The three common states of matter—solid, liquid, and gas—are distinguished by their properties of shape and volume. A liquid maintains a fixed volume but conforms to its container, while a gas possesses neither a fixed volume nor a fixed shape. By scientific definition, a solid does not take the shape of its container. Instead, it maintains its own fixed form, a characteristic resulting directly from its internal atomic structure.
The Molecular Structure That Holds the Shape
A solid’s ability to retain its shape is rooted in the arrangement and interaction of its constituent particles (atoms, ions, or molecules). Strong intermolecular forces, such as ionic bonds or van der Waals forces, lock these particles into fixed positions. In many solids, this arrangement forms a highly ordered, repeating pattern known as a crystal lattice structure.
The particles within this rigid structure are not immobile, but their movement is restricted to vibrating about these fixed points. This restricted motion prevents the particles from moving or sliding past one another under normal conditions, which stops the material from deforming. Significant external force or increased thermal energy, such as melting, is required to overcome these strong attractive forces. Because the internal bonds resist rearrangement, the solid maintains its shape and volume regardless of the container.
How Solids Differ From Fluids
Fluids include both liquids and gases, clarifying the unique properties of the solid state. Liquids have a definite volume but an indefinite shape because their particles have enough kinetic energy to partially overcome intermolecular attraction. This allows the particles to move freely and slide past each other, enabling the liquid to flow and occupy the lower portion of any container.
Gases exhibit even greater particle mobility, with particles separated by large distances and moving randomly and independently. Since kinetic energy almost entirely overcomes intermolecular forces, a gas expands completely to fill both the volume and the shape of any closed container. The fundamental difference is that a solid’s particles are spatially fixed, while a fluid’s particles have the freedom to move relative to one another.
The Exception: Granular and Amorphous Solids
Certain materials may appear to challenge the rule that a solid does not conform to its container. Granular solids, like sand, flour, or salt, are collections of countless tiny, individual solid particles. When poured, the bulk material flows and settles into the container’s shape, mimicking a fluid’s behavior.
However, each individual grain remains a solid that maintains its own fixed shape. The collective flow and rearrangement of these discrete units create the illusion of conformance. Amorphous solids, such as glass or certain plastics, lack the long-range, strict order of a crystalline lattice. Despite this internal disorder, the bonds holding their particles together are strong enough to prevent them from flowing or conforming to a container’s shape at typical temperatures, confirming their status as materials with a definite shape.