine if the weight moved up or down. Using your pen, mark the new position of the bottom of the weight. Is what you observe what you expected? How do you think you can explain your observations?
• Wait a few seconds to allow the rubber band to return to room temperature.
• Now, rub the ice cube on the surface of the rubber band. Watch the weight carefully and determine whether it moves up or down. Mark the new position of the bottom of the weight. Is what you observe what you expected? How do you think you can explain your observations? What do you notice about the relative positions of all the marks you made?