A solid is defined as a state of matter with a definite shape and volume. In contrast, liquids can change shapes, while gases can change both shape and volume. The particles in a solid (atoms, molecules, ions) are tightly-packed compared to liquids and gases. The arrangement may be a regular lattice called a crystal or an irregular arrangement called an amorphous solid.
Properties of Solids
Properties of solids include:
- Particles are packed closely together. This allows atoms and molecules to form chemical bonds.
- Solids are rigid.
- Solids don’t flow.
- Solids aren’t readily compressible.
Examples of Solids
Anything with a fixed shape and volume is an example of a solid. Examples of solids include:
- Most metals (coins, tools, cutlery, nails)
- Building materials (bricks, wood, glass, concrete)
- Everyday objects (pots and pans, desk, toys, computer, automobile)
- Rocks and minerals
- Gems and most crystals (diamond, sapphire, ruby)
- Most chemical elements (exceptions include many nonmetals)
Examples of things that are not solids include air, water, liquid crystals, the elements mercury and helium, and steam.
Classes of Solids
There are different ways to classify solids.
Solids may be categorized as crystalline, polycrystalline, or amorphous.
- Crystalline Solid: Particles in a crystalline solid are arranged in a regular lattice. A good example is a salt crystal (sodium chloride).
- Polycrystalline Solid: In polycrystalline solids, tiny crystals called crystallites join together to form a larger structure. Many ceramics are polycrystalline.
- Amorphous Solid: In an amorphous solid, particles are packed together in an irregular manner. Glass and polystyrene are examples of amorphous solids.
Another way to classify solids is by the nature of their chemical bonds.
- Ionic Solids: Some solids contain ionic bonds, such as sodium chloride. These solids consist of positively-charged cations and negatively-charged anions that are strongly attracted to each other. Ionic solids tend to form brittle crystals with high melting points. Because of the polarity of the ionic bond, many ionic solids dissolve in water, forming solutions that conduct electricity.
- Molecular Solids: Molecular solids form using covalent bonds. Examples of molecular solids include ice and sugar. Molecular solids tend to be nonpolar and have lower melting points than ionic solids. Most molecular solids are softer than ionic solids.
- Network Covalent Solids: Particles in a network covalent solid form a continuous network with each atom bonded to the surrounding atoms (basically a giant single molecule). Network solids have properties similar to ionic solids. They tend to be hard and brittle, with high melting points. Unlike ionic compounds, they don’t dissolve in water and are poor electrical conductors. Diamonds and rubies are examples of network covalent solids.
- Metallic Solids: Atoms in metals are held together by metallic bonding. Because the electrons are relatively free to move, metals conduct heat and electricity. Metallic solids are opaque, malleable, and ductile.
A third way to classify solids is by their composition. Major classes of solids are:
- Metals: Except for mercury, elemental metals are solids. Most alloys are also solids. Metals are hard, malleable, ductile, and usually good electrical and thermal conductors. Examples of solid metals include silver, brass, and steel.
- Minerals: Minerals are natural inorganic solids. Examples include salts, mica, and diamond.
- Ceramics: Ceramics are solids made from inorganic compounds, usually oxides. Ceramics are hard, brittle, and corrosion-resistant.
- Organic Solids: Organic solids include waxes, plastics, polymers, hair, fingernails, and wood. Most organic solids are electrical and thermal insulators with lower melting and boiling points than metals or ceramics.
- Composite Materials: Composite materials are solids consisting of two or more phases. For example, plastic with carbon fibers is a composite material.
- Semiconductors: Semiconductors are solids with electrical properties in between those of insulators and conductors. They may be elements, compounds, or doped materials. Examples of semiconductors include gallium arsenide and silicon.
- Biomaterials: Biomaterials are a special class of organic solids that are made by living organisms. Some biomaterials are capable of self-assembly. Examples include collagen and bone.
- Nanomaterials: Nanomaterials are tiny solids that measure nanometers in size. These extremely small solids display different chemical and physical properties compared with their larger counterparts. For example, gold nanoparticles are red rather than gold and melt at a lower temperature than ordinary gold.
- Holley, Dennis (2017). General Biology I: Molecules, Cells and Genes. Dog Ear Publishing. ISBN 9781457552748.
- Narula, G. K.; Narula, K. S.; Gupta, V. K. (1989). Materials Science. Tata McGraw-Hill Education. ISBN 9780074517963.