chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

and materials science, impurities are

chemical substance A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wit ...

s inside a confined amount of liquid, gas, or

solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structur ...

, which differ from the chemical composition of the material or compound. Firstly, a pure chemical should appear thermodynamically in at least one chemical phase and can also be characterized by its one-component- phase diagram. Secondly, practically speaking, a pure chemical should prove to be

homogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

(i.e., will show no change of properties after undergoing a wide variety of consecutive analytical chemical procedures). The perfect pure chemical will pass all attempts and tests of further separation and purification. Thirdly, and here we focus on the common chemical definition, it should not contain any trace of any other kind of chemical species. In reality, there are no absolutely 100% pure chemical compounds, as there is always some minute

contamination Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc. Types of contamination ...

. Indeed, as detection limits in analytical chemistry decrease, the number of impurities detected tends to increase. Impurities are either naturally occurring or added during

synthesis Synthesis or synthesize may refer to: Science Chemistry and biochemistry * Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors **Organic synthesis, the chemical synthesis of organ ...

of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance. The levels of impurities in a material are generally defined in relative terms.

Standards Standard may refer to: Symbols * Colours, standards and guidons, kinds of military signs * Standard (emblem), a type of a large symbol or emblem used for identification Norms, conventions or requirements * Standard (metrology), an object t ...

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 Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non-gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...

and debris in

metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typical ...

and leaf pieces in blank white papers. The removal of impurities is usually done chemically. For example, in the manufacturing of

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

, calcium carbonate is added to the blast furnace to remove

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

from the

iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the ...

. 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 is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

can be separated by

distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the he ...

, with water as the distillate and salt as the solid residue. 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 impurities completely is of thermodynamic nature and is predicted by the second law of thermodynamics. Removing impurities completely means reducing their concentration to zero. This would require an infinite amount of work and energy as predicted by the

second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unle ...

. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible. Impurities in pharmaceuticals and therapeutics are of special concern and the last couple of decades have witnessed a fair number of scandals, from insecure ingredients and incorrect dosage forms to intentionally fortified medications and accidental contaminations.

Impurities and nucleation

When an impure liquid is cooled to its

melting point The melting point (or, rarely, liquefaction point) of a substance 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 depen ...

the liquid, undergoing a

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states ...

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

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 occurs, but it forms into an

amorphous solid In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek ''a'' (" ...

– a

glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...

, 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 In physics, the Kondo effect describes the scattering of conduction electrons in a metal due to magnetic impurities, resulting in a characteristic change i.e. a minimum in electrical resistivity with temperature. The cause of the effect was fi ...

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 In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in ...

defects. Point defects can nucleate reversed domains in ferromagnets and dramatically affect their coercivity. In general impurities are able to serve as initiation points for

s because the energetic cost of creating a finite-size

domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function ** Natural domain of a partial function **Domain of holomorphy of a function * ...

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.

References

* Longman's English-Chinese Dictionary of Chemistry, Hong Kong, 1997. * Cheng, E. et al., ''Chemistry – A Modern View'', Aristo-Wilson, Hong Kong, 2004 {{Combined substance use and adulteration Materials Environmental chemistry Adulteration ru:Примесь (металлургия)

Destructive impurities

Impurities and nucleation

See also

References