In the February 2018 edition of ChemShorts For Kids you investigated how to make a chemical hand warmer. Although this month’s activity may not be particularly effective if you need to warm up, it describes an interesting phenomenon.

Materials:

• Thick rubber band
• Blow dryer
• Hammer
• Ice

Experiment:

With the rubber band taut, place it on your forehead noticing the temperature. Now, stretch the rubber band quickly and put it back to your forehead. Do you notice a temperature change? Now stretch the rubber band fully and keep it pulled fully for about 10 seconds. Place it against your forehead and let the rubber band go slack. Do you again notice a temperature change? Now hang the hammer from the rubber band and notice where the bottom of the hammer is. As you heat the rubber band with the hot hair dryer do you notice that the hammer is moving? You can also try to ice the rubber band and see if the hammer moves.

What’s happening?

Normally when you heat a substance, the substance gets larger or it expands. Water is a little odd here in that when you freeze it the ice takes up more space than the water did originally, but most substances don’t behave that way. A thermometer works because the mercury or alcohol in the thermometer expands as it gets heated and contracts when it is cooled. It’s why bridges have expansion joints in them so that when it gets hot, the surface won’t buckle. Rubber bands however are made up of very long chain polymers that are all twisted up on each other as seen in the diagram below. When you stretch the rubber band the molecules line up with each other better.

The reason that most things expand when heated is because when you heat a substance you give the atoms more motion or kinetic energy. Now imagine the stretched rubber from above and you cause those atoms to move more quickly. You might perceive that as the atoms in the middle move around more that you might shrink the rubber band. This is what happened with the hammer pulling on the rubber band and heating it, the rubber band shortened. The same is true when you start with a tight rubber band and you allow it to go slack, your forehead put heat into the rubber band and the rubber band felt cool as it pulled heat from you. This allowed the molecular strands to jumble up with the increased amount of kinetic energy in the molecules.

References:
https://www.scientificamerican.com/article/bring-science-home-rubber-band-heat/
https://revisionworld.com/a2-level-level-revision/physics/force-motion/solid-materials/rubber

To view all past “ChemShorts for Kids”, go to:
http://chicagoacs.org/articles/article_category/1

Paul Brandt