The basic reason behind the existence of three states of matter—solid, liquid, and gas—is the behavior of particles at the molecular or atomic level.
The state of matter of a substance is determined by the relative strength of the forces between its particles (atoms, molecules, or ions) and the amount of energy they possess.
Solid: In a solid, the particles are closely packed together in a regular arrangement. The intermolecular forces between the particles are strong enough to keep them fixed in their positions, giving solids a definite shape and volume. The particles vibrate around their fixed positions but do not move freely.
Liquid: In a liquid, the particles are still in close proximity but have more freedom of movement compared to solids. The intermolecular forces in liquids are weaker than in solids, allowing the particles to move past each other while remaining in contact. Liquids take the shape of their container but still have a definite volume.
Gas: In a gas, the particles have significant separation between them and move freely in all directions. The intermolecular forces in gases are very weak, allowing the particles to move independently. Gases have neither a definite shape nor a definite volume and expand to fill the container they are in.
The state of matter can be changed by altering the temperature or pressure. Heating a solid can increase the energy of its particles, causing them to overcome the intermolecular forces and transition into the liquid state (melting), and further heating can lead to a transition into the gaseous state (vaporization). Cooling a gas or applying pressure can cause the particles to come closer together and form a liquid (condensation) or solid (freezing).
The existence of these three states of matter arises from the balance between the attractive and repulsive forces between particles and the energy they possess. The behavior of particles at the molecular level determines the macroscopic properties and characteristics of each state.