3.1 Solids, Liquids, and Gases

States of matter and particle arrangement

Matter exists as solid, liquid, or gas, distinguished by particle spacing, arrangement, and motion. In solids, particles are tightly packed with fixed positions and only vibrate. In liquids, particles are close but can move past each other. In gases, particles are far apart with weak interactions and move independently at high speeds.

Intermolecular forces and phase

The strength of intermolecular forces (IMFs) relative to kinetic energy determines the phase. Strong IMFs pull particles into fixed, ordered solids; weaker IMFs allow liquids to flow; when kinetic energy dominates over IMFs, particles separate into gases. Temperature and pressure changes shift this balance, driving phase changes.

Macroscopic properties reflect particle behavior

Solids have definite shape and volume; liquids have definite volume but variable shape; gases have neither definite shape nor volume, expanding to fill containers. Gases are highly compressible because of large spaces between particles, while solids and liquids are nearly incompressible due to close particle packing.

Key terms

Solid (particle behavior)

Particles are tightly packed in fixed positions, only vibrating; results in definite shape and volume.

Liquid (particle behavior)

Particles are close together but can move past each other; definite volume, variable shape.

Gas (particle behavior)

Particles are far apart with weak interactions, moving independently; no definite shape or volume.

Why gases are compressible

Large spaces between gas particles allow them to be pushed closer together under pressure.

What determines phase of matter

The balance between intermolecular forces (attraction) and particle kinetic energy (motion), influenced by temperature and pressure.

Try an AP-style question

Which substance has the highest boiling point?

1. CH₄
2. Ne
3. H₂O
4. H₂S