The Kinetic Theory of Matter: Definition & The Four States of Matter

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  • 0:01 The Four Phases of Matter
  • 1:34 Kinetic Theory of Matter
  • 3:38 Phase Changes
  • 5:36 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Everything on Earth is made of matter, but that matter isn't always the same. Matter can exist in four different phases, and the kinetic theory of matter helps us understand the differences between those phases.

The Four Phases of Matter

Solids, liquids, gases, and plasmas: these words should be quite familiar to you because they are the four phases of matter, which are simply the different forms matter can take on. What's neat is that many substances can exist as more than one phase. Take water, for example: water can exist as a solid (ice), a liquid (liquid water), and a gas (water vapor).

The difference between these states is the amount of energy. Solids have the least amount of energy, which is part of why their particles hang so tightly together. Liquids have more energy than solids, which is why they will take on the shape of their container but only up to the surface.

Gases have even more energy than liquids. So much more in fact that their particles spread out to fill the entire space of their container. Gas particles have so much energy that they just can't keep still. They fly around in all directions, putting as much distance as possible between themselves and the rest of the gas particles.

Plasmas are ionized gases, and in their natural form are uncommon on Earth. You've seen them as man-made things, like neon signs and fluorescent light bulbs. But in the rest of the universe, plasma is actually the most common phase of matter! Most stars are plasma, as are the northern lights you see around the Polar Regions. Plasma only exists under certain conditions though, so we'll end our discussion of it here for this lesson.

Kinetic Theory of Matter

The kinetic theory of matter states that all matter is made of small particles that are in random motion and that have space between them. This means that no matter what phase matter is in, it is made of separate, moving particles.

This theory sounds pretty simple, but it actually explains a lot about the physical properties of matter and how it behaves. You might be surprised to learn that the particles of a solid are actually moving, just not enough for you to see. This type of vibrational movement is why a solid won't change shape no matter what kind of container you put it in.

Remember how liquid particles have more energy than solids? The extra energy in this state allows the particles to move around more freely, and they spread out more than those of a solid, putting more space between those particles. This is why a liquid will take the shape of its container up to its surface.

And since gases have even more energy than liquids, their particles are moving around a lot more, too. This is why a gas will expand to fill its entire container, not just to its surface like a liquid. Not only do the particles of a solid not move very much, but they're also held very close to each other by strong attractive forces. These forces are what hold the particles in place and are what give a solid its fixed size and shape.

On the other hand, the particles of a gas are so far apart that the attractive forces between them are assumed to be negligible. The particles of a gas are viewed as independent from each other, meaning that the gas is the opposite of a solid and has neither a fixed size nor shape.

Since the movement of liquid particles is in between a solid and a gas, the attractive forces between its particles are also in a middle range of the other two phases. Liquid particles have more freedom than solid particles, which is why a liquid can flow freely. This means that like a gas, a liquid has no fixed shape. But because the particles aren't quite as free as those of a gas, a liquid does have a fixed volume.

Phase Changes

The kinetic theory of matter is also useful for explaining why substances can change phase under certain conditions. You know that water can be solid, liquid, or gas, but how does this happen? A phase change occurs when energy is added to or taken away from matter, usually in the form of heat.

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