What Is a Solid?
A solid is one of the three main states of matter. Unlike liquids or gases, a solid keeps its own shape and volume no matter what container it is placed in. A wooden block stays a block; a rock stays a rock.
Solids are all around you — chairs, books, coins, ice cubes, and bones are all solids at room temperature.
Particles in a Solid
Everything is made of tiny particles (atoms or molecules). In a solid, these particles are:
- Packed very closely together — there is almost no empty space between them.
- Arranged in a regular pattern — many solids, like salt crystals and metals, have particles in a neat grid called a crystal lattice.
- Held by strong forces — the particles cannot move past each other.
- Vibrating in place — the particles are not completely still; they shake back and forth around fixed positions, but they do not move freely.
Because the particles cannot change their positions, a solid does not flow or change shape on its own.
Ice is a solid form of water. Its particles are arranged in a hexagonal (six-sided) crystal lattice, which is why snowflakes always have six sides or six branches.
Physical Properties of Solids
| Property | Description |
|---|---|
| Shape | Fixed — does not change unless a force is applied |
| Volume | Fixed — stays the same |
| Compressibility | Very hard to compress (squeeze smaller) |
| Density | Usually high — particles are closely packed |
| Flow | Does not flow |
Some solids are hard (diamond, steel), while others are soft (chalk, wax). The hardness depends on how strongly the particles are bonded together.
Types of Solids
Crystalline solids have particles in a repeating, ordered pattern. Examples include table salt (sodium chloride), sugar, and quartz.
Amorphous solids have particles arranged randomly with no regular pattern. Glass and rubber are amorphous solids. They do not have sharp melting points like crystalline solids do.
You can test whether a solid is crystalline by looking at it under a magnifying glass. Many crystals have flat faces and sharp edges because of their ordered particle arrangement.
Heating and Changing State
When a solid is heated, its particles vibrate faster and faster. Eventually, if enough heat energy is added, the particles break free from their fixed positions and the solid melts into a liquid. The temperature at which this happens is called the melting point.
Different solids melt at different temperatures. Ice melts at 0 °C, while iron melts at around 1538 °C.
Real world: Ice cubes in your drink
Ice cubes are solid water. They keep their fixed shape in your glass — they do not flow or spread out like liquid water would. As they absorb heat from the drink, they eventually melt, showing how solids change state at their melting point (0 °C for ice).
Real world: Steel beams in construction
Steel is a hard crystalline solid. The particles in steel are tightly packed in a regular lattice, making it rigid and very difficult to compress. This is why steel beams in buildings and bridges can support enormous weight without changing shape.
Real world: Car tyres
Rubber used in car tyres is an amorphous solid — its particles have no regular arrangement. This gives rubber flexibility, allowing it to absorb bumps. Unlike crystalline solids, rubber has no sharp melting point; it gradually softens when heated.
Where do you see this in real life? Look around you — every object with a fixed shape (your phone, a coin, a piece of chalk) is a solid. Notice how solids hold their shape even when moved between containers.
Practice Problems
Why does a solid keep its shape when placed in a container, unlike a liquid?
Show answer
What happens to the particles in a solid when it is heated until it melts?