II). Kinetic Particle Theory:
The kinetic particle theory states that all matter is made up of tiny particles, and that these particles are in constant, random motion. However, the movement of these particles varies according to the state of matter.
The Solid State:
The forces of attraction between the particles of solid are very strong, so they are held very close together and cannot move about freely. They have very less kinetic energy, enough to enable them to vibrate about their fixed positions. This is why a solid has a fixed shape.
As these particles are already very closely packed, a solid cannot be further compressed, which is why it has a fixed volume.
The Liquid State:
The forces of attraction between the particles of a liquid are weaker than those in a solid. These particles are not held in fixed positions, but instead arranged in a disorderly manner, and, due to higher kinetic energy as compared to solid, can move freely by sliding over each other. This is why a liquid does not have a fixed shape.
The particles of a liquid are further away from each other as compared to those in a solid, but are still packed quite closely together. Thus a liquid cannot be compressed and has a fixed volume.
The Gaseous State:
The particles of a gas have high kinetic energy and very low forces of attraction, therefore they are not held in fixed positions. They can move about rapidly in any direction, therefore a gas has no fixed shape.
A gas takes up as much free space as provided to it, which is why there are large distances between particles. These spaces make it possible for the gas to be compressed, or forced into coming closer. Therefore, a gas has no fixed volume.