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A substance in a solid phase is relatively rigid, has a definite volume and shape. The atoms or molecules that comprise a solid are packed close together and are not compressible. Because all solids have some thermal energy, its atoms do vibrate. However, this movement is very small and very rapid, and cannot be observed under ordinary conditions.
Liquids have a definite volume, but are able to change their shape by flowing. Liquids are similar to solids in that the particles touch. However the particles are able to move around. Since particles are able to touch, the densities of liquid will be close to that of a solid. Since the liquid molecules can move, they will take the shape of their container.
Gases have no definite volume or shape. If unconstrained gases will spread out indefinitely. If confined they will take the shape of their container. This is because gas particle have enough energy to overcome attractive forces. Each of the particles are well separated resulting in a very low density.
What are the shape, volume, and particle motion of solids, liquids, and gases?