Editing Mixture (section)

==Homogeneous and heterogeneous mixtures==

[[File:Mixtures and Pure Substances 2x2.svg|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 "[[Solution (chemistry)|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 [[Solubility|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 [[Amount of substance|amounts]] of both the liquid medium and dissolved solid (solvent and solute)

===Homogeneous mixtures===

====Solutions====
{{Main articles|Solution (chemistry)}}
A [[Solution (chemistry)|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. <!--| access-date =2010–01-01-->}}</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===
Examples of heterogeneous mixtures are [[emulsion]]s and [[foam]]s. 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 theory|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>