States of Matter

Learning Objective 3.3.A Represent the differences between solid, liquid, and gas phases using a particulate-level model.

Quick Notes

Solids: Particles are packed closely in fixed positions. Can be crystalline (ordered) or amorphous (disordered). Particles vibrate but do not move past each other.
Liquids: Particles are close together but can move past one another. Constant motion and collisions. Intermolecular forces influence flow and shape.
Gases: Particles are far apart and in constant rapid motion. Forces between particles are negligible. Gases expand to fill container and do not have fixed volume or shape.
Molar volumes of solids and liquids are similar because particles remain in close contact.
Differences in phase behavior are explained by intermolecular forces, energy (temperature), and space between particles.

Full Notes

The three main phases of matter—solid, liquid, and gas—are distinguished by how their particles are arranged and how they move. A particulate-level understanding helps explain the physical behavior of substances in each state.

Solids

Particles are closely packed and have very limited motion—mainly vibrations. Solids have definite shape and volume due to fixed particle positions.

AP Chemistry Particle diagram of a solid

Two types:

Crystalline solids: Regular, repeating 3D arrangement (e.g. NaCl, ice)
Amorphous solids: No long-range order (e.g. glass, rubber)

Influencing factors:

Intermolecular forces (IMFs) determine how strongly particles hold together.
The ability to pack efficiently affects structure and density.

Liquids

Particles are still close together, but they can move past one another. Liquids have definite volume, but no fixed shape—they take the shape of their container.

AP Chemistry Particle diagram of a liquid

Particles are in constant motion, colliding and rearranging due to:

Temperature (higher temp = more motion)
Intermolecular forces (e.g. polarity, hydrogen bonding)
Size and shape of molecules

Liquids are influenced by:

Cohesive forces (between particles of the liquid)
Adhesive forces (between liquid and container surface)

AP Chemistry Diagram of adhesion and cohesion

Molar Volume Comparison: Solids vs. Liquids

For most substances, solid and liquid molar volumes are similar because the particles remain in close contact. This is unlike gases, where particles are far apart and the molar volume is much greater.

Gases

Particles are far apart and move freely and rapidly in all directions. Gases have no definite shape or volume. They expand to fill their container.

Collisions between gas particles and with container walls explain:

Pressure (force per unit area)
Temperature dependence of speed and kinetic energy

Key features:

Negligible IMF under normal conditions
Particles move in random straight-line motion
Spacing and frequency of collisions depend on:

Temperature: higher temperature → faster motion
Pressure: more collisions at higher pressure
Volume: less space = more frequent collisions

Matt’s exam tip

Think visually:
Solids: particles tightly packed, fixed in position
Liquids: particles close together but free to move
Gases: particles spaced out, moving in all directions
Be ready to relate these pictures to properties like compressibility, shape, and volume.

Summary

Solids: Fixed shape and volume, ordered structure (especially crystalline), limited motion
Liquids: Fixed volume but no fixed shape, mobile particles, significant IMFs
Gases: No fixed shape or volume, random motion, particles far apart, negligible IMFs

Understanding these differences at the particulate level helps explain key physical behaviors such as compressibility, density, and phase changes.