Solid

Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the specific material under consideration. Solids also always possess the least amount of kinetic energy per atom/molecule relative to other phases or, equivalently stated, solids are formed when matter in the liquid / gas phase is cooled below a certain temperature. This temperature is called the melting point of that substance and is an intrinsic property, i.e. independent of how much of the matter there is. All matter in solids can be arranged on a microscopic scale under certain conditions.

Solids are characterized by structural rigidity and resistance to applied external forces and pressure. Unlike liquids, solids do not flow to take on the shape of their container, nor do they expand to fill the entire available volume like a gas. Much like the other three fundamental phases, solids also expand when heated, the thermal energy put into increasing the distance and reducing the potential energy between atoms. However, solids do this to a much lesser extent. When heated to their melting point or sublimation point, solids melt into a liquid or sublimate directly into a gas, respectively. For solids that directly sublimate into a gas, the melting point is replaced by the sublimation point. As a rule of thumb, melting will occur if the subjected pressure is higher than the substance's triple point pressure, and sublimation will occur otherwise. Melting and melting points refer exclusively to transitions between solids and liquids. Melting occurs across a great extent of temperatures, ranging from 0.10 K for helium-3 under 30 bars (3 MPa) of pressure, to around 4,200 K at 1 atm for the composite refractory material hafnium carbonitride.

The atoms in a solid are tightly bound to each other in one of two ways: regular geometric lattices called crystalline solids (e.g. metals, water ice), or irregular arrangements called amorphous solids (e.g. glass, plastic). Molecules and atoms forming crystalline lattices usually organize themselves in a few well-characterized packing structures, such as body-centered cubic. The adopted structure can and will vary between various pressures and temperatures, as can be seen in phase diagrams of the material (e.g. that of water, see left and upper). When the material is composed of a single species of atom/molecule, the phases are designated as allotropes for atoms (e.g. diamond / graphite for carbon), and as polymorphs (e.g. calcite / aragonite for calcium carbonate) for molecules.

Non- porous solids invariably strongly resist any amount of compression that would otherwise result in a decrease of total volume regardless of temperature,DEGENERACY PRESSURE IN A SOLID
/ref> owing to the mutual-repulsion of neighboring electron clouds among its constituent atoms. In contrast to solids, gases are very easily compressed as the molecules in a gas are far apart with few intermolecular interactions. Some solids, especially metallic alloys, can be deformed or pulled apart with enough force. The degree to which this solid resists deformation in differing directions and axes are quantified by the elastic modulus, tensile strength, specific strength, as well as other measurable quantities.

For the vast majority of substances, the solid phases have the highest density, moderately higher than that of the liquid phase (if there exists one), and solid blocks of these materials will sink below their liquids. Exceptions include water ( icebergs), gallium, and plutonium. All naturally occurring elements on the periodic table has a melting point at standard atmospheric pressure, with three exceptions: the noble gas helium, which remains a liquid even at absolute zero owing to zero-point energy; the metalloid arsenic, sublimating around 900 K; and the life-forming element carbon, which sublimates around 3,950 K.

When applied pressure is released, solids will (very) rapidly re-expand and release the stored energy in the process in a manner somewhat similar to those of gases. An example of this is the (oft-attempted) confinement of freezing water in an inflexible container (of steel, for example). The gradual freezing results in an increase in volume, as ice is less dense than water. With no additional volume to expand into, water ice subjects the interior to intense pressures, causing the container to explode with great force.

Solids' properties on a macroscopic scale can also depend on whether it is contiguous or not. Contiguous (non-aggregate) solids are characterized by structural rigidity (as in rigid bodies) and strong resistance to applied forces. For solids aggregates (e.g. gravel, sand, dust on lunar surface), solid particles can easily slip past one another, though changes of individual particles (quartz particles for sand) will still be greatly hindered. This leads to a perceived softness and ease of compression by operators. An illustrating example is the non-firmness of coastal sandand of the lunar regolith.

The branch of physics that deals with solids is called solid-state physics, and is a major branch of condensed matter physics (which includes liquids). Materials science, also one of its numerous branches, is primarily concerned with the way in which a solid's composition and its properties are intertwined.

Microscopic description

The atoms, molecules or ions that make up solids may be arranged in an orderly repeating pattern, or irregularly. Materials whose constituents are arranged in a regular pattern are known as crystals. In some cases, the regular ordering can continue unbroken over a large scale, for example diamonds, where each diamond is a single crystal. Solid objects that are large enough to see and handle are rarely composed of a single crystal, but instead are made of a large number of single crystals, known as crystallites, whose size can vary from a few nanometers to several meters. Such materials are called polycrystalline. Almost all common metals, and many ceramics, are polycrystalline.

In other materials, there is no long-range order in the position of the atoms. These solids are known as amorphous solids; examples include polystyrene and glass.

Whether a solid is crystalline or amorphous depends on the material involved, and the conditions in which it was formed. Solids that are formed by slow cooling will tend to be crystalline, while solids that are frozen rapidly are more likely to be amorphous. Likewise, the specific crystal structure adopted by a crystalline solid depends on the material involved and on how it was formed.

While many common objects, such as an ice cube or a coin, are chemically identical throughout, many other common materials comprise a number of different substances packed together. For example, a typical rock is an aggregate of several different minerals and mineraloids, with no specific chemical composition. Wood is a natural organic material consisting primarily of cellulose fibers embedded in a matrix of organic lignin. In materials science, composites of more than one constituent material can be designed to have desired properties.

Classes of solids

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

Metals

Metals typically are strong, dense, and good conductors of both electricity and heat.
The bulk of the elements in the periodic table, those to the left of a diagonal line drawn from boron to polonium, are metals.
Mixtures of two or more elements in which the major component is a metal are known as alloys.

People have been using metals for a variety of purposes since prehistoric times.
The strength and reliability of metals has led to their widespread use in construction of buildings and other structures, as well as in most vehicles, many appliances and tools, pipes, road signs and railroad tracks. Iron and aluminium are the two most commonly used structural metals. They are also the most abundant metals in the Earth's crust. Iron is most commonly used in the form of an alloy, steel, which contains up to 2.1% carbon, making it much harder than pure iron.

Because metals are good conductors of electricity, they are valuable in electrical appliances and for carrying an electric current over long distances with little energy loss or dissipation. Thus, electrical power grids rely on metal cables to distribute electricity. Home electrical systems, for example, are wired with copper for its good conducting properties and easy machinability. The high thermal conductivity of most metals also makes them useful for stovetop cooking utensils.

The study of metallic elements and their alloys makes up a significant portion of the fields of solid-state chemistry, physics, materials science and engineering.

Metallic solids are held together by a high density of shared, delocalized electrons, known as " metallic bonding". In a metal, atoms readily lose their outermost ("valence") electrons, forming positive ions. The free electrons are spread over the entire solid, which is held together firmly by electrostatic interactions between the ions and the electron cloud. The large number of free electrons gives metals their high values of electrical and thermal conductivity. The free electrons also prevent transmission of visible light, making metals opaque, shiny and lustrous.

More advanced models of metal properties consider the effect of the positive ions cores on the delocalised electrons. As most metals have crystalline structure, those ions are usually arranged into a periodic lattice. Mathematically, the potential of the ion cores can be treated by various models, the simplest being the nearly free electron model.

Minerals

Minerals are naturally occurring solids formed through various geological processes under high pressures. To be classified as a true mineral, a substance must have a crystal structure with uniform physical properties throughout. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms. In contrast, a rock sample is a random aggregate of minerals and/or mineraloids, and has no specific chemical composition. The vast majority of the rocks of the Earth's crust consist of quartz (crystalline SiO₂), feldspar, mica, chlorite, kaolin, calcite, epidote, olivine, augite, hornblende, magnetite, hematite, limonite and a few other minerals. Some minerals, like quartz, mica or feldspar are common, while others have been found in only a few locations worldwide. The largest group of minerals by far is the silicates (most rocks are ≥95% silicates), which are composed largely of silicon and oxygen, with the addition of ions of aluminium, magnesium, iron, calcium and other metals.

Ceramics

Ceramic solids are composed of inorganic compounds, usually oxides of chemical elements. They are chemically inert, and often are capable of withstanding chemical erosion that occurs in an acidic or caustic environment. Ceramics generally can withstand high temperatures ranging from . Exceptions include non-oxide inorganic materials, such as nitrides, borides and carbides.

Traditional ceramic raw materials include clay minerals such as kaolinite, more recent materials include aluminium oxide ( alumina). The modern ceramic materials, which are classified as advanced ceramics, include silicon carbide and tungsten carbide. Both are valued for their abrasion resistance, and hence find use in such applications as the wear plates of crushing equipment in mining operations.

Most ceramic materials, such as alumina and its compounds, are formed from fine powders, yielding a fine grained polycrystalline microstructure that is filled with light-scattering centers comparable to the wavelength of visible light. Thus, they are generally opaque materials, as opposed to transparent materials. Recent nanoscale (e.g. sol-gel) technology has, however, made possible the production of polycrystalline transparent ceramics such as transparent alumina and alumina compounds for such applications as high-power lasers. Advanced ceramics are also used in the medicine, electrical and electronics industries.

Ceramic engineering is the science and technology of creating solid-state ceramic materials, parts and devices. This is done either by the action of heat, or, at lower temperatures, using precipitation reactions from chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components, and the study of their structure, composition and properties.

Mechanically speaking, ceramic materials are brittle, hard, strong in compression and weak in shearing and tension. Brittle materials may exhibit significant tensile strength by supporting a static load. Toughness

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