Particle Theory, also known as the Kinetic Molecular Theory, is fundamentally related to the modeling of different states of matter and their properties. This theory provides a framework for understanding the physical properties of matter in different states — solid, liquid, and gas — by considering the behavior of the particles (atoms and molecules) that compose the matter.
Solids: According to Particle Theory, particles in a solid are closely packed together in a fixed arrangement and vibrate in place. This explains solids' properties such as definite shape and volume, and incompressibility.
Liquids: Particles in a liquid are close together but not in a fixed arrangement. They can move/slide past each other, which accounts for the fluidity of liquids, their ability to take the shape of their container while maintaining a definite volume, and relative incompressibility.
Gases: Gas particles are much farther apart than in solids or liquids and move freely in all directions. This explains why gases fill any container they are in, have no definite shape or volume, and are easily compressible.
Gases and liquids are made of molecules or inert atoms that are moving about relative to each other.
In a liquid, the molecules are constantly in contact with others; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and may vibrate in position but do not change relative locations.
The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter.
The term “heat” as used in everyday language refers both to thermal energy (the motion of atoms or molecules within a substance) and the transfer of that thermal energy from one object to another. In science, heat is used only for this second meaning; it refers to the energy transferred due to the temperature difference between two objects.
The temperature of a system is proportional to the average internal kinetic energy and potential energy per atom or molecule (whichever is the appropriate building block for the system’s material). The details of that relationship depend on the type of atom or molecule and the interactions among the atoms in the material. Temperature is not a direct measure of a system's total thermal energy. The total thermal energy (sometimes called the total internal energy) of a system depends jointly on the temperature, the total number of atoms in the system, and the state of the material.