The Three States of Matter

Everything on this topic follows from how the particles are arranged, how they move and how far apart they are.

Solid

• Arrangement: regular, closely packed pattern.
• Motion: vibrate about fixed positions; they cannot move past one another.
• Spacing: very close.
• Result: fixed shape and fixed volume, and almost incompressible because there is no space to squeeze out.

Liquid

• Arrangement: close together but random.
• Motion: move past one another, so the liquid can flow.
• Spacing: close, similar to a solid.
• Result: fixed volume but no fixed shape — it takes the shape of its container. Nearly incompressible.

Gas

• Arrangement: random and far apart.
• Motion: fast, random motion in all directions.
• Spacing: very far apart, so most of a gas is empty space.
• Result: no fixed shape or volume; it fills its container completely, and it is easily compressed because of that empty space.

Why gases are compressible and liquids are not. A gas has large gaps between particles, so a pressure squeezes those gaps smaller. In a liquid the particles are already touching, so there is nothing to squeeze out. This is why hydraulic systems use liquid rather than gas — the fluid must not compress if the force is to be transmitted.

Brownian motion — pollen grains in water, or smoke particles in air viewed under a microscope, are seen to jiggle about in a random, jerky path. The grains are far too large to be moving on their own. They are being struck unevenly by the invisible, fast-moving molecules of the fluid, and the random path is the evidence that those molecules exist and move randomly.

This was historically important: it is direct visual evidence for the particle model itself.

Example: 1 cm³ of water becomes about 1600 cm³ of steam at atmospheric pressure. Nothing has been created — the same particles are simply far more widely spread.