Kinetic Theory of Matter
Overview
Kinetic Theory of Matter explains macroscopic thermal behaviour using a microscopic particle model.
Matter is modelled as huge numbers of atoms, molecules, or ions in constant motion and interacting through intermolecular forces.
Definition
The kinetic theory of matter explains properties of matter in terms of discrete particles, random motion, intermolecular forces, collisions, and energy transfer.
Why It Matters
This theory explains why solids keep shape, liquids flow, gases fill containers, heating raises temperature, melting and boiling occur, evaporation causes cooling, and gases exert pressure.
Key Representations
Solids have particles closely packed and vibrating about mean positions. They have fixed shape and volume and are difficult to compress.
Liquids have particles close together but randomly arranged and able to move past one another. They have fixed volume but no fixed shape.
Gases have particles far apart, moving rapidly and randomly. They have no fixed shape or volume and are easily compressed.
Higher temperature means greater average random kinetic energy. For an ideal gas:
Internal energy is:
During heating without change of state, average kinetic energy increases:
During melting or boiling, supplied energy increases intermolecular potential energy while average kinetic energy remains unchanged, so temperature remains constant.
Evaporation occurs at the liquid surface at any temperature. More energetic molecules escape, leaving lower average kinetic energy in the liquid and causing cooling.
Diffusion occurs because particles move randomly and spread from higher concentration to lower concentration. Brownian motion is random zig-zag motion of small suspended particles caused by collisions with fluid molecules.
Gas pressure arises because gas molecules collide with container walls and change momentum. More frequent or stronger collisions produce higher pressure. See Ideal Gases.
Common Exam Traps
Temperature relates to average kinetic energy per particle, not total energy.
Molecules usually do not significantly expand when heated; average separation and motion change.
Particles in solids are not motionless; they vibrate continuously.
Evaporation occurs at any temperature, not only at boiling point.
Internal energy is not always just kinetic energy. That approximation is mainly for ideal gases.
Use precise phrases such as average kinetic energy increases, intermolecular forces are overcome, particles move further apart, more energetic molecules escape, and collision frequency with walls increases.