Particle Theory

Basic Particle Theory

All matter is made up of tiny particles that are too small to see. These particles are often modelled as solid spheres to simplify understanding. They are arranged with spaces between them, which explains why substances can be compressed or expanded.

Particles are constantly moving, even in solids where they vibrate in fixed positions. The way particles are arranged and how they move differs depending on the state of the substance (solid, liquid, or gas).

Particle Movement and Energy

Particles have energy, which affects how fast they move. When a substance is heated, its particles gain energy and move faster. When cooled, particles lose energy and move more slowly.

In solids, particles vibrate around fixed positions because they have low energy. In liquids, particles have more energy, so they can move past each other and flow. In gases, particles have the most energy and move freely in all directions.

For example, heating a metal causes its particles to vibrate more vigorously, which can make the metal expand slightly.

The relationship between particle energy and temperature can be summarised as:

• Higher temperature 12 particles have more energy 12 move faster
• Lower temperature 12 particles have less energy 12 move slower

Particle Arrangement in States

The arrangement of particles varies between solids, liquids, and gases:

• Solids: Particles are tightly packed in a regular, fixed pattern. They cannot move from their positions but vibrate about fixed points.
• Liquids: Particles are close together but arranged randomly. They can move around each other, allowing liquids to flow and take the shape of their container.
• Gases: Particles are far apart and move randomly at high speeds. This explains why gases fill any container completely.

This explains why solids have a fixed shape and volume, liquids have a fixed volume but no fixed shape, and gases have neither fixed shape nor volume.

Learning Example: Particle Movement and Temperature

If a solid is heated, its particles vibrate faster because they gain energy. For instance, if a block of ice is heated from -10C to 0C, the particles vibrate more intensely but remain in fixed positions until melting begins (covered in Changes of State topic).