Jump to main content

Boyle's Law: Pressure vs. Volume of a Gas at Constant Temperature

1
2
3
4
5
213 reviews

Abstract

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. This project shows you a simple method for re-creating this famous experiment.

Summary

Areas of Science
Difficulty
 
Time Required
Very Short (≤ 1 day)
Prerequisites
None
Material Availability
Specialty items
Cost
Very Low (under $20)
Safety
Minor injury possible: be careful when stacking bricks on syringe apparatus.
Credits
Andrew Olson, Ph.D., Science Buddies

Sources

  • Gabel, Dorothy, 1996. "Learning Sequence Item 929: Gas Laws" in Scope, Sequence and Coordination: A National Curriculum Project for High School Science Education. Arlington, VA: National Science Teachers Association.
  • Gardner, Robert, 1990. Famous Experiments You Can Do. New York, NY: Franklin Watts (pp. 61-65).
  • Blauch, D., 2004. Gas Laws: Pressure. Department of Chemistry, Davidson College.
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.

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.

Drawing of a manometer shows a glass tube filled with liquid and bent into a U shape

"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 and Concepts

Questions

Bibliography

Here is the sequence of three Chemistry Applet webpages mentioned in the Introduction. These will really help your understanding of Boyle's Law if you take the time to do the virtual experiments!

The Sizes.com website has an exhaustive index of units of measure, including the SI definition for the pascal, and a link to a calculator to convert between various units of pressure:

  • Sizes.com, (2003). pascal. Retrieved December 13, 2005.

Materials and Equipment

Disclaimer: Science Buddies participates in affiliate programs with Home Science Tools, Amazon.com, Carolina Biological, and Jameco Electronics. Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity, and keep our resources free for everyone. Our top priority is student learning. If you have any comments (positive or negative) related to purchases you've made for science projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

Experimental Procedure

Making the Measurements
Diagram of a syringe sandwiched by two wood blocks has a wire inserted between the plunger and inner syringe wall

Diagram of a syringe held vertically in pre-drilled wood block support. A 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. A second pre-drilled wood block is placed atop the syringe plunger and acts as a platform 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.
  • Do your results agree?
  • What method(s) of graphing your results will show the inverse relationship between pressure and volume? (If you need ideas, check out the virtual experiment links in the Bibliography section.) You should be able to describe in your own words how your graph illustrates an equation for Boyle's Law.
  • What is the pressure on the air in the syringe before you start adding weight to the the plunger? Can you extrapolate from your measurements to give a value for this pressure? Can you think of a way to check the results of your extrapolation?
Questions
  1. Was the assumption of constant temperature valid?
  2. What are the possible sources of error in your experiment?
icon scientific method

Ask an Expert

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Variations

Careers

If you like this project, you might enjoy exploring these related careers:

Career Profile
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. Read more
Career Profile
The role that the chemical technician plays is the backbone of every chemical, semiconductor, and pharmaceutical manufacturing operation. Chemical technicians conduct experiments, record data, and help to implement new processes and procedures in the laboratory. If you enjoy hands-on work, then you might be interested in the career of a chemical technician. Read more
Career Profile
When you hear the word chemicals, you might think of laboratories and scientists in white coats; but actually, chemicals are all around you, as well as inside of you. Everything in the world is made up of chemicals, also known as matter, or stuff that takes up space. Chemistry is the study of matter—what it is made of, how it behaves, its structure and properties, and how it changes during chemical reactions. Chemistry teachers are the people who help students understand this physical… Read more
Career Profile
Chemical engineers solve the problems that affect our everyday lives by applying the principles of chemistry. If you enjoy working in a chemistry laboratory and are interested in developing useful products for people, then a career as a chemical engineer might be in your future. Read more

News Feed on This Topic

 
, ,

Cite This Page

General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Science Buddies Staff. "Boyle's Law: Pressure vs. Volume of a Gas at Constant Temperature." Science Buddies, 21 Oct. 2023, https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p011/chemistry/boyles-law-pressure-versus-volume-of-a-gas-at-constant-temperature. Accessed 19 Mar. 2024.

APA Style

Science Buddies Staff. (2023, October 21). Boyle's Law: Pressure vs. Volume of a Gas at Constant Temperature. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p011/chemistry/boyles-law-pressure-versus-volume-of-a-gas-at-constant-temperature


Last edit date: 2023-10-21
Top
We use cookies and those of third party providers to deliver the best possible web experience and to compile statistics.
By continuing and using the site, including the landing page, you agree to our Privacy Policy and Terms of Use.
OK, got it
Free science fair projects.