4.Main Content
experiments
Air expanding at constant pressure: Charles’ Law
Class experiment
This experiment leads to the concept of Absolute Zero. Studetns investigate the effect of temperature on the volume of a gas at fixed pressure (Charles' Law).
Apparatus and materials
- Beaker, deep
- Tripod
- Thermometer -10°C to 110°C
- Waterproof scale, marked in millimetres, preferably glass
- Rubber bands
- Quill tube (see technical notes)
- Access to mixed crushed ice, cold water and hot water from a kettle
Technical notes
The quill tubes should be about 20 cm long, sealed at one end with a liquid index between 0.5 and1.0 cm long placed just over half way along. The index could be mercury but to help ensure the trapped air is dry, concentrated sulphuric acid is better.
Prepare the apparatus by taking the unsealed tubes, dipping an end into some acid, placing a gloved finger over the other end, lifting the tube out of the liquid and tilting it to get the index in about the right place. Then seal one end in a Bunsen flame. With a bit of practice this can be done quite quickly.
The scale and tube can be used to stir the water in the beaker. If not, wire stirrers will be needed.
Safety
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Read our standard health & safety guidance
Procedure
a Attach the thermometer to the scale and its tube with rubber bands.
b 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.
cRecord the temperature and the length of the air column.
d Repeat for about four more temperatures, plotting a graph as the experiment proceeds.
Teaching notes
1 Students need to appreciate that the way the length of the column changes reveals how the volume of the air alters (volume = length x cross-sectional area).
2 Students should get straight-line graphs, although there may be a need to discuss the uncertainties in the readings.
3 Once students recognise 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.
4 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.
5 With a good 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.