States of Matter - Kinetic particle theory

All matter is composed of tiny particles (atoms, molecules, or ions) that are in constant, random motion.

In a solid, particles are packed in a regular lattice, vibrating about fixed positions with very low kinetic energy.

In a liquid, particles are close together but can slide past one another, allowing the substance to flow and take the shape of its container.

In a gas, particles are far apart and move rapidly in random directions; collisions between particles and container walls exert pressure.

Phase changes occur with energy transfer: melting ($solid \rightarrow liquid$), boiling/evaporation ($liquid \rightarrow gas$), condensation ($gas \rightarrow liquid$), freezing ($liquid \rightarrow solid$), and sublimation ($solid \rightarrow gas$ as seen in $CO_2$ or $I_2$).

Brownian motion refers to the random movement of visible particles (like smoke or pollen) suspended in a fluid, caused by collisions with invisible, fast-moving atoms or molecules.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, driven by the random motion of particles.

The rate of diffusion depends on temperature (higher $T$ leads to faster diffusion) and the relative molecular mass ($M_r$) of the particles (lighter particles diffuse faster).