Variation of gas pressure with temperature

Class practical

This experiment leads to the concept of absolute zero. Students investigate the effect of temperature on the pressure of a fixed volume of gas.

Apparatus and Materials

For each student group

• Bourdon gauge
• Flask, 250 ml, fitted with bung, glass tube and polythene or rubber tubing
• Metal container for flask, fitted with wire stirrers
• Thermometer -10°C to 110°C
• Crushed ice
• Electric kettle to provide hot water OR Bunsen burner and tripod
• Retort stand, boss, and clamp

Health & Safety and Technical Notes

Ensure that the can support can be removed, so that the can may be lowered safely without moving the flask.

Read our standard health & safety guidance

School kitchens might be a source of deep tins large enough to contain the flasks, stirrers and thermometers.

The flasks need to be immersed up to their bungs. Wire each bung to the neck of its flask so it cannot easily be dislodged. The stirrers need to be able to move past the flasks; their use is vital to the success of this experiment.

Procedure

1. Ensure the tubing from the flask fits tightly onto its Bourdon gauge and is as narrow and short as possible. Using stands, bosses and clamps, if necessary, hold the bung and the Bourdon gauge.
2. Put a small amount of ice into the container, add the stirrer and bring it up around the flask before adding some more ice. Then top up the water so all the air in the flask is immersed.
3. Stir and record the temperature and corresponding pressure.
4. Carefully remove the can and empty it of ice. Replace and add cold water.
5. Stir and record the temperature and corresponding pressure.
6. Repeat with mixtures of cold and hot water, and finally with just hot water. Aim to get five sets of readings with roughly evenly separated temperatures.
7. Plot a graph of pressure against temperature.

Teaching Notes

• The data should give straight-line graphs, although there may be a need to discuss the uncertainties in the readings.
• Once students recognize that the pressure falls as the temperature decreases, ask whether we could predict a temperature at which the pressure would be zero. Get them to consider what happens to the motion of the molecules as the temperature falls.
• Either by calculation or - better - by drawing onto an extra sheet of graph paper, get them to extrapolate to find values of the temperature at which the pressure would be zero. Discuss its significance.
• With an able group, this provides an opportunity to discuss the benefits and dangers of both extrapolation and interpolation. Interpolation can be safe but dull. Extrapolation is rash but sometimes very fruitful.
• A discussion with students could follow:
• "What happens to the pressure of air when heated like that? If pressure is just the result of molecules bombarding the walls, why does the pressure change?"
• "How can the molecules make a bigger pressure? There are just as many as before. (Heating cannot manufacture more molecules.) So the same lot of molecules make more pressure. How?"
• You hope that students will say "the molecules must bombard the walls more often and more violently".
• Coax students to statements like: "thus if more often, the molecules must be moving faster; and if more violently, something must have changed to make the collision more violent".
• Suggest that "if a gas molecule makes a more violent collision when it hits the wall, that must be because it is moving faster."
• Both suggestions, more frequent collisions and more violent collisions lead to the idea of molecules moving faster in a hotter gas.
• Then reverse the story: "If molecules move faster, they will make more violent collisions (change more momentum) and they will arrive back at the wall for another bang more often. So you might expect the speed to appear twice over as a factor in the pressure. Pressure varies as temperature, and as (speed) ² . Energy stored kinetically also varies as (speed) ² . Temperature is related to the energy stored kinetically.

This experiment was safety-checked in August 2006