Impurity


Impurity

Impurities are substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound.

Impurities are either naturally occurring or added during synthesis 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 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.

Contents

Destructive impurities

Impurities can be destructive when they obstruct the working nature of the material. Examples include ash and 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, calcium carbonate is added to the blast furnace to remove silicon dioxide from the iron ore. 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 can be separated by distillation, 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 imputities completely is of thermodynamic nature and is predicted by the second law of thermodynamics. Removing impurities completely means reducing the entropy of the system to zero. This would require an infinite amount of work and energy as predicted by the second law of thermodynamics. 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 the liquid, undergoing a phase transition, crystallizes 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 — a glass, 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 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 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 phase transitions 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

Line notes

References

  1. Longman's English-Chinese Dictionary of Chemistry, Hong Kong, 1997.
  2. Cheng, E. et al., Chemistry - A Modern View, Aristo-Wilson, Hong Kong, 2004

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  • Impurity — Im*pu ri*ty, n.; pl. Impurities . [L. impuritas: cf. F. impuret[ e].] [1913 Webster] 1. The condition or quality of being impure in any sense; defilement; foulness; adulteration. [1913 Webster] Profaneness, impurity, or scandal, is not wit.… …   The Collaborative International Dictionary of English

  • impurity — index vice Burton s Legal Thesaurus. William C. Burton. 2006 …   Law dictionary

  • impurity — (n.) mid 15c., thing which makes or is impure; c.1500, fact or quality of being impure, from M.Fr. impurité, from impur (see IMPURE (Cf. impure)). Related: Impurities …   Etymology dictionary

  • impurity — [n] contaminant adulteration, contamination, corruption, defilement, dirt, filth, foreign matter, grime, pestilence, poisoning, pollutant, pollution, scum, stain, taint, uncleanness; concepts 230,306,674 …   New thesaurus

  • impurity — ► NOUN (pl. impurities) 1) the quality or condition of being impure. 2) a thing which impairs the purity of something …   English terms dictionary

  • impurity — [im pyoor′ə tē] n. [OFr impurité < L impuritas] 1. the state or quality of being impure 2. pl. impurities an impure thing or element …   English World dictionary

  • Impurity — (Roget s Thesaurus) < N PARAG:Impurity >N GRP: N 1 Sgm: N 1 impurity impurity Sgm: N 1 uncleanness uncleanness &c.(filth) 653 Sgm: N 1 immodesty immodesty Sgm: N 1 grossness grossness &c. >Adj. Sgm: N 1 indelicacy …   English dictionary for students

  • impurity — UK [ɪmˈpjʊərətɪ] / US [ɪmˈpjʊrətɪ] noun Word forms impurity : singular impurity plural impurities 1) [countable] an amount of a substance, especially one that is not valuable or is of low quality, that is present in another substance new methods… …   English dictionary

  • impurity — noun 1) the impurity of the cast iron Syn: adulteration, debasement, degradation, corruption; contamination, pollution 2) the impurities in beer Syn: contaminant, pollutant, foreign body, foreign matter; dross …   Thesaurus of popular words

  • impurity — noun /ɪmˈpjɝɪti,ɪmˈpjuɹɪti/ a) The condition of being impure; because of contamination, pollution, adulteration or insufficient purification. Even animals in the Jewish system cause impurity only when they are dead …   Wiktionary


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