Boyle’s Law Science Fair Experiment: Pressure vs. Volume of a Gas at Constant Temperature shows you a simple method for re-creating Boyle’s famous experiment.
This is a modern version of a classic experiment by Robert Boyle on the compressibility of gases. Boyle discovered the relationship between pressure and volume of gases that now bears his name.
The goal of this project is to measure the relationship between the volume of a gas and its pressure, when the temperature of the gas is held constant.
This project re-creates a study begun in 1662 by Robert Boyle. Now that’s a classic! Unlike liquids, gases are compressible. Boyle systematically studied the compression of air, sealed in a glass tube with a U-shaped curve. The air was trapped by a column of mercury, added to the open end of the tube. By changing the amount of mercury in the tube, Boyle could change the pressure exerted on the trapped air. Boyle’s apparatus was an example of a manometer, a device used to measure pressure.
Materials and Equipment
- Syringe that can hold at least 30 mL. If the syringe does not come with a tight-fitting cap, you will also need epoxy or silicone sealant
- Wood blocks with center holes drilled partway through (2)
- The first block will hold the syringe upright, and will need a hole that is just slightly larger than the diameter of the syringe, plus a smaller hole to accommodate the syringe tip. For greater stability, this block can be clamped in place (optional).
- The second block will be placed on top of the syringe plunger, and will act as a shelf for the bricks; the diameter of the hole should fit the top of the syringe plunger snugly.
- A small piece of wire
- Bricks with approximately uniform size and weight of about 1 kg (4 or 5 bricks)
- Scale for weighing bricks; a bathroom scale should be adequate.
- Graph paper
- Optional: “C” clamps (2). These are available at hardware stores and online through suppliers.
- Lab notebook
Diagram of experimental setup showing syringe with sealed tip held vertically in predrilled wood block support. The thin wire between the plunger tip and the inner syringe wall allows air to escape from in front of the plunger in order to equalize pressure. It is removed before starting the experiment. The second pre-drilled wood block is placed atop the syringe plunger and acts as a shelf for bricks to increase the pressure on the plunger. Diagram from Gabel, 1996.
- With the plunger removed from the syringe, seal the tip of the syringe with a tight-fitting cap. If a suitable cap is not available, you can try epoxy or silicone sealant. Allow the epoxy or silicone the recommended curing time before proceeding with the experiment. (Note: if you seal the tip with the plunger in place, you will probably not be able to remove the plunger unless you destroy the seal.)
- When your sealed syringe is ready for use, insert the syringe firmly, tip down, into the pre-drilled hole in the bottom wood block support, as shown in the diagram. The syringe should fit snugly, so it does not wobble when you load it up with bricks. You may wish to clamp the block in place. (Note: clamp to a workbench, not a piece of fine furniture!)
- Insert the plunger to the 30 ml mark of the syringe along with a thin wire as shown in the diagram. The wire will allow air to escape from beneath the plunger, equalizing the pressure in the syringe with the atmosphere. Use the lower ring of the plunger as your indicator.
- Hold the plunger in place and carefully withdraw the wire.
- Make sure that the plunger can move freely in the syringe, and that the tip of the syringe is well-sealed. Give the plunger a small downward push, and verify that it springs back. If it does not, you may need to lubricate the side of the plunger with a small amount of silicone lubricant or you may not have sealed the tip of your syringe properly.
- When you are satisfied with the results of the previous step, record the initial volume of air in the syringe.
- Place second wood block over the top of the plunger, as shown in the diagram. This wood block will act as a shelf to hold bricks in order to exert downward force on the plunger. Make sure that the shelf is leveled and well-seated on the plunger.
- Place the first brick on the shelf. You may need to tap on the brick to free the plunger. Note the resulting volume of air in the syringe.
- Repeat the previous step until you have 4 or 5 bricks stacked on the syringe. With each added brick, note the volume of air, and the number of bricks.
- Next, you will remove the bricks, one at a time, noting the volume of air in the syringe each time. Again, you may need to tap on the shelf to free the plunger.
- Take the average of your two values (from loading and unloading) for each number of bricks.
- Remove the plunger and repeat steps 4–12 so that you have at least 5 trials.
Analyzing your Data and Converting to Standard Units
- Calculate the average and standard deviation of the volume for each of your data points over the repeated trials.
- At this point, you have measured the volume of air in the syringe as a function of the number of bricks pushing down on the plunger. The next step is to convert from bricks to units of pressure.
- Pressure is defined as force per unit area. The SI unit for pressure is the pascal (Pa), which is defined as the force of 1 newton acting over an area of 1 square meter. So you will need to know the force exerted by the bricks, plus the area of the plunger.
- You can calculate the downward force, F of the brick(s) by multiplying the mass, m, of the brick(s) times the acceleration, a, due to gravity (F = ma), where a = 9.83 N/kg.
- You can calculate the area of the plunger (in units of square meters) by measuring the diameter, and recalling the formula for calculating the area of a circle.
- You can then calculate the pressure, in Pa, by dividing the force, F, generated by each stack of bricks by the area, A, of the plunger.
Boyle found that, when temperature is held constant, the pressure and volume of a gas are inversely related. In mathematical terms, we can write PV = k, where P, naturally, is pressure, and V is volume. Alternatively, we can write P1V1 = P2V2, where the subscripts indicate measurements of different pressure-volume pairs. The equation shows that as pressure increases, volume must decrease, and vice versa.