Expansion of a gas at constant pressure

Class practical

Apparatus and Materials

For each group of students

• Washing-up bowls with hot and cold water from taps
• Round-bottomed flask
• Bung with narrow-bore tubing to fit
• Access to lubricating oil

Health & Safety and Technical Notes

Warn the class to handle the long glass tubes carefully as they are (relatively) easily broken.

Read our standard health & safety guidance

Glass test-tubes and corks with capillary tubing can be used in place of the flasks. The drop of oil is added to the bung-end of the tube before insertion into the flask.

Procedure

1. Trap the air in the flask with a small bead of oil in the glass tubing. Gently heat the flask with your hand. This will produce a sufficient temperature rise for the oil index to move up the tubing.
2. Plunge the flask first into cold and then into warm (not hot) water.

Teaching Notes

• The oil plug will rise up the glass tube as the air in the flask expands (in warm water) and fall as the air contracts (in cold water). The volume expansion of a gas is approximately 500 times that of glass, so it is unlikely that the expansion of the flask will have any noticeable effect.
• Students may already know that a gas is made up of rapidly moving molecules which hit the surfaces of the container and exert a force on the container so creating a pressure. If the temperature of the container is raised, molecules move faster. The pressure exerted by the gas will therefore increase.
• Plunging the flask into hot water may increase the volume of the gas so much that either the oil plug flies out of the tube, or it breaks up and runs down the inside of the tube.

This experiment was safety-tested in August 2006