States of Matter: Solids, Liquids, Gases & Plasma

Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn the four states of matter in the universe: solid, liquid, gas and plasma. Learn about the differences in shape and volume among the four states, which of the states is the most common and why plasma conducts electricity.


Open a bottle of perfume in one corner of a room, then move to the opposite corner and wait. In only a few minutes, you will be able to smell that perfume. Why is that? Because air is a gas and the perfume molecules evaporate into a gas and disperse into the gaseous air. Gases consist of tiny particles or molecules that are really far apart relative to their size. These molecules have so much room to move around that pretty soon the two gases have mixed, and the perfume has spread throughout the room.

Liquid molecules are more tightly compacted than gas molecules.
Molecules Liquid Gas

Gas is a form of matter that does not have a definite volume or shape. Gases have low density compared to the same substance in other states. Gases are also able to diffuse easily, as shown in the perfume example.

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  • 0:08 Gas
  • 1:00 Liquid
  • 2:02 Solid
  • 3:45 Plasma
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Now, look at a bottle of water. What properties do you know about that liquid? You can tell that the liquid takes the shape of the bottle, and you probably know that if you pour the liquid water into a different container, it will take the shape of the new container. You also probably know that no matter how hard you push on the water, you can never change its volume. Whether it is in a tall, narrow bottle or spread out over a table top, liquid has a definite volume, but not a definite shape. Contrast this with gas, which has no definite fixed shape or volume.

In a liquid, the molecules are more tightly packed but can still move and flow past each other. Liquids are able to diffuse and mix with other liquids, but it is a slower mixing than in gases. Another property common to all liquids is surface tension. Surface tension is a force of attraction that keeps molecules on the surface of a liquid together, causing tension. This is why bugs can walk on water.


Higher density solids have a definite shape and volume.
Solid Properties

Now, look at a block of wood. Try to push on it. Have an elephant stand on it. What can you say about it? You can say that this solid block of wood has a definite volume and a definite shape. It doesn't change shape even with the elephant standing on it - unless of course, he disintegrates it, but let's pretend this is a baby elephant. These are characteristics of a solid - it has a definite volume and shape. A solid has molecules too, they are just so tightly packed that they have limited movement and are incompressible and hold their volume and shape. In general, substances are most dense in the solid state.

Solids have a definite shape and volume. They also have high densities and are incompressible. Solids do not diffuse well, but they can diffuse. Their diffusion rate is millions of times slower than diffusion in liquids.

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