Solid
Summary: The state of matter with a fixed shape and fixed volume, in which particles vibrate about fixed positions in a regular arrangement held by strong forces of attraction. Tags: igcse chemistry Created: 2026-07-17
A solid is the state of matter in which a substance has both a fixed shape and a fixed volume, and it is the state with the lowest energy of the three states of matter. According to the kinetic particle theory, the particles in a solid are packed closely together in a regular, ordered arrangement, often described as a lattice, and they cannot move from place to place. Instead, the particles vibrate continuously about fixed positions, and the amplitude of these vibrations increases as the solid is heated. The particles are held in place by strong forces of attraction between them, which explains why solids cannot flow and are extremely difficult to compress. Because the particles are so closely packed, solids generally have high densities compared with gases of the same substance. When a solid is heated to its melting point, the particles gain enough energy to partially overcome the forces holding them in their fixed positions, and the solid melts into a liquid.
Properties of Solids
Solids have a set of characteristic macroscopic properties, each of which can be explained by the arrangement, motion, and forces of the particles within them.
| Property | Description | Particle explanation |
|---|---|---|
| Fixed shape | A solid keeps its own shape and does not take the shape of its container | Particles are held in fixed positions in a regular arrangement and cannot move past one another |
| Fixed volume | The volume of a solid does not change when it is moved between containers | Particles are already touching and cannot spread out or be pushed significantly closer |
| Incompressible | Solids cannot be squashed into a smaller volume | There is almost no empty space between the closely packed particles |
| High density | A given volume of solid contains a large mass | A very large number of particles is packed into a small volume |
| Cannot flow | Solids do not pour or flow | Particles cannot slide past each other because they are locked in position by strong forces |
Particle arrangement and motion
The particles in a solid are arranged in a regular, repeating pattern and are in direct contact with their neighbours. Although the particles cannot change position, they are not stationary: they vibrate about fixed points, and the energy of these vibrations depends on temperature. At higher temperatures the vibrations become more vigorous, causing the solid to expand slightly; at the melting point the vibrations become strong enough for the regular structure to break down.
Forces between particles
The forces of attraction between particles in a solid are strong — stronger than in the liquid state and far stronger than in the gaseous state. These strong forces hold the particles in their fixed positions and give the solid its rigidity. The strength of these forces varies between substances, which is why different solids have very different melting points: substances with very strong forces between particles (such as ionic lattices or giant covalent structures) melt at much higher temperatures than substances held together by weak intermolecular forces.
Comparison with liquids and gases
| Feature | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes shape of container below its surface | Fills whole container |
| Volume | Fixed | Fixed | Not fixed |
| Compressibility | Incompressible | Very slightly compressible | Highly compressible |
| Density | High | High (usually slightly lower than solid) | Low |
| Particle arrangement | Regular, closely packed | Random, closely packed | Random, far apart |
| Particle motion | Vibrate about fixed positions | Slide past one another | Rapid, random motion in straight lines |
| Forces between particles | Strong | Weaker than in solids | Negligible |
Shape of Solid
A solid has a fixed shape — it does not change shape when placed in a different container. This is because the particles in a solid are held in fixed positions by strong forces of attraction and are arranged in a regular, repeating pattern (a lattice). Since particles cannot move past one another, the solid retains its own shape regardless of the container. This contrasts with liquids and gases, whose particles can move freely and therefore take the shape of their container.
Exam point: “Fixed shape” means the solid keeps its own shape; it does NOT take the shape of its container.
Volume of Solid
A solid has a fixed volume — its volume does not change when it is moved between containers. The particles in a solid are already closely packed and touching, with almost no empty space between them. They cannot spread out to occupy more volume, nor can they be pushed significantly closer together under ordinary pressure. Therefore the volume of a solid remains constant.
Exam point: “Fixed volume” means the volume stays the same no matter what container the solid is in.
Density of Solid
Solids generally have high density — a given volume of solid contains a large mass. This is because the particles are packed very closely together in a regular arrangement, so a very large number of particles occupies a small volume. Different solids have different densities depending on the mass of their individual particles and how closely they are packed.
Exam point: Solids have high density because particles are tightly packed with almost no empty space.
Incompressibility of Solid
Solids are incompressible — they cannot be squashed into a smaller volume. This is because there is almost no empty space between the closely packed particles. Any attempt to compress a solid simply forces already-touching particles against each other, which requires enormous pressure and produces almost no change in volume.
Exam point: “Cannot be compressed” means the particles are already touching with no space to squeeze into.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| The particles in a solid do not move at all | The particles in a solid cannot change position, but they constantly vibrate about fixed points; the vibrations increase in energy as temperature rises |
| The particles themselves are hard and rigid, or expand when a solid is heated | Individual particles do not change size or hardness; the properties of a solid arise from the arrangement of particles and the strong forces between them, and expansion on heating is due to particles vibrating more and moving slightly further apart |