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Cool Hot Rod

Science Snack
Cool Hot Rod
Objects change size when heated or cooled.
Cool Hot Rod
Objects change size when heated or cooled.

Changes in temperature cause objects to expand or contract. Try this experiment and directly observe the expansion and contraction of a metal tube.

Tools and Materials
  • Three to six feet (1 to 2 meters) of straight 1/4-inch (6-millimeter) copper tubing
  • Small funnel
  • A four-inch (10-centimeter) length of plastic tubing that will fit snugly over the end of the copper tubing and over the end of the funnel
  • Ring stand
  • A table
  • Two small blocks of smooth wood
  • A C-clamp
  • Large needle
  • Toothpick
  • Bucket
  • Very hot tap water
  • Cold water
  • Optional: Microscope slide, rubber band
Assembly
  1. Insert one end of the copper tubing into the plastic tubing, then slip the plastic tubing over the end of the funnel. Make sure that the tubes all fit snugly together. (See photo above for detail.)
  2. Place the ring stand near the edge of a table, positioning the ring so it supports the funnel a few inches above the tabletop.
  3. Position the copper tubing so that the end farthest from the funnel sticks out beyond the edge of the table by a few inches. 
  4. Place a small block under the copper tubing at the end near the funnel, then clamp the tubing and block to the table so that they can’t move.
  5. Put the second block under the other end of the tubing. Place the needle between the copper tubing and the second block, positioned perpendicular to the tubing. Make sure that the eye of the needle extends past the block.
  6. Stick the toothpick through the eye of the needle (see diagram; click to enlarge). As the tubing expands and contracts, the needle will rotate, rolled along by the movement of the tubing. The toothpick will shift from an upright position to a slanted position as the needle rotates, making the rotation more evident.
  7. Finally, place the bucket under the end of the copper tubing that sticks out beyond the edge of the table. The bucket will catch the water that you will pour through the copper tubing.
To Do and Notice

Pour very hot tap water into the funnel to heat the tubing. 

When you pour the hot water into the funnel, notice the direction in which the needle rotates. Immediately pour cold water through the funnel, and watch the needle again. Notice the direction in which it rotates.

What’s Going On?

The copper tubing, like everything else in the world, is made of atoms that are constantly vibrating. The higher the temperature, the faster the atoms vibrate. When you pour hot water into the tubing, heat flows from the water to the copper, giving energy to the copper atoms, which vibrate faster. This increase in vibration causes the atoms to collide with each other more often and more violently, so the space between the atoms increases. As a result, the whole tube gets longer and thicker. The needle turns as the tube expands.

When you pour cold water into the tube, the copper atoms give up some of their heat energy to the water, vibrate less violently, and move closer together. The tube shrinks and the needle turns in the opposite direction as the tube contracts.

The copper tube expands by 1.7 × 10–5 of its length for every 1.8 degrees Fahrenheit (1 degree Celsius) of temperature increase. So, a copper tube that is 3.3 ft (1 m) long will expand by 5.6 × 10–3 ft (1.7 × 10–3 m) over a 180-degree Fahrenheit (100-degree Celsius) temperature change, lengthening by almost 0.06 inch (1.7 mm).

As the copper tube expands, it will make the needle roll over this distance of 0.06 in (1.7 mm). When an average-sized needle rolls 0.06 in (1.7 mm), it makes more than two complete revolutions. The toothpick in the eye of the needle dramatically amplifies the motion of the expanding or contracting rod.

Going Further

You may have noticed the effects of this kind of change around your house. For example, if you run cool water on a hot glass, it may break, as some parts of the glass expand more rapidly than others. You can also loosen a tight jar lid by running hot water over it, causing the metal lid to expand more than the glass. The expansion and contraction of materials when they are heated or cooled is commonly used to make thermometers and control thermostats.