What Is the Definition of a Solid?

Hand with metal rings resting against a brick structure.

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A solid is a state of matter characterized by particles arranged such that their shape and volume are relatively stable. The constituents of a solid tend to be packed together much closer than the particles in a gas or liquid. The reason a solid has a rigid shape is that the atoms or molecules are tightly connected via chemical bonds. The bonding may produce either a regular lattice (as seen in ice, metals, and crystals) or an amorphous shape (as seen in glass or amorphous carbon). A solid is one of the four fundamental states of matter, along with liquids, gases, and plasma.

Solid-state physics and solid-state chemistry are two branches of science dedicated to studying the properties and synthesis of solids.

Examples of Solids

The matter with a defined shape and volume is solid. There are many examples:

  • A brick
  • A penny
  • A piece of wood
  • A chunk of aluminum metal (or any metal at room temperature except mercury)
  • Diamond (and most other crystals)

Examples of things that are not solids include liquid water, air, liquid crystals, hydrogen gas, and smoke.

Classes of Solids

The different types of chemical bonds that join the particles in solids exert characteristic forces that can be used to classify solids. Ionic bonds (e.g. in table salt or NaCl) are strong bonds that often result in crystalline structures that may dissociate to form ions in water. Covalent bonds (e.g., in sugar or sucrose) involve the sharing of valence electrons. Electrons in metals seem to flow because of metallic bonding. Organic compounds often contain covalent bonds and interactions between separate portions of the molecule due to van der Waals forces.

Major classes of solids include:

  • Minerals: Minerals are natural solids formed by geological processes. A mineral has a uniform structure. Examples include diamond, salts, and mica.
  • Metals: Solid metals include elements (e.g., silver) and alloys (e.g., steel). Metals are typically hard, ductile, malleable, and excellent conductors of heat and electricity.
  • Ceramics: Ceramics are solids consisting of inorganic compounds, usually oxides. Ceramics tend to be hard, brittle, and corrosion-resistant.
  • Organic Solids: Organic solids include polymers, wax, plastics, and wood. Most of these solids are thermal and electrical insulators. They typically have lower melting and boiling points than metals or ceramics.
  • Composite Materials: Composite materials are those which contain two or more phases. An example would be a plastic containing carbon fibers. These materials yield properties not seen in the source components.
  • Semiconductors: Semiconducting solids have electrical properties intermediate between those of conductors and insulators. The solids may be either pure elements, compounds, or doped materials. Examples include silicon and gallium arsenide.
  • Nanomaterials: Nanomaterials are tiny solid particles at the nanometer size. These solids may display very different physical and chemical properties from large-scale versions of the same materials. For example, gold nanoparticles are red and melt at a lower temperature than gold metal.
  • Biomaterials: Biomaterials are natural materials, such as collagen and bone, that are often capable of self-assembly.
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Helmenstine, Anne Marie, Ph.D. "What Is the Definition of a Solid?" ThoughtCo, Aug. 29, 2020, thoughtco.com/definition-of-solid-604648. Helmenstine, Anne Marie, Ph.D. (2020, August 29). What Is the Definition of a Solid? Retrieved from https://www.thoughtco.com/definition-of-solid-604648 Helmenstine, Anne Marie, Ph.D. "What Is the Definition of a Solid?" ThoughtCo. https://www.thoughtco.com/definition-of-solid-604648 (accessed June 8, 2023).