Teach lesson
Half the volume, double the pressure?
Test a memorable inverse-proportion claim with the 20 mL and 10 mL readings from a real Boyle run.
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Learning Outcomes
Measure pressure at 20 mL and 10 mL in the same syringe run.
Calculate the pressure ratio after volume is halved.
Use real evidence to distinguish an approximately doubled pressure from an exactly doubled pressure.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
A claim worth testing
7 min
"If you compress a gas to half its volume, its pressure doubles." That sentence is easy to remember—but does it describe a real experiment exactly or only approximately? Today you will not collect a huge table. You will test the claim using two key readings from the same run.
Use a closed syringe at approximately constant temperature. Compare 20 mL with 10 mL: the second syringe-scale volume is exactly half the first. Pressure is read in kPa from the LabQuest display. The available lab source does not identify the display as showing absolute or gauge pressure. In addition, gas in the tubing or sensor may add volume that is not shown on the syringe scale. Use the displayed pressure and syringe-scale volume consistently as working measurements. The result is an approximate test of the model, not proof of a perfect ideal-gas relationship.
If volume changes from 20 mL to 10 mL, what do you predict for pressure?
Give one brief reason for your prediction. Say whether you expect a small, large, or zero change.
Record the two key readings
11 min
Use the 20 mL syringe, trial 1. Record the initial pressure at 20 mL. Then continue through the pauses at 18, 16, 14, and 12 mL without recording them until you reach 10 mL. Record the second pressure there. If the display uses a decimal comma, use a decimal point in numeric fields.
The measurement display
The large number beside kPa is pressure; it appears red in the current interface. Always check the current volume before copying it.
Lab-screen note: the introduction may mention pressure and temperature. In this Boyle run, syringe volume changes, pressure is measured, and temperature is treated as approximately constant.
Open the 20 mL run
Open the lab from this activity.
Select the 20 mL syringe and trial 1.
Read and record the pressure at 20 mL.
At each pause, select Decrease by 2 mL until you reach 10 mL.
Read and record the pressure at 10 mL. Replay that pause if a digit is unclear.
Complete only the two prepared rows. Each needs one pressure in kPa; leave any extra row shown by the interface empty.
Before and after halving the volume
Enter the observed pressures at 20 mL and 10 mL.
| Volume mL | Observed pressure kPa |
|---|---|
Test the word ‘double’
7 min
Compare the change by dividing pressure at 10 mL by pressure at 20 mL:
Pressure ratio
\text{ratio}=\frac{P_{10}}{P_{20}}
A ratio of 2 means that the final pressure is exactly twice the initial pressure. For this lesson, use 1.90–2.10 as the class range for a pressure ratio of approximately 2. Make the decision from the unrounded ratio, then report it to two decimal places. The ratio is dimensionless because both pressures use the same unit.
Calculate P10 ÷ P20. Enter the dimensionless ratio rounded to two decimal places, explain which two readings you used, and say whether the unrounded result lies within the range 1.90–2.10.
Which verdict best describes your measurements?
One test of an inverse-proportion model
5 min
In an inverse proportion, multiplying the two variables gives an approximately constant product: PV ≈ constant. Two readings cannot establish that rule across every volume, but the model makes a clear prediction when the volume is halved.
Write a three-sentence conclusion:
1. State that volume fell from 20 mL to 10 mL, and cite both measured pressures.
2. Report your ratio and say whether the unrounded value lies within 1.90–2.10.
3. Decide whether your data support the prediction that pressure approximately doubled. Make clear that two readings test this prediction but do not establish the rule across all volumes.