Quick answer: Gas pressure is caused by particles colliding with the walls of their container. This Physics 9-10 lab on kinetic theory of gases gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Does squeezing a gas increase its pressure in a predictable way? You will compress trapped air in a syringe, record the pressure, and test whether p × V stays constant. Then you will plot a graph that gives a straight line and use it to make a prediction.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: IGCSE 2.1, HS-PS3-2
Aim
To test whether the pressure of a fixed mass of air is inversely proportional to its volume at constant temperature.
Equipment
- 100 cm³ gas syringe or sturdy plastic syringe with a sealed nozzle
- pressure gauge or sensor (absolute pressure in kPa) with a short rigid tube
- silicone grease for the plunger
- thermometer (room temperature)
- clamp stand
Safety
Do not exceed the maximum reading of the gauge or syringe rating. Push the plunger slowly and keep your face away from the nozzle in case a joint pops off. Wear safety glasses.
Method
- Smear a thin film of grease on the plunger and set it at 50 cm³. Connect the syringe to the gauge with the short tube so that no air can escape.
- Record the room temperature and the volume V and pressure p.
- Push the plunger in slowly to 40 cm³. Wait 30 s so the air returns to room temperature, then record p.
- Repeat for 30, 25 and 20 cm³.
- Calculate p × V and 1 / V for each reading.
Results
Sample measurements at 21 °C (V in cm³, p in kPa):
| Volume V (cm³) | Pressure p (kPa) | p × V (kPa cm³) | 1 / V (cm⁻³) |
|---|---|---|---|
| 50 | 101 | 5050 | 0.0200 |
| 40 | 126 | 5040 | 0.0250 |
| 30 | 168 | 5040 | 0.0333 |
| 25 | 202 | 5050 | 0.0400 |
| 20 | 251 | 5020 | 0.0500 |
Analysis questions
Question 1 Checking the products
Verify the p × V value for the 30 cm³ reading and calculate the mean of the five values.Question 2 Spread of the data
Calculate the largest difference from the mean as a percentage of the mean.Question 3 Graph shape
Describe the graph of p against V and the graph of p against 1 / V that you expect.Question 4 Gradient
Calculate the gradient of the p against 1 / V line using the first and last points. State what it represents.Question 5 Prediction
Use the mean value of p × V to predict the pressure at 15 cm³.Question 6 Control of temperature
Explain why the plunger is pushed slowly and why you wait before reading.Question 7 Source of error
The tube and gauge contain about 2 cm³ of extra air that is not read on the syringe scale. Explain how this affects the p × V values.Question 8 Conclusion and improvement
State your conclusion and suggest one improvement.
