Boyle's law
Verify Boyle's law (PV = constant at fixed temperature) by compressing and expanding gas with the piston in the FizziQ Web Ideal Gas simulation.
In 1662, Robert Boyle discovered a remarkable law: when you compress a gas while keeping its temperature constant, the pressure increases inversely proportional to the volume. In other words, the product P × V remains constant. This elegantly simple relationship is one of the foundation stones of thermodynamics and the ideal gas model. The FizziQ Web Ideal Gas simulation lets you verify this law by moving a virtual piston and watching how the pressure responds in real time.
Activity overview:
The student uses the FizziQ Web Ideal Gas simulation while maintaining constant temperature. They slowly move the piston to vary the gas volume while recording pressure and volume simultaneously. They plot the PV isotherm and verify PV = constant.
Level:
High school
FizziQ Web
Author:
Duration (minutes) :
25
What students will do :
- Experimentally verify Boyle's law (PV = constant at constant T)
- Record pressure and volume simultaneously during an isothermal transformation
- Plot a P(V) isotherm and identify its shape as a hyperbola
- Verify the constancy of the PV product
- Linearize the relationship by plotting P versus 1/V
Scientific concepts:
- Boyle's law (Boyle-Mariotte)
- Isothermal transformation
- Gas pressure and volume
- Inverse proportionality
- Ideal gas law PV = nRT
- Hyperbola
Sensors:
- FizziQ Web Ideal Gas simulation
Material needed:
- Computer, tablet, or smartphone with FizziQ Web
Experimental procedure:
Open the Ideal Gas simulation in FizziQ Web (Experiment → Simulations → Ideal Gas).
Set the temperature to 293 K (20°C). This temperature will remain fixed throughout the experiment.
Select the quantities to record: enable Pressure and Volume. Choose table mode to export data easily.
Position the piston at its maximum (largest volume). Note the initial P and V values.
Click REC to start recording. Slowly move the piston inward to reduce the volume in small steps.
Move the piston across its full range, from maximum to minimum volume, taking time to cover the entire range evenly.
Stop the recording (REC). The P and V data are automatically exported to the experiment notebook.
Plot the graph of P versus V. Is the curve a straight line? What shape do you recognize?
Add a calculated column to the table: PV (pressure × volume product). Is this quantity constant?
Add a column 1/V and plot P versus 1/V. Do you get a straight line through the origin? The slope of this line gives nRT.
Expected results:
The P(V) graph is a decreasing hyperbola: when V decreases, P increases. The product PV is constant for all measurements (within reading precision), confirming Boyle's law. The P(1/V) graph is a straight line through the origin with slope nRT. The product PV equals nRT, consistent with the ideal gas law.
Scientific questions:
- If you halve the volume, by what factor is the pressure multiplied?
- What does the product PV physically represent? Why is it constant at fixed temperature?
- How does the isotherm curve change if the temperature increases?
- What would the P(V) graph look like for a real gas at very high pressure?
- Can you determine the number of moles from the slope of P(1/V)?
- Why is Boyle's law only valid at constant temperature?
Scientific explanations:
Boyle's law states that for a fixed amount of gas at constant temperature, the product of pressure and volume is constant: PV = constant. This means that halving the volume doubles the pressure.
Physically, when gas is compressed (V decreases), the molecules are confined in a smaller space. They hit the container walls more frequently, which increases the pressure. The total kinetic energy stays the same (constant temperature), but the impacts are more concentrated.
The P(V) curve at constant temperature is called an isotherm. Its mathematical form is P = constant/V, which is a hyperbola. Different temperatures give different isotherms, all with the same shape but shifted upward for higher temperatures.
To linearize the relationship (transform the curve into a straight line), plot P versus 1/V. If P = nRT/V, then P is proportional to 1/V: the graph is a straight line through the origin with slope nRT.
This law is an idealization: real gases deviate from it at high pressure and low temperature, when intermolecular interactions become significant. The ideal gas model works well at moderate conditions.
Extension activities:
- If you halve the volume, by what factor is the pressure multiplied?
- What does the product PV physically represent? Why is it constant at fixed temperature?
- How does the isotherm curve change if the temperature increases?
- What would the P(V) graph look like for a real gas at very high pressure?
- Can you determine the number of moles from the slope of P(1/V)?
- Why is Boyle's law only valid at constant temperature?
Frequently asked questions:
Q: The P(V) graph does not look like a perfect hyperbola.
R: Check that the temperature truly stayed constant during the entire manipulation. If you accidentally moved the temperature slider, the data points will not follow a single isotherm.
Q: The PV product is not exactly the same for all measurements.
R: Small variations (1-2%) are normal due to reading precision and the speed at which you moved the piston. The important thing is that PV is approximately constant.
Q: Why plot P versus 1/V instead of just checking PV = constant?
R: A straight-line graph is easier to verify visually and allows you to extract the slope (nRT) with better precision than checking whether a product is constant.
Q: Can I observe Boyle's law with a real gas at home?
R: Yes. A sealed syringe connected to a pressure sensor (or smartphone barometer via tubing) lets you compress air and measure P versus V. The results closely match the ideal gas prediction at room conditions.