Solid

A solid is one of the main states of matter. In this state, atoms are packed closely together and are hard to move past each other. Solids stay in one shape and do not flow like liquids or spread out like gases. They resist being squished or stretched, which is why many everyday objects, like tables and chairs, keep their shape.

Solids are formed when liquids or gases cool down below a special temperature called the melting point. At this temperature, the atoms slow down enough to stick together in fixed positions. Different solids have different melting points. For example, water turns into ice at 0°C, while some metals need very high temperatures before they melt.

The atoms in solids can be arranged in two main ways: in regular patterns called crystalline solids, like diamonds or ice, or in irregular patterns called amorphous solids, like glass or plastic. These arrangements affect how the solid looks and behaves. Most solids are denser than their liquid forms, but there are exceptions, like water, which is less dense when frozen into ice.

The study of solids is important in physics and materials science. Understanding how solids work helps scientists and engineers create new materials with special properties, such as stronger metals or better electronic devices. Solids are all around us, and their unique properties make the world we live in possible.

Microscopic description

Solids are made up of tiny particles like atoms, molecules, or ions that are packed closely together. These particles can be arranged in a regular, repeating pattern, which we call a crystal. For example, diamonds are a type of crystal where the pattern continues for a very long distance. Most solid objects we see, like metal objects or ceramics, are made of many tiny crystals called crystallites, and these materials are known as polycrystalline.

Some solids, however, do not have a regular pattern at all. These are called amorphous solids, and examples include materials like polystyrene and glass. Whether a solid is crystalline or amorphous depends on how it was made. Solids that cool slowly are more likely to form crystals, while those that cool quickly tend to be amorphous. Many everyday materials, like rocks or wood, are mixtures of different substances working together.

Classes of solids

Further information: Bonding in solids

The forces between atoms in a solid can take many forms. For example, a crystal of sodium chloride (common salt) is made of ionic sodium and chlorine, held together by ionic bonds. In diamond or silicon, atoms share electrons and form covalent bonds. In metals, electrons are shared in metallic bonding. Some solids, especially organic compounds, are held together with van der Waals forces from the polarization of electronic charge on each molecule. The differences between types of solids come from the differences in their bonding.

Metals

Main article: Metal

Metals are usually strong, dense, and good conductors of both electricity and heat. Most elements in the periodic table, those to the left of a diagonal line from boron to polonium, are metals. Mixtures of two or more elements where the main part is a metal are called alloys.

People have used metals for many purposes since ancient times. Their strength and reliability has led to their use in building structures, vehicles, appliances, tools, pipes, road signs, and railroad tracks. Iron and aluminium are the two most common structural metals. Iron is often used as an alloy called steel, which contains up to 2.1% carbon, making it harder than pure iron.

Because metals conduct electricity well, they are important in electrical devices and for carrying electric current over long distances with little energy loss. For example, home wiring uses copper for its good conducting properties. Metals' ability to conduct heat also makes them useful for cooking utensils.

Minerals

Main article: Minerals

Minerals are natural solids formed by geological processes under high pressure. To be a true mineral, a substance must have a crystal structure with uniform properties. Minerals can be pure elements, simple salts, or complex silicates. Rocks, in contrast, are mixtures of minerals and have no specific chemical composition. Most rocks in the Earth's crust contain minerals like quartz, feldspar, mica, chlorite, and others. Some minerals, like quartz or mica, are common, while others are rare.

Ceramics

Main article: Ceramic engineering

Ceramic solids are made of inorganic compounds, usually oxides of elements. They resist chemicals and can handle high temperatures, from 1,000 to 1,600 °C. Examples include traditional materials like clay and modern ones like alumina. Advanced ceramics, such as silicon carbide, are used for their hardness in tools and wear-resistant parts.

Ceramics are usually brittle but strong when compressed. They are used in many applications, such as knife blades made from zirconia, and in bulletproof vests to stop bullets. Ceramics can also be used in engines to improve fuel efficiency.

Glass ceramics

Main article: Glass-ceramic

Glass-ceramic materials have properties of both glass and ceramics. They are made by heating glass until it partly turns into a crystal. This creates materials that can handle quick temperature changes without breaking. They are used in cookware and stovetops.

Organic solids

Main article: Organic chemistry

Organic chemistry studies compounds made of carbon and hydrogen, sometimes with other elements like nitrogen or oxygen. Examples of organic solids include wood, paraffin wax, and many polymers and plastics.

Wood

Main article: Wood

Wood is a natural material made of cellulose fibers in a lignin matrix. It is strong and has been used for building and making tools for a long time. Wood is also used to make paper and packaging materials.

Polymers

Main article: Polymer

Polymers are chains of repeating molecules. Natural polymers include cellulose and natural rubber. Man-made polymers, called plastics, include polyethylene, polypropylene, and many others. These materials are used in many everyday products.

Composite materials

Main article: Composite material

Composite materials combine two or more different materials to get new properties. For example, concrete mixed with steel makes it stronger. Composites are used in buildings, aircraft, and everyday items like plastic casings for electronics.

Semiconductors

Main article: Semiconductors

Semiconductors have electrical properties between metals and insulators. They are used to make electronic devices like transistors, solar cells, and computers. Common semiconductors include silicon, germanium, and gallium arsenide.

Nanomaterials

Main article: Nanotechnology

When materials are made very small, at the nanometer scale, they can have new properties. For example, tiny particles of gold can be red instead of yellow. These nanomaterials are used in energy applications, like improving batteries and solar cells.

Physical properties

Physical properties help us understand what materials are made of. These properties include things like color, shape, how heavy something is, and how it changes with temperature. For solids, we look at how they stay strong and keep their shape.

Solids can be stretched, squeezed, or bent, but they usually stay the same size and shape. Some solids, like steel, are very strong and don’t bend easily. Others might bend or change shape more easily. Scientists study how solids react to forces to learn about their strength and flexibility.

Solids also have special ways of handling heat and electricity. Some let heat pass through easily, while others keep it trapped. The same goes for electricity—some solids conduct electricity well, like metals, while others don’t, like glass. These properties make solids useful in many everyday items, from wires to windows.

Fields of study

Solids are studied in different areas of science. Solid-state physics looks at how atoms are arranged and how they move in solid materials. Solid-state chemistry focuses on the chemical properties of solids and how they react. Materials science explores how we can use solids to create new products and technologies, helping us understand both natural and man-made materials.