States of Matter at the Macroscopic Level

Solid - In a solid the particles are packed closely together in a regular manner. Motion is restricted by adjacent particles, and movement is limited to vibrations. This arrangement explains the difficulty solids have in being compressed and their definite shape and volume.

Liquid - At the melting point of the solid the particles break loose from fixed positions and can move relative to each other. The particles are still packed closely together, but there is more space for movement. The liquid takes up more space than the solid (usually). The increased movement allows liquids to flow. The ability of the particles to move relative to each other accounts for variable shape liquids can take.

In the gas phase the particles are well separated and move rapidly and randomly about. Because the gas particles no longer attracted to each other the gas expands to fill the container. The large amount of space between gas particles explain the compressibility of gases.