Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

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 Technial notes

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

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.



round-bottomed flasha 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. 
b Plunge the flask first into cold and then into warm (not hot) water.


Teaching notes

1 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. 

2 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, energy is transferred to the gas as faster movement of its molecules. The pressure exerted by the gas will therefore increase. 
3 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-checked in August 2006


Related guidance

Theoretical thoughts: extrapolation


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