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Project Summary

Difficulty  8 
Time required Very Short (a day or less)
Prerequisites None
Material Availability Specialty items
Cost Very Low (under $20)
Safety Minor injury possible: be careful when stacking bricks on syringe apparatus.

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Sponsor

Sponsored by a generous grant from the Camille and Henry Dreyfus Foundation

Objective

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.

Introduction

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.

The following diagram and description, from Prof. David N. Blauch, of Davidson College, explain how a manometer works.

Diagram of a manometer

"A manometer is a device employed to measure pressure. There are a variety of manometer designs. A simple, common design is to seal a length of glass tubing and then bend the glass tube into a U-shape. The glass tube is then filled with a liquid, typically mercury, so that all trapped air is removed from the sealed end of the tube. The glass tube is then positioned with the curved region at the bottom. The mercury settles to the bottom (see the picture at the left).

"After the mercury settles to the bottom of the manometer, a vacuum is produced in the sealed tube (the left tube in the picture). The open tube is connected to the system whose pressure is being measured. In the sealed tube, there is no gas to exert a force on the mercury. In the tube connected to the system, the gas in the system exerts a force on the mercury. The net result is that the column of mercury in the left (sealed) tube is higher than that in the right (unsealed) tube. The difference in the heights of the columns of mercury is a measure of the pressure of gas in the system.

"In the example at the left, the top of the left column of mercury corresponds to 875 mm on the scale. The top of the right column of mercury corresponds to 115 mm. The difference in heights is 875 mm − 115 mm = 760. mm, which indicates that the pressure is 760. mm Hg or 760. torr." (Blauch, 2004).

You can learn more about how manometers work, and even run a simulated Boyle's Law experiment by visiting the Chemistry Applet website (see Bibliography). This would be excellent preparation for doing the experiment on your own, so we highly recommend it.

You can repeat Boyle's experiments with an inexpensive, modern apparatus based on a disposable plastic syringe. Mercury is a dangerous neurotoxin, so we'll avoid working with it. Instead, you will compress the air in the syringe with . . . bricks!

Pressure is force exerted over a unit area, so the units are those of force divided by area. The larger area of a syringe (compared to a narrow glass tube) means that you will need a larger force to compress the gas. Bricks, which are stackable and weigh about 1 kg apiece, will do the job nicely.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

Questions

Bibliography

Materials and Equipment

Experimental Procedure

Making the Measurements

Diagram of experimental setup showing syringe with sealed tip held vertically in pre-drilled 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 of experimental setup showing syringe with sealed tip held vertically in pre-drilled 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.

  1. Before starting the experiment, do your background research so that you are knowledgeable about the terms, concepts and questions, above.
  2. 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. Why?)
  3. 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!)
  4. 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.
  5. Hold the plunger in place and carefully withdraw the wire.
  6. 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.
  7. When you are satisfied with the results of the previous step, record the initial volume of air in the syringe.
  8. 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 level and well-seated on the plunger.
  9. 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.
  10. 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.
  11. 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.
  12. Take the average of your two values (from loading and unloading) for each number of bricks.
  13. Remove the plunger and repeat steps 4–12 so that you have at least 5 trials.

Analyzing Your Data and Converting to Standard Units

  1. Calculate the average and standard deviation of the volume for each of your data points over the repeated trials.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.

Presenting Your Results

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.

Questions

  1. Was the assumption of constant temperature valid?
  2. What are the possible sources of error in your experiment?

Variations

Credits

Written by: Andrew Olson, Ph.D., Science Buddies

Sources

Grateful acknowledgment is also made to Prof. Blauch, Department of Chemistry, Davidson College for making his Gas Law and other Chemistry Applets freely available on the Web.


Last edit date: 2005-12-29 14:12:13


Career Focus

science career image If you like this project, you might want to think about career opportunities in Chemistry.

Everything in the environment, whether naturally occurring or of human design, is composed of chemicals. Chemists search for and use new knowledge about chemicals to develop new processes or products. Learn more about this career: Chemist.




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