Impurity
Encyclopedia
Impurities are substances inside a confined amount of liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

, gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

, or solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...

, which differ from the chemical composition of the material or compound.

Impurities are either naturally occurring or added during synthesis
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...

 of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance.

The level of impurities in a material are generally defined in relative terms. Standards
Standardization
Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....

 have been established by various organizations that attempt to define the permitted levels of various impurities in a manufactured product. Strictly speaking, then, a material's level of purity can only be stated as being more or less pure than some other material.

Destructive impurities

Impurities can be destructive when they obstruct the working nature of the material. Examples include ash and debris
Debris
Debris is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, in geology, large rock fragments left by a melting glacier etc. The singular form of debris is debris...

 in metals and leaf pieces in blank white papers. The removal of impurities is usually done chemically. For example, in the manufacturing of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, calcium carbonate is added to the blast furnace
Blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...

 to remove silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

 from the iron ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....

. Zone refining is an economically important method for the purification of semiconductors.

However, some kinds of impurities can be removed by physical means. A mixture of water and salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 can be separated by distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

, with water as the distillate and salt as the solid residue
Residue (chemistry)
In chemistry, residue is the material remaining after a distillation or an evaporation, or to a portion of a larger molecule, such as a methyl group. It may also refer to the undesired byproducts of a reaction....

. Impurities are usually physically removed from liquids and gases. Removal of sand particles from metal ore is one example with solids.

No matter what method is used, it is usually impossible to separate an impurity completely from a material. The reason that it is impossible to remove imputities completely is of thermodynamic nature and is predicted by the second law of thermodynamics. Removing impurities completely means reducing the entropy
Entropy
Entropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...

 of the system to zero. This would require an infinite amount of work and energy as predicted by the second law of thermodynamics
Second law of thermodynamics
The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...

. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible.

Impurities and nucleation

When an impure liquid is cooled to its melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

 the liquid, undergoing a phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

, crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

lizes around the impurities and becomes a crystalline solid. If there are no impurities then the liquid is said to be pure and can be supercooled below its melting point without becoming a solid. This occurs because the liquid has nothing to condense around so the solid cannot form a natural crystalline solid. The solid is eventually formed when dynamic arrest or glass transition
Glass transition
The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass...

 occurs, but it forms into an amorphous solid
Amorphous solid
In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....

 — a glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, instead, as there is no long-range order in the structure.

Impurities play an important role in the nucleation of other phase transitions. For example, the presence of foreign elements may have important effects on the mechanical and magnetic properties of metal alloys. Iron atoms in copper cause the renowned Kondo effect
Kondo effect
In physics, the Kondo effect describes the scattering of conduction electrons in a metal due to magnetic impurities. It is a measure of how electrical resistivity changes with temperature....

 where the conduction electron spins form a magnetic bound state with the impurity atom. Magnetic impurities in superconductors can serve as generation sites for vortex
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 defects. Point defects can nucleate reversed domains in ferromagnets and dramatically affect their coercivity
Coercivity
In materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...

. In general impurities are able to serve as initiation points for phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

s because the energetic cost of creating a finite-size domain of a new phase is lower at a point defect. In order for the nucleus of a new phase to be stable, it must reach a critical size. This threshold size is often lower at an impurity site.

See also

  • Adulterant
    Adulterant
    An adulterant is a chemical substance which should not be contained within other substances for legal or other reasons. Adulterants may be intentionally added to more expensive substances to increase visible quantities and reduce manufacturing costs or for some other deceptive or malicious purpose...

  • Dross
    Dross
    Dross is a mass of solid impurities floating on a molten metal. It appears usually on the melting of low-melting-point metals or alloys such as tin, lead, zinc or aluminium, or by oxidation of the metal. It can also consist of impurities such as paint leftovers...

  • Fineness
    Fineness
    The fineness of a precious metal refers to the ratio of the primary metal to any additives or impurities.A piece of alloy metal containing a precious metal may have the weight of its precious component referred to as its fine weight. For example, 1 troy ounce of 18 karat gold may be said to have a...

  • Pollution
    Pollution
    Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...

  • Semiconductor
    Semiconductor
    A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

  • Slag
    Slag
    Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...

  • Spin wave
    Spin wave
    Spin waves are propagating disturbances in the ordering of magnetic materials. These low-lying collective excitations occur in magnetic lattices with continuous symmetry. From the equivalent quasiparticle point of view, spin waves are known as magnons, which are boson modes of the spin lattice...

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