The kinetic model explains the physical properties of matter in terms of the motion of particles. It is based on the following key ideas:

Basic Assumptions of the Kinetic Theory

• All matter is made up of a large number of small particles (atoms or molecules).
• These particles are in constant random motion.
• There are spaces between the particles.
• Particles exert forces on each other (attractive or repulsive), except in ideal gases where these are negligible.
• Collisions between particles and with container walls are elastic (no net loss of kinetic energy).

States of Matter

The kinetic model describes the behavior of matter in different states:

1. Solids

• Particles are closely packed in a fixed, regular arrangement.
• Strong intermolecular forces keep particles in place.
• Particles vibrate about fixed positions but do not move freely.
• Solids have fixed shape and volume.

2. Liquids

• Particles are close together but not in fixed positions.
• Weaker intermolecular forces than in solids allow particles to slide over one another.
• Liquids have a fixed volume but take the shape of their container.
• Particles move more freely than in solids but less than in gases.

3. Gases

• Particles are far apart and move freely at high speeds.
• Intermolecular forces are negligible.
• Gases have neither fixed shape nor fixed volume; they expand to fill the container.
• Particles collide frequently with each other and with container walls, exerting pressure.

Changes of State

• When heat is added or removed, particles gain or lose energy.
• Melting, boiling, and sublimation involve particles gaining energy and overcoming attractive forces.
• Freezing, condensation, and deposition involve particles losing energy and coming closer together.
• Temperature remains constant during a change of state, as energy goes into changing potential energy, not kinetic energy.

Gas Pressure and Temperature

• Gas pressure is caused by collisions of particles with container walls.
• Increasing temperature increases average kinetic energy of particles.
• At higher temperatures, particles collide more frequently and with greater force, increasing pressure (if volume is constant).

Brownian Motion

• Random movement of particles suspended in a fluid, observed under a microscope.
• Evidence for the existence of tiny, fast-moving particles (like gas molecules) that collide with larger particles (e.g., smoke or pollen grains).

Written by Thenura Dilruk