Homogeneous and heterogeneous mixtures
A homogeneous mixture is a solid, liquid or gaseous mixture that has the same proportions of its components throughout a given sample (or multiple samples of different proportion). Homogeneous mixture can have a variable composition. Conversely, a heterogeneous mixture is not uniform in composition, but proportions of its components vary throughout the sample. "Homogeneous" and "heterogeneous" are not absolute terms, but depend on context and size of the sample. In chemistry, a homogeneous suspension of material means that when dividing the volume in half, the same amount of material is suspended in both halves of the substance. However, it might be possible to see the particles under the microscope. An example of a homogeneous mixture is air.
A solution is a special type of homogeneous mixture. Solutions are homogeneous because the ratio of solute to solvent remains the same throughout the solution even if homogenized with multiple sources, and stable because the solute will not settle out after any period of time, and it cannot be removed by a filter or by centrifuge. This type of mixture is very stable, i.e., its particles do not settle, or separate. As a homogeneous mixture, a solution has one phase (liquid) although the solute and solvent can vary: for example, salt water.
Air can be more specifically described as a gaseous solution (oxygen and other gases dissolved in the major component, nitrogen). Since interactions between molecules play almost no role, dilute gases form rather trivial solutions. In part of the literature, they are not even classified as solutions.
In chemistry, a mixture is a substance containing two or more elements or compounds that are not covalently bound to each other and which retain their own chemical and physical identities; – a substance which has two or more constituent physical substances. Mixtures, in the broader sense, are two or more substances physically in the same place, but these are not chemically combined, and therefore ratios are not necessarily considered.
Homogeneous mixtures have the same proportions of the various components throughout a given sample (or multiple samples of different proportion), creating a consistent mixture. However, two homogeneous mixtures of the same pair of substances may differ widely from each other and can be homogenized to make a constant. Homogeneous mixtures always have the same composition. Mixtures can be characterized by being separable by mechanical means e.g. heat, filtration, gravitational sorting, etc.
During the sampling of heterogeneous mixtures of particles, the variance of the sampling error is generally non-zero. Gy's sampling theory  quantitatively defines the heterogeneity of a particle as:
- , the heterogeneity of the th particle of the population
- , the mass concentration of the property of interest in the th particle of the population
- ,the mass concentration of the property of interest in the population
- , the mass of the th particle in the population
- , the average mass of a particle in the population
- Lew, Kristi (2009). "Homogeneous". Acids and Bases, Essential Chemistry. New York: Chelsea House Publishing. Online publisher: Science Online. Facts On File, Inc. ISBN 978-0-7910-9783-0. access date: 2010-01-01
- "Solution (chemistry)" (authors: William Ashworth and Charles E. Little). Encyclopedia of Environmental Studies, New Edition. Online publisher:Science Online. Facts On File, Inc. 2001. access date: 2010-01-01
- "Mixture" (authors: William Ashworth and Charles E. Little). Encyclopedia of Environmental Studies. Online publisher:Science Online. Facts On File, Inc. 2001. access date: 2010-01-01
- Gy, P. (1979), Sampling of Particulate Materials: Theory and Practice, Elsevier: Amsterdam, 431 pp.