States of Matter

All materials are made of tiny particles. Whether something is a solid, liquid or gas depends on how these particles are arranged and how they move.

Distinguishing properties

• Solids: fixed shape and volume; cannot flow; very hard to compress.
• Liquids: fixed volume; take the shape of their container; flow; hard to compress.
• Gases: no fixed shape or volume; fill their container; easy to compress.

Particle model

• Solid: particles very close, in a regular pattern; vibrate in place.
• Liquid: particles close but irregular; slide past each other; move more freely.
• Gas: particles far apart, random arrangement; move quickly in all directions.

Changes of state

• Melting: solid → liquid
• Freezing: liquid → solid
• Boiling: liquid → gas throughout the liquid at its boiling point
• Evaporation: liquid → gas at the surface, at any temperature
• Condensation: gas → liquid

Heating gives particles more kinetic energy. During melting or boiling, temperature stays constant while energy breaks attractions. Cooling curves show the reverse.

Gases: temperature and pressure

Warmer gas particles move faster, collide more, and a gas expands if pressure is kept constant. Squeezing (higher pressure) forces particles closer, so volume decreases (at constant temperature).

$$V \propto T \;\text{(at constant pressure; T in kelvin)}$$

$$V \propto \tfrac{1}{p} \;\text{(at constant temperature)}$$