Thermal expansion of air - Charles' law

Class practical

Expansion of air at constant pressure and an indication of absolute zero (-273°C).

Apparatus and Materials

For each group of students

• Concentrated sulfuric acid
• Beaker, deep (see technical note)
• Bunsen burner
• Capillary tube with liquid index (see technical note)
• Tripod
• Thermometer -10°C to 110°C
• Ruler, 300 mm
• Lab oven (if available)
• Rubber bands

Health & Safety and Technical Notes

When you seal the tubes, avoid a long taper at the end.

Take care when handling concentrated sulfuric acid.

Read our standard health & safety guidance

It is almost impossible to obtain tubing with this bore and not a very thick wall: 3 mm outer diameter and 1 mm inner diameter is ideal. If suppliers will not stock it, this experiment could be omitted.

The beaker should be deep enough for all the air column in the tube to be immersed in water.

The capillary tubes should be about 20 cm long and 1 mm bore, closed at one end and open at the other.

To insert the index:

1. Put all the tubes into a deep beaker of strong brine and bring it to the boil. This will ensure that the tubes are dry internally as well as hot.
2. Whilst hot, dip the open end of each tube into a small beaker containing concentrated sulfuric acid, so that a 5 mm length of acid is drawn in to the tube as it cools.
3. Leave the tube to cool further. The index will be drawn to a suitable position just over halfway along.

If a laboratory oven is available, the tubes can be heated to 120°C for about 20 minutes instead of using brine.

Sulphuric acid is hydrophilic; it removes water vapour from the air trapped in the capillary tube. If you do not like working with acid, the experiment could be tried with an oil plug but the water vapour pressure in the air column would then distort the results.

Procedure

1. Fix the capillary tube and thermometer to the ruler with rubber bands at each end. Measurements are easier if the end of the air column inside the tube coincides with the zero of the centimetre scale on the rule.
2. Put the tubes into the deep beakers with the open end free to the air.
3. Add water, or water and crushed ice, to the beaker, always ensuring the trapped air column is under the surface of the water in the beaker. Stir.
4. Record the temperature and length of the air column.
5. Repeat for about four more temperatures, plotting the graph as the experiment proceeds.

Teaching Notes

• Students need to appreciate that the way the length of column changes reveals how the volume of the air alters ( volume = length x cross-sectional area).
• The data should give straight-line graphs, although there may be a need to discuss the uncertainties in the readings.
• Once students recongnize that the length/volume of the air column decreases as the temperature falls, ask whether they could predict a temperature at which the pressure would be zero.
• An alternative method could use a test-tube with a capillary tube through its bung, as in the experiment:

  Expansion of a gas at constant pressure

This experiment was safety-tested in August 2006