Investigating Science 11–12 · Year 12

Pressure and volume of a gas: Boyle’s law and a hidden systematic error

Module 6: Technologies

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

For a fixed amount of gas at constant temperature, pressure times volume stays constant; air hidden in the connecting tube is a systematic error that bends the data until it is counted.

What you need

a 60 mL plastic syringe connected by a short tube to a digital pressure sensor (0 to 250 kPa) with data logger, or to an analogue Bourdon gauge
a way to measure the tube and sensor volume (fill with water from a 5 mL syringe)

How to do it

Measure the internal volume of the tube and sensor fitting.
Set the syringe at 60.0 mL open to the air, then connect it to the sensor.
Push the plunger in 5 mL steps to 30 mL, waiting 20 s at each step for the temperature to settle, and record the pressure.
Plot P against 1/V twice: once with the syringe reading and once adding the tube volume.
Repeat with the analogue gauge and compare resolution, accuracy and ease of reading.

What you should see

Ideally 60.0 mL at 101.3 kPa becomes 202.6 kPa at 30.0 mL, and PV stays constant. If the tube and sensor hold 3.0 mL that is not counted, the gas in fact goes from 63.0 to 33.0 mL, so the sensor reads 193.4 kPa at the 30 mL mark, 4.5 % below the uncorrected prediction. Adding the dead volume straightens the P against 1/V line through the origin and makes PV constant within the sensor’s accuracy; the analogue gauge shows the same trend with coarser resolution.

What changes

What you change: syringe volume
What you measure: gas pressure
What you keep the same: fixed amount of air (sealed system)
temperature (wait at each step)
same connecting tube

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Pressure and volume change by the same amount.
Squeezing a gas heats it permanently.
A digital sensor cannot have a systematic error.

Safety card

Low riskLearners carry it out

Hazards

syringe plunger springing out or tube popping off under pressure

Controls

compress no further than half the starting volume
secure the tube with a clip
safety glasses

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS12-13 INS11/12-1 INS11/12-2 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities