Solids, liquids, and gases differ in the arrangement, motion, and interactions of their particles. In solids, particles vibrate around fixed positions; in liquids, they remain close but move past one another; in gases, they move freely and are widely separated.
This molecular model is part of B.1 Thermal energy transfers and applies to both SL and HL.
| State | Particle arrangement and motion | Macroscopic properties |
|---|---|---|
| Solid | Particles are closely packed in an ordered or disordered structure. Strong intermolecular interactions hold them near fixed equilibrium positions, but they continuously vibrate. | Fixed shape and volume; usually difficult to compress. |
| Liquid | Particles remain close together, but their positions are not fixed. They move randomly and rearrange by sliding past one another. | Fixed volume but no fixed shape; difficult to compress and able to flow. |
| Gas | Particles are widely separated and move rapidly in random directions. Intermolecular forces are usually negligible except during collisions. | No fixed shape or volume; easily compressed and expands to fill its container. |
The state of matter also depends on internal energy, which is the sum of the particles' random kinetic energy and intermolecular potential energy. Heating generally increases internal energy. During a phase change, energy can increase intermolecular potential energy without increasing temperature because particles become more separated or less strongly bound.
A common misconception is that particles in a solid do not move. They do move: they vibrate around equilibrium positions, and their average vibrational kinetic energy generally increases as temperature rises.
For an IB Explain question, link microscopic behaviour to macroscopic properties. For example: a gas is compressible because its particles are widely separated, leaving substantial empty space between them. Avoid merely listing properties; state the particle-level cause of each property and use “particles” unless the substance is specifically molecular.
