Science corner

24th June 2005 at 01:00
Sophie Duncan looks at some effects of heating air

This easy experiment helps your students explore what happens when you heat air.

Take a glass bottle and place it in a bucket of cold water (making sure you don't get water into the bottle). Rub the bottle opening with water or oil. Remove the bottle from the bucket and place a copper coin over the mouth of the bottle. Place your hands around the bottle and observe what happens. You should find that the coin jumps up and down.

This experiment is effective as a classroom demonstration. Explain to your students what you are going to do, and ask them to predict what they think will happen and why. Once you have done the experiment, ask them to discuss their predictions.

The bottle is cooled down when it is placed in the cold water. Air can move freely into and out of the bottle so the air pressure inside and outside the bottle are the same. The coin over the bottle opening seals it. Placing your hands around the bottle warms the air inside. This causes the air to expand. However the air cannot escape from the bottle, so the pressure inside the bottle increases. When the pressure inside the bottle is greater than the combination of the air pressure outside the bottle and the weight of the coin, the coin is pushed upwards. It then falls again, and the process is repeated. This happens until the temperature of the air inside the bottle is the same as the air temperature outside the bottle. (If the coin remains stationary, the air may be escaping around the rim of the bottle. Put more oil or water on the lip of the bottle, to make sure the coin makes a seal with the glass.) To see how much the air inside the bottle expands, repeat the experiment, but this time place a balloon over the mouth of the bottle.

Warming the air inside the bottle will cause the air to expand and fill the balloon. If you then cool the bottle down again, the balloon will deflate.

Noc-5 Sophie Duncan is project manager for science at the BBC

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