KMT states that matter consists of tiny particles (atoms, molecules) in constant motion. The average kinetic energy of particles is proportional to absolute temperature. Pressure results from particles colliding with container walls. The theory explains properties of solids, liquids, and gases based on particle motion and spacing.
Matter exists in three main states: solid, liquid, and gas. The kinetic molecular theory explains these states by looking at particle arrangement and motion. In solids, particles are tightly packed in a fixed structure and vibrate in place. In liquids, particles move freely but are still close together. In gases, particles are far apart and move rapidly in random directions. Temperature and pressure determine which state is stable.
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Intermolecular forces (van der Waals, hydrogen bonds, ionic attractions) hold particles together. In solids, these forces are strong and particles are tightly packed. In liquids, forces are moderate and particles can move. In gases, forces are weak and kinetic energy overcomes them. Temperature and pressure determine which state is stable.
The ideal gas law is PV = nRT, where P = pressure, V = volume, n = moles, R = 8.31 J/(mol·K), T = absolute temperature (Kelvin). It works well for gases at low pressure and high temperature. Real gases deviate at high pressure (molecules take up volume) and low temperature (intermolecular forces matter).
Melting is the phase change from solid to liquid; vaporization is from liquid to gas. Melting breaks apart the fixed crystal structure but molecules stay close. Vaporization requires molecules to overcome intermolecular forces and fly apart. Vaporization is much more energy-intensive than melting for the same substance.
The critical point is the temperature and pressure above which liquid and gas become indistinguishable — there's no phase boundary. Above the critical point, a substance is called a supercritical fluid. This is important for industrial applications (supercritical CO₂ for extraction) and explains unusual behavior of matter at extreme conditions.
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