Boyle's Law: Gas Pressure and Volume Relationship

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  • 0:06 Boyle's Law
  • 3:30 Boyle's Law Examples
  • 5:49 Lesson Summary
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Lesson Transcript
Instructor: Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

Have you ever wondered how an air powered water gun works? It uses the fantastic properties of gases to make a summer day more enjoyable! In this lesson, we will be discussing Boyle's Law and the relationship between pressure and volume of a gas.

Boyle's Law

The deeper a scuba diver goes, the more the volume in the ear decreases because of pressure.
Ear Air Volume Decreases Scuba Diving

Johnny Dalton has a very big day ahead of him. He is vacationing on Ideal Island, where all gases behave ideally. Johnny and his family are going to start off the morning by scuba diving around the island. As we follow Johnny during his dive, we will be paying close attention to the role that gases play in this activity. More specifically, we will be seeing how Boyle's Law helps us explain and predict some of the behaviors of gases.

Johnny puts on his wetsuit and tank and jumps into the water. As he lowers himself under the surface of the water, he feels a little pressure in his ears, so he plugs his nose and swallows to equalize them. The reason he is feeling that pressure is because the surrounding water is pushing in on his ear. The deeper he gets, the more water pushes on his ears, which causes even more pressure to be put on his ears. As that pressure increases on the outside of his ear, the air inside the ear gets squished and the volume inside decreases. The air doesn't go anywhere, it just gets squeezed together, which causes that uncomfortable feeling he had before he equalized. When a scuba diver descends, he will need to let some of the air inside the ear out, which reduces this pressure. This is usually done by pinching your nostrils and trying to blow gently out your nose. If you try it right now, you may hear a little popping sound. That's the sound of air going in or out of your ear!

Another thing Johnny notices as he descends is that his buoyancy compensator (which is just a vest filled with air) is getting smaller and smaller! Even his wetsuit is shrinking! What is causing this? Well, a scientist named Robert Boyle discovered that the pressure on a gas and its volume are inversely proportional to each other. This works for any closed system of gas, such as the inside of your ear, the buoyancy compensator, or even the thousands of tiny bubbles embedded in a neoprene wetsuit. If the temperature is held constant, the pressure of a gas increases as its volume decreases. The reverse is also true; as the pressure of a gas decreases, its volume increases. This is Boyle's Law.

Robert Boyle discovered how pressure and volume are related.
Boyle Diagram and Portrait

Now, as Johnny was lowering himself under the water, all of those little 'containers' of gas got smaller. The reason they did lies in the Kinetic Molecular Theory, which we previously covered. One thing the kinetic molecular theory states is that gas particles are very far away from each other. This distance allows for the compression of gases. If the particles are far apart, they have room to move closer together. If the particles are already close together (like they would be in a liquid or solid), there is very little room for compression. Now we also know that gas particles move randomly and rapidly, and each time they collide with the walls of their container, they apply a little bit of pressure. If we keep the same number of particles in a container, all with the same kinetic energy, but we decrease the size of the container, the particles will more frequently hit the walls of that container, causing the pressure to increase.

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