Mixture

{{Short description|Substance formed when two or more constituents are physically combined}} {{Other uses|Mixture (disambiguation)}} {{Use dmy dates|date=July 2021}} In chemistry, a '''mixture''' is a material made up of two or more different chemical substances which can be separated by physical method. It is an impure substance made up of two or more elements or compounds mechanically mixed together in any proportion.<ref>{{GoldBookRef|title=mixture |file=M03949 }}</ref> A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in the form of solutions, suspensions or colloids.<ref>{{cite book|author=Whitten K.W., Gailey K. D. and Davis R. E.|year= 1992|title= General chemistry |edition=4th |publisher= Saunders College Publishing|location= Philadelphia|isbn= 978-0-03-072373-5}}{{page needed|date=October 2021}}</ref><ref>{{cite book |last1 = Petrucci |first1 = Ralph H. |last2 = Harwood |first2 = William S. |last3 = Herring |first3 = F. Geography |date=2002 |title = General chemistry: principles and modern applications |url = https://archive.org/details/generalchemistry00hill |url-access = registration |edition=8th |location=Upper Saddle River, N.J |publisher=Prentice Hall |isbn = 978-0-13-014329-7 |lccn=2001032331 |oclc=46872308 }}{{page needed|date=October 2021}}</ref>

Mixtures are one product of mechanically blending or mixing chemical substances such as elements and compounds, without chemical bonding or other chemical change, so that each ingredient substance retains its own chemical properties and makeup.<ref>{{Cite book|title=Atkins' Physical Chemistry|last1=De Paula|first1=Julio|last2=Atkins|first2=P. W.|year=2002|publisher=Oxford University Press |isbn=978-0-19-879285-7|edition=7th}}{{page needed|date=October 2021}}</ref> Despite the fact that there are no chemical changes to its constituents, the physical properties of a mixture, such as its melting point, may differ from those of the components. Some mixtures can be separated into their components by using physical (mechanical or thermal) means. Azeotropes are one kind of mixture that usually poses considerable difficulties regarding the separation processes required to obtain their constituents (physical or chemical processes or, even a blend of them).<ref name="Alberts">{{cite book|author=Alberts B.|display-authors=etal|year=2002|title= Molecular Biology of the Cell, 4th Ed.|publisher= Garland Science|isbn=978-0-8153-4072-0}}{{page needed|date=October 2021}}</ref><ref name="Laider">{{cite book|author=Laidler K. J.|year=1978|title=Physical chemistry with biological applications |publisher=Benjamin/Cummings|location= Menlo Park|isbn=978-0-8053-5680-9}}{{page needed|date=October 2021}}</ref><ref name="Weast">{{cite book|author=Weast R. C., Ed.|year= 1990|title=CRC Handbook of chemistry and physics|publisher= Chemical Rubber Publishing Company|location= Boca Raton|isbn=978-0-8493-0470-5}}{{page needed|date=October 2021}}</ref>

==Characteristics of mixtures== All mixtures can be characterized as being separable by mechanical means (e.g. purification, distillation, electrolysis, chromatography, heat, filtration, gravitational sorting, centrifugation).<ref>{{cite book |doi=10.5744/florida/9780813054254.003.0003 |title=Home Front |year=2017 |isbn=978-0-8130-5425-4 |editor1-last=Pleasants |editor1-first=Julian M |chapter=A Call to Duty: The Selective Service Act of 1940 }}</ref><ref name=mixture>{{Cite encyclopedia| title =Mixture| encyclopedia =The Encyclopedia of Environmental Studies| publisher =Facts on File | year =2001| editor-first=William |editor-last=Ashworth |editor2-first=Charles E. |editor2-last=Little}}</ref> Mixtures differ from chemical compounds in the following ways: * The substances in a mixture can be separated using physical methods such as '''filtration''', '''freezing''', and '''distillation.''' * There is little or no energy change when a mixture forms (see Enthalpy of mixing). *The substances in a mixture keep their separate properties. In the example of sand and water, neither one of the two substances changed in any way when they are mixed. Although the sand is in the water it still keeps the same properties that it had when it was outside the water. * mixtures have variable compositions, while compounds have a fixed, definite formula. * when mixed, individual substances keep their properties in a mixture, while if they form a compound their properties can change.<ref>{{Cite web|url=https://www.chemicool.com/definition/mixture.html|title=Definition of mixture - Chemistry Dictionary|website=www.chemicool.com|access-date=2018-11-30}}</ref>

The following table shows the main properties and examples for all possible phase combinations of the three "families" of mixtures :

{| class="wikitable" |+Mixtures Table |- ! Dispersion medium (mixture phase) || Dissolved or dispersed phase || Solution || Colloid || Suspension (coarse dispersion) |- ! rowspan="3" | Gas | Gas || Gas mixture: air (oxygen and other gases in nitrogen) || {{CNone|None}} || {{CNone|None}} |- | Liquid || {{CNone|None}} || Liquid aerosol:<ref name="PDF1">{{Cite report|url=https://www.iupac.org/publications/pac/pdf/1972/pdf/3104x0577.pdf|title=Manual of Symbols and Terminology for Physicochemical Quantities and Units. Appendix II Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part I|last=Everett|first=D. H.|date=23 July 1971|publisher=International Union of Pure and Applied Chemistry: Division of Physical Chemistry|location=London|access-date=28 October 2016|url-status=live|archive-url=https://web.archive.org/web/20161028084759/https://www.iupac.org/publications/pac/pdf/1972/pdf/3104x0577.pdf|archive-date=28 October 2016}}</ref> fog, mist, vapor, hair sprays || Spray |- | Solid || {{CNone|None}} || Solid aerosol:<ref name="PDF1"/> smoke, ice cloud, air particulates || Dust |- ! rowspan="3" | Liquid | Gas || Solution: oxygen in water || Liquid foam: whipped cream, shaving cream || Sea foam, beer head |-

| Liquid || Solution: alcoholic beverages || Emulsion: milk, mayonnaise, hand cream || Vinaigrette |- | Solid || Solution: sugar in water || Liquid sol: pigmented ink, blood || Suspension: mud (soil particles suspended in water), chalk powder suspended in water |- ! rowspan="3" | Solid | Gas || Solution: hydrogen in metals || Solid foam: aerogel, styrofoam, pumice || Foam: dry sponge |-

| Liquid || Solution: amalgam (mercury in gold), hexane in paraffin wax || Gel: agar, gelatin, silicagel, opal || Wet sponge |-

| Solid || Solution: alloys, plasticizers in plastics || Solid sol: cranberry glass || Clay, silt, sand, gravel, granite |}

==Homogeneous and heterogeneous mixtures==

thumb|A diagram representing at the microscopic level the differences between homogeneous mixtures, heterogeneous mixtures, compounds, and elements. Mixtures can be either ''homogeneous'' or ''heterogeneous'': a mixture of uniform composition and in which all components are in the same phase, such as salt in water, is called homogeneous, whereas a mixture of non-uniform composition and of which the components can be easily identified, such as sand in water, it is called heterogeneous.

In addition, "'''uniform mixture'''" is another term for '''homogeneous mixture''' and "'''non-uniform mixture'''" is another term for '''heterogeneous mixture'''. These terms are derived from the idea that a '''homogeneous mixture''' has a '''uniform appearance''', or '''only one phase''', because the particles are evenly distributed. However, a '''heterogeneous mixture''' has constituent substances that are in different phases and '''easily distinguishable''' from one another. In addition, a heterogeneous mixture may have a uniform (e.g. a colloid) or non-uniform (e.g. a pencil) composition.

Several solid substances, such as salt and sugar, dissolve in water to form homogeneous mixtures or "solutions", in which there are both a solute (dissolved substance) and a solvent (dissolving medium) present. Air is an example of a solution as well: a homogeneous mixture of gaseous nitrogen solvent, in which oxygen and smaller amounts of other gaseous solutes are dissolved. Mixtures are not limited in either their number of substances or the amounts of those substances, though in most solutions, the solute-to-solvent proportion can only reach a certain point before the mixture separates and becomes heterogeneous.

A homogeneous mixture is characterized by uniform dispersion of its constituent substances throughout; the substances exist in equal proportion everywhere within the mixture. Differently put, a homogeneous mixture will be the same no matter from where in the mixture it is sampled. For example, if a solid-liquid solution is divided into two halves of equal volume, the halves will contain equal amounts of both the liquid medium and dissolved solid (solvent and solute)

===Homogeneous mixtures===

====Solutions==== {{Main articles|Solution (chemistry)}} A solution is equivalent to a "homogeneous mixture". In solutions, solutes will not settle out after any period of time and they cannot be removed by physical methods, such as a filter or centrifuge.<ref name=solution>{{Cite encyclopedia | title =Solution (chemistry)| encyclopedia =Encyclopedia of Studies, New Edition| publisher = Online publisher: Science Online. Facts on File, Inc. | year =2001| last1 =Ashworth | first1 =William | last2 =Little | first2 =Charles E. }}</ref> As a homogeneous mixture, a solution has one phase (solid, liquid, or gas), although the phase of the solute and solvent may initially have been different (e.g., salt water).

====Gases==== Gases exhibit by far the greatest space (and, consequently, the weakest intermolecular forces) between their atoms or molecules; since intermolecular interactions are minuscule in comparison to those in liquids and solids, dilute gases very easily form solutions with one another. Air is one such example: it can be more specifically described as a gaseous solution of oxygen and other gases dissolved in nitrogen (its major component).

===Heterogeneous mixtures=== thumb|left|Making an heterogeneous mixture of water and oilExamples of heterogeneous mixtures are emulsions and foams. In most cases, the mixture consists of two main constituents. For an emulsion, these are immiscible fluids such as water and oil. For a foam, these are a solid and a fluid, or a liquid and a gas. On larger scales both constituents are present in any region of the mixture, and in a well-mixed mixture in the same or only slightly varying concentrations. On a microscopic scale, however, one of the constituents is absent in almost any sufficiently small region. (If such absence is common on macroscopic scales, the combination of the constituents is a dispersed medium, not a mixture.) One can distinguish different characteristics of heterogeneous mixtures by the presence or absence of continuum percolation of their constituents. For a foam, a distinction is made between reticulated foam in which one constituent forms a connected network through which the other can freely percolate, or a closed-cell foam in which one constituent is present as trapped in small cells whose walls are formed by the other constituents. A similar distinction is possible for emulsions. In many emulsions, one constituent is present in the form of isolated regions of typically a globular shape, dispersed throughout the other constituent. However, it is also possible each constituent forms a large, connected network. Such a mixture is then called '''bicontinuous'''.<ref>{{cite book |title=Heterogeneous Materials I: Linear Transport and Optical Properties |last=Sahimi |first=Muhammad |date=26 May 2006 |page=31 |publisher=Springer |isbn=978-0-387-21705-5}}</ref>

==Distinguishing between mixture types== Making a distinction between homogeneous and heterogeneous mixtures is a matter of the scale of sampling. On a coarse enough scale, any mixture can be said to be homogeneous, if the entire article is allowed to count as a "sample" of it. On a fine enough scale, any mixture can be said to be heterogeneous, because a sample could be as small as a single molecule. In practical terms, if the property of interest of the mixture is the same regardless of which sample of it is taken for the examination used, the mixture is homogeneous.

Gy's sampling theory quantitatively defines the '''heterogeneity''' of a particle as:<ref>{{Cite book|title=Sampling of Particulate Materials: Theory and Practice|last=Gy|first=P|publisher=Elsevier|year=1979|location=Amsterdam}}</ref>

:<math>h_i = \frac{(c_i - c_\text{batch})m_i}{c_\text{batch} m_\text{aver}},</math>

where <math>h_i</math>, <math>c_i</math>, <math>c_\text{batch}</math>, <math>m_i</math>, and <math>m_\text{aver}</math> are respectively: the heterogeneity of the <math>i</math>th particle of the population, the mass concentration of the property of interest in the <math>i</math>th particle of the population, the mass concentration of the property of interest in the population, the mass of the <math>i</math>th particle in the population, and the average mass of a particle in the population.

During sampling of heterogeneous mixtures of particles, the variance of the sampling error is generally non-zero.

Pierre Gy derived, from the Poisson sampling model, the following formula for the variance of the sampling error in the mass concentration in a sample:

:<math>V = \frac{1}{(\sum_{i=1}^N q_i m_i)^2} \sum_{i=1}^N q_i(1-q_i) m_{i}^{2} \left(a_i - \frac{\sum_{j=1}^N q_j a_j m_j}{\sum_{j=1}^N q_j m_j}\right)^2,</math>

in which ''V'' is the variance of the sampling error, ''N'' is the number of particles in the population (before the sample was taken), ''q'' ''i'' is the probability of including the ''i''th particle of the population in the sample (i.e. the first-order inclusion probability of the ''i''th particle), ''m'' ''i'' is the mass of the ''i''th particle of the population and ''a'' ''i'' is the mass concentration of the property of interest in the ''i''th particle of the population.

The above equation for the variance of the sampling error is an approximation based on a linearization of the mass concentration in a sample.

In the theory of Gy, correct sampling is defined as a sampling scenario in which all particles have the same probability of being included in the sample. This implies that ''q'' ''i'' no longer depends on ''i'', and can therefore be replaced by the symbol ''q''. Gy's equation for the variance of the sampling error becomes:

:<math>V = \frac{1-q}{q M_\text{batch}^2} \sum_{i=1}^N m_{i}^{2} \left(a_i - a_\text{batch} \right)^2,</math>

where ''a''batch is that concentration of the property of interest in the population from which the sample is to be drawn and ''M''batch is the mass of the population from which the sample is to be drawn.

== Toxicology == Air pollution research shows biological and health effects after exposure to mixtures of pollutants are more potent than effects from exposures of individual components.<ref>{{Cite web |last=US EPA |first=ORD |title=Exposure and Health Effects of Mixtures of Air Pollutants |url=https://19january2017snapshot.epa.gov/air-research/exposure-and-health-effects-mixtures-air-pollutants_.html |access-date=2022-11-10 |website=19january2017snapshot.epa.gov |language=en}}</ref><ref>{{Cite web |last=Institute |first=Health Effects |date=2016-03-09 |title=Multipollutant Mixtures |url=https://www.healtheffects.org/air-pollution/multipollutant-mixtures |access-date=2022-11-10 |website=Health Effects Institute |language=en}}</ref><ref>{{Cite journal |last1=Majumder |first1=Nairrita |last2=Kodali |first2=Vamsi |last3=Velayutham |first3=Murugesan |last4=Goldsmith |first4=Travis |last5=Amedro |first5=Jessica |last6=Khramtsov |first6=Valery V |last7=Erdely |first7=Aaron |last8=Nurkiewicz |first8=Timothy R |last9=Harkema |first9=Jack R |last10=Kelley |first10=Eric E |last11=Hussain |first11=Salik |date=2022-10-27 |title=Aerosol physicochemical determinants of carbon black and ozone inhalation co-exposure induced pulmonary toxicity |journal=Toxicological Sciences |volume=191 |issue=1 |pages=61–78 |doi=10.1093/toxsci/kfac113 |pmid=36303316 |pmc=9887725 |issn=1096-6080 }}</ref>

==Homogenization== {{Main|Homogenization (chemistry)|Mixing (process engineering)}}

==Properties of a mixture == * Chemical substance * Mixing (process engineering)

== References == {{Reflist}} * {{GoldBookRef|title=mixture|file=M03949}}

Category:Chemical mixtures Category:Physical chemistry Category:Chemistry