4.3.5Properties of Matter
Gas Law Experiments — SQA National 5 Physics Revision Notes & Topical Questions
5 min read 5 parts Lesson 11 of 11
Part 1 of 3Reliability and Evaluation
Good experimental technique improves the reliability and accuracy of the results.
1
Allow time for the gas to reach the same temperature as the water or the room before reading each value.
2
Stir the water bath so that the thermometer reading matches the true gas temperature throughout the flask.
3
Repeat each measurement and calculate an average to reduce the effect of random errors in reading the gauge.
4
Keep tubing between the flask and gauge short, because gas in the tubing is at a different temperature and adds error.
5
Check for leaks, since escaping gas changes the mass of gas and invalidates every gas law relationship.
| Experiment | Variable changed | Variable kept constant |
|---|---|---|
| Pressure–volume | Volume | Temperature and mass |
| Pressure–temperature | Temperature | Volume and mass |
| Volume–temperature | Temperature | Pressure and mass |
Part 2 of 3Worked Example
Question
- In a pressure–temperature experiment the gauge reads 100 kPa at 20 °C and 120 kPa at a higher temperature.
- Calculate that temperature in °C.
- Convert the starting temperatureT₁ = 20 + 273 = 293 K.
- Volume is constant, so p₁/T₁ = p₂/T₂ and T₂ = p₂T₁ ÷ p₁.
- T₂ = 120 × 293 ÷ 100 = 351.6 K.
- Convert back351.6 − 273 = 78.6 °C.
Answer T₂ ≈ 79 °C (352 K).
Part 3 of 3Exam Checklist & Tips
4.3.5 – Key Points for the Exam
- Independent variable The quantity deliberately changed during an investigation.
- Dependent variable The quantity measured as a result of the change.
- Control variable A quantity kept constant to make the test fair.
- Reliability How consistent repeated measurements are.
- Extrapolate Extend a line on a graph beyond the measured points.
Common Mistakes
- Failing to wait for the gas to reach the same temperature as its surroundings before reading.
- Plotting temperature in °C and expecting a line through the origin.
- Forgetting to state which quantities are kept constant in the experiment.
- Using a leaking syringe, so the mass of gas does not stay fixed.
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