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Chemical impurity
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<h2 id="unwanted-impurities">Unwanted impurities</h2> <p>Impurities can become unwanted when they prevent the working nature of the material. Examples include ash and <a href="/facts/Debris/V10YQYym">debris</a> in <a href="/facts/Metals/PT8hNX6R">metals</a> and leaf pieces in blank white papers. The removal of impurities is usually done chemically. For example, in the manufacturing of <a href="/facts/Iron/QK1JYy8E">iron</a>, calcium carbonate is added to the <a href="/facts/Blast_furnace/DJzUB2EG">blast furnaces</a> to remove <a href="/facts/Silicon_dioxide/A3WILp6J">silicon dioxide</a> from the <a href="/facts/Iron_ore/rQMEvgMh">iron ore</a>. <a href="/facts/Zone_refining/y5qBd8RB">Zone refining</a>, another purification method, is an economically important method for the purification of semiconductors. </p> <p>However, some kinds of impurities can be removed by physical means. A mixture of water and <a href="/facts/Salt/SCplalQ3">salt</a> can be separated by <a href="/facts/Distillation/plDvUMxn">distillation</a>, with water as the distillate and salt as the solid <a href="/facts/Residue_(chemistry)/xE6sUTDa">residue</a>. This is done by heating the water so it boils and leaves behind the salt. The water is cooled and the gas turns back to a pure liquid.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> Impurities are usually physically removed from liquids and gases. Removal of sand particles from metal ore is one example with solids. </p><p>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 <a href="/facts/Second_law_of_thermodynamics/m7SJ2eeH">second law of thermodynamics</a>. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p><p>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.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <h2 id="wanted-impurities">Wanted impurities</h2> <p>Occasionally, we may want to include impurities in a material to change its properties. These impurities can be naturally occurring and left unaltered in a material or be intentionally added during synthesis. These types of impurities can show up in our day-to-day lives such as different colors in gemstones or by doping to tune the conductivity of semiconductors.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p><p>An example of when impurities are wanted is shown in gems. These gems have slight impurities that act as <a href="/facts/Chromophore/9RykVkek">chromophores</a> and give the stone its color. An example is the gem family <a href="/facts/Beryl/Fl7q1Hl9">beryl</a> which has the base chemical formula of Be3Al2(SiO3)6. Pure beryl will appear colorless but this rarely occurs and the presence of trace elements change its color. The green of <a href="/facts/Emerald/pr52yTAD">emeralds</a> are from impurities such as chromium, vanadium, or iron. A manganese impurity will give a pink gem called morganite and iron creates the blue gem <a href="/facts/Aquamarine_(gem)/GI2x1pzu">aquamarine</a>.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> </p> <p><a href="/facts/Doping_(semiconductor)/WrGdtWnm">Doping</a> is a process where impurities are purposefully added to semiconductors to increase electrical conductivity and improve a <a href="/facts/Semiconductor/zFf3mGTE">semiconductors</a> function. The <a href="/facts/Dopant/DvChmeRL">dopants</a>, the elements added to the original crystal structure, contain a different number of electrons then the base formula. Semiconductors that are p-doped contains a small amount of elements that have less valence electrons then the other elements in the crystal. N-doping is the opposite and the dopant contains more valence electrons.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h2 id="impurities-and-nucleation">Impurities and nucleation</h2> <p>When an impure liquid is cooled to its <a href="/facts/Melting_point/MY0w0h0k">melting point</a> the liquid, undergoing a <a href="/facts/Phase_transition/eLP2BY8R">phase transition</a>, <a href="/facts/Crystal/e2R2mk3E">crystallizes</a> around the impurities and becomes a crystalline solid. If there are no impurities then the liquid is said to be pure and can be <a href="/facts/Supercooled/VdSgseJU">supercooled</a> 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 <a href="/facts/Dynamic_arrest/yVl3QzsT">dynamic arrest</a> or <a href="/facts/Glass_transition/yVl3QzsT">glass transition</a> occurs, but it forms into an <a href="/facts/Amorphous_solid/YTIzmrR0">amorphous solid</a> – a <a href="/facts/Glass/hezRsAGf">glass</a>, instead, as there is no <a href="/facts/Long-range_order/VAqeaphv">long-range order</a> in the structure.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>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 <a href="/facts/Kondo_effect/VCfEKDKU">Kondo effect</a> where the conduction electron spins form a magnetic bound state with the impurity atom. Magnetic impurities in <a href="/facts/Superconductors/pqQAlSzJ">superconductors</a> can serve as generation sites for <a href="/facts/Vortex/K6oeqcuX">vortex</a> defects. Point defects can nucleate reversed domains in <a href="/facts/Ferromagnet/fH3HFnSl">ferromagnets</a> and dramatically affect their <a href="/facts/Coercivity/duBA23fB">coercivity</a>. In general impurities are able to serve as initiation points for <a href="/facts/Phase_transition/eLP2BY8R">phase transitions</a> because the energetic cost of creating a finite-size <a href="/facts/Domain_wall_(magnetism)/4f76pEXb">domain</a> 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. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Dross/ACyc00Ic">Dross</a></li> <li><a href="/facts/Fineness/Wc5HBM0I">Fineness</a></li> <li><a href="/facts/Pollution/JRagxumQ">Pollution</a></li> <li><a href="/facts/Semiconductor/zFf3mGTE">Semiconductor</a></li> <li><a href="/facts/Slag/LD3qEAlV">Slag</a></li> <li><a href="/facts/Spin_wave/bgSrTqEc">Spin wave</a></li></ul> <ul><li>Cheng, E. et al., <i>Chemistry – A Modern View</i>, Aristo-Wilson, Hong Kong, 2004</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Definition of IMPURITY". www.merriam-webster.com. 2024-03-24. Retrieved 2024-04-01. <a href="https://www.merriam-webster.com/dictionary/impurity" target="_blank">https://www.merriam-webster.com/dictionary/impurity</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Definition of HOMOGENEOUS". www.merriam-webster.com. 2024-03-10. Retrieved 2024-03-27. <a href="https://www.merriam-webster.com/dictionary/homogeneous" target="_blank">https://www.merriam-webster.com/dictionary/homogeneous</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Robbins, Lanny (2011-05-27). Distillation Control, Optimization, and Tuning: Fundamentals and Strategies. Boca Raton: CRC Press. doi:10.1201/b10875. ISBN 978-0-429-10729-0. <a href="978-0-429-10729-0" target="_blank">978-0-429-10729-0</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Abdin, Ahmad Yaman; Yeboah, Prince; Jacob, Claus (February 2020). "Chemical Impurities: An Epistemological Riddle with Serious Side Effects". International Journal of Environmental Research and Public Health. 17 (3): 1030. doi:10.3390/ijerph17031030. ISSN 1661-7827. PMC 7038150. PMID 32041209. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7038150" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7038150</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Abdin, Ahmad Yaman; Yeboah, Prince; Jacob, Claus (February 2020). "Chemical Impurities: An Epistemological Riddle with Serious Side Effects". International Journal of Environmental Research and Public Health. 17 (3): 1030. doi:10.3390/ijerph17031030. ISSN 1661-7827. PMC 7038150. PMID 32041209. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7038150" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7038150</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Smith, George Frederick Herbert. "Gem-Stones and Their Distinctive Characters". gutenberg.org. Retrieved 2024-04-15. <a href="https://www.gutenberg.org/ebooks/60990/pg60990-images.html" target="_blank">https://www.gutenberg.org/ebooks/60990/pg60990-images.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Kar, Pradip (2013-08-23). Doping in Conjugated Polymers (1 ed.). Wiley. doi:10.1002/9781118816639. ISBN 978-1-118-57380-8. <a href="978-1-118-57380-8" target="_blank">978-1-118-57380-8</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Smith, George Frederick Herbert. "Gem-Stones and Their Distinctive Characters". gutenberg.org. Retrieved 2024-04-15. <a href="https://www.gutenberg.org/ebooks/60990/pg60990-images.html" target="_blank">https://www.gutenberg.org/ebooks/60990/pg60990-images.html</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Kar, Pradip (2013-08-23). Doping in Conjugated Polymers (1 ed.). Wiley. doi:10.1002/9781118816639. ISBN 978-1-118-57380-8. <a href="978-1-118-57380-8" target="_blank">978-1-118-57380-8</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Urwin, Stephanie J.; Levilain, Guillaume; Marziano, Ivan; Merritt, Jeremy M.; Houson, Ian; Ter Horst, Joop H. (2020-08-21). "A Structured Approach To Cope with Impurities during Industrial Crystallization Development". Organic Process Research & Development. 24 (8): 1443–1456. doi:10.1021/acs.oprd.0c00166. ISSN 1083-6160. PMC 7461122. PMID 32905065. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7461122" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7461122</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> </ol>