The Kinetic Molecular Theory: Properties of Gases

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  • 0:51 Properties of Gases
  • 2:31 Kinetic Molecular…
  • 6:09 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.

What makes a gas ideal? What types of characteristics do ideal gases have? In this lesson, we will discuss the many characteristics of gases and how knowing the microscopic properties of gas particles will help you understand the macroscopic properties of a gas.

I would like to introduce you to Johnny Dalton. Johnny is a quiet kid; he wears glasses and is fascinated by the weather. School just got out for the summer, and Johnny and his family members are packing their bags to travel to their favorite destination: Ideal Island. Johnny loves traveling there because he loves to do science experiments, and experimenting is much easier on Ideal Island, where all gases are ideal.

Properties of Gases

Today, we are going to discuss what makes a gas ideal and why Johnny would even want to travel there in the first place. But first, let's do a little review on what a gas is and explain how gases are different from solids and liquids. One major characteristic of gases is that they expand on their own to completely fill their container. So, if you ever want to know the volume of a gas, all you need to know is the volume of the container that it's in!

The volume of gas is easy to calculate.
Volumn Gas Equals Volumn Container

Keep in mind here: the particles themselves don't actually expand. What happens is the particles spread out evenly, completely filling the container. The volumes of solids and liquids, on the other hand, are quite fixed, meaning they can't really change a lot. Liquids and solids don't expand to fill their containers. A rock is the same size whether it's in a small container or a large one. Liquids may take the shape of their container, but their volume will never increase or decrease.

Another characteristic of gases is that they can be compressed. This is because the particles in gases are very far apart. The particles in solids and liquids are so close together that they are difficult (if not impossible) to compress. Brake hydraulics work so well because of this. If liquids were easy to compress, the brake fluid in your car would just 'squish together' instead of working to stop your car. So, the more distance between the particles of a substance, the easier it is to compress. And gas particles are very far apart, so they are easy to compress.

Kinetic Molecular Theory and Ideal Gases

Gases differ from other matter in that they are easy to compress.
Compression of Gases

These are just a couple of the properties of gases, but how can we explain some of these properties? For example, how does a gas fill its container? Well, when scientists try to explain the world around us, they tend to oversimplify things, and in many situations that is perfectly acceptable. When you were learning about the different planets that orbit around the sun, you probably learned from pictures in a book or three-dimensional models. Those pictures were not 100% accurate (there are not actual rings that the planets sit on), but they didn't have to be 100% accurate. They still served the purpose of providing a simple model so that the average person would get an idea of the make-up of the solar system.

Well, on Ideal Island, gases are also simplified. They are all ideal. That's why Johnny likes visiting this place. The gases are perfect. The carbon dioxide, oxygen and nitrogen all behave ideally. So, then, what is an ideal gas? An ideal gas is a theoretical gas that follows a set of principles. These principles are part of a model called the kinetic molecular theory. It sounds very complicated, but this theory is just a description of moving molecules. It explains why gases behave the way they do.

One thing ideal gas particles do is move rapidly and randomly. They are constantly flying around, and their direction can change at any moment. This is how gases are able to fill their containers; they fly around in random directions.

Ideal gas particles are also so small that they are said to have no volume. This is referring to each individual particle, not the gas as a whole. Another property of an ideal gas is that there are no attractive or repulsive forces between particles. As you may remember, solids and liquids have very high intermolecular forces. The intermolecular forces between gases are extremely small - and in an ideal gas, they are nonexistent.

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