Author: Sandy Van Natta
Source: "The Rubber Band Stretch", Chain Gang - The Chemistry of Polymers, Sarquis, M., editor, Terrific Science Press, Center for Chemical Education, Miami University Middletown, Middletown, Ohio, 1995, pgs. 41- 46.
To show students how metals react when heated, either:
Pass the ball through the ring of a ball and ring apparatus when both ball and ring are at room temperature. Heat the ball and try to pass it through the ring again. This time it will not fit through.
Ball and Ring Demonstration Video
Set up a ring stand with a small ring clamp attached to it. Demonstrate that the beaker or jar will not pass through the ring. (Make sure the diameter of the beaker or jar is just barely larger than the ring.) Heat the ring with a burner. Pass the bottle or beaker through the ring and be sure to catch it to prevent it from breaking.
Ask students what prevented the ball from passing through the ring or what allowed the glass jar or beaker to pass through the ring. Next, ask students what they think will happen if an elastic object such as a rubber band is heated.
Assessment: Assessment is informal at this stage. Make sure all students are contributing to the discussion and that they make note of the class ideas at this time. Most students will be able to explain that metals expand upon heating. If the ball and ring demonstration is preformed, they will conclude that the metal ball expanded, preventing it from passing through the ring. If the ring clamp and glass jar are used, they will conclude that the ring expanded upon heating. However, students have had little personal experience with heating rubber, a long chain-like molecule. Most likely they will predict that the rubber band will stretch.
Procedure: (Can be done in groups or as a demonstration if only one heat gun is available)
1. Make one cut in the rubber band so that it forms one long, straight strip.
2. Tie the rubber band to a ring clamp attached to a ring stand.
3. Tie a weight of about 100 g to the opposite end of the rubber band.
4. Position the height of the ring clamp so that the weight just barely touches the base of the ring stand or top of the table.
5. Heat the entire rubber band with a heat gun or hair dryer by moving the gun up and down the length of the rubber band. (Note - if the rubber band is heated too much in one spot, it may melt and/or break)
6. Observe the position of the weight. (When the rubber band is heated, it will contract and the weight rises.)
7. Optional: Use a meter stick to measure the amount of contraction of the rubber band by measuring the height of the weight from the ring stand base or table top. You may also wish to try rubber bands of various thicknesses to look for changes in length. However, if you try a thicker rubber band, you will need a larger weight, 500 g to 1 kg, to provide an easily measurable difference in length.
Assessment: See that students are following proper laboratory procedures. Monitor the groups and make sure they are recording all their observations and data.
Most substances, like the metal used in the engagement activity, expand when heated and contract when cooled. An increase in temperature causes particles in a substance to jiggle faster and move farther apart. The result is thermal expansion. Some examples of this are telephone wires that sag more on a hot summer day than in winter, the loosening of a metal jar lid when placed under hot water, and the closing of expansion gaps on bridges in the summertime. Rubber, however, is different due to the chain-like makeup of its molecules.
Many students will be surprised to learn that a rubber band shortens when heated. Explain to students that rubber is a long chain molecule made up of repeating units called a polymer. The following enactment will help students understand what is going on when the rubber band is heated.
Have students hold onto a rope, at regular intervals, and stretch it across the room. Explain that the rope represents part of the polymer chain in the rubber band and note that the polymer chain is stretched to its limit.
Explain that warming the rubber band gives energy to the atoms in the chain, which in turn, causes increased molecular motion. To simulate this motion, students in the middle of the rope should start to move back and forth, perpendicular to the length of the rope. Students at the ends of the rope will need to move inwards to accommodate this motion. This movement can only occur if the distance between the two ends decreases. This is analogous to the rubber band becoming shorter when heated.
For the rubber band to stretch again, it must lose energy. The loss of energy decreases molecular motion and the ends move further apart. To simulate this loss of energy, have the students return to their original positions.
Point out to students that many materials expand during heating, but many polymers, such as rubber bands, contract when heated because of increased molecular motion within the polymer. As the rubber band is heated, the energy that is added causes components of the polymer chain to move and become less ordered.
Assessment: Ask students to write a paragraph explaining the shortening of the polymer chains when heated. You may also assess their understanding by asking them to predict what will happen to the length of a rubber band placed in a freezer. After students have made their predictions, you may choose to place a rubber band, cut into one long strip, into a freezer. Have students measure the length of the rubber band before placing it in the freezer and after it has cooled in the freezer for at least 1/2 hour. The rubber band will increase in length by a few millimeters. The band must be measured quickly however, to prevent it from warming and returning to its original length.
Part I. Extend students' understanding of temperature changes to rubber by conducting the following activity:
Warn students not to stretch the rubber band so far that it may break when held against their skin. No other special safety precautions are needed here.
Have each student touch a rubber band to the skin above his or her upper lip or forehead. Ask students to determine if it feels hot or cold and have them record their observations.
Have students predict what will happen to the temperature of the rubber band when it stretches.
Ask students to hold the rubber band firmly against the skin of their upper lip or forehead and have them quickly stretch the rubber band. Ask them to describe the temperature as warmer or colder to their skin and record their observations.
Have students predict what will happen to the temperature of the rubber band when it contracts.
Ask students to again stretch the rubber band against their skin in the same location, hold the stretched rubber band stationary for 1 to 2 seconds, and then allow the rubber band to contract while still holding it against the skin. Again, ask students to describe the temperature as warmer or colder to their skin and record their observations.
When asked to describe their results to the rest of the class, most, if not all students, will describe a temperature increase experienced while stretching the rubber band and a temperature decrease while allowing the rubber band to contract.
Assessment: Ask students how their observations from the rubber band stretch can be explained by their results and knowledge gained in the previous sections of this activity. Depending on the age of the students they may need some guidance formulating the following explanation. To stretch a rubber band, the band must lose energy. This energy is transferred to the skin on the lip or forehead accounting for the warmth that was felt. For a rubber band to contract, it must gain energy. The heat from the heat gun applied it in the exploration section. In this section, the heat from the lip or forehead supplies the energy. That is why the contracted rubber band feels cooler than the stretched rubber band.
Part II. Interesting changes to rubber can be observed when it is placed in liquid nitrogen.
Effects of Liquid Nitrogen on Rubber
Effects of Liquid Nitrogen on Natural Rubber