Homogeneous and heterogeneous mixtures

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A homogeneous mixture is a solid, liquid, or gaseous mixture that has the same proportions of its components throughout any given sample. Conversely, a heterogeneous mixture has components whose proportions vary throughout the sample. "Homogeneous" and "heterogeneous" are not absolute terms but depend on context and the 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 a microscope. An example of a homogeneous mixture is air.

In physical chemistry and materials science this refers to substances and mixtures which are in a single phase. This is in contrast to a substance that is heterogeneous.[1]

A figure reprinting at the microscopic level the differences between homogeneous mixtures, heterogeneous mixtures, compounds, and elements.

Homogeneous mixtures

Solution

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.[2] 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.

Gases

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 gas intermolecular space is greatest and intermolecular force of attraction is least.

Solids

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.[3]

Metrics

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.[3]

During the sampling of heterogeneous mixtures of particles, the variance of the sampling error is generally non-zero. Gy's sampling theory[4] quantitatively defines the heterogeneity of a particle as:

${\displaystyle h_{i}={\frac {(c_{i}-c_{\text{batch}})m_{i}}{c_{\text{batch}}m_{\text{aver}}}},}$

where

${\displaystyle h_{i}}$ is the heterogeneity of the ${\displaystyle i}$th particle of the population,
${\displaystyle c_{i}}$ is the mass concentration of the property of interest in the ${\displaystyle i}$th particle of the population,
${\displaystyle c_{\text{batch}}}$ is the mass concentration of the property of interest in the population,
${\displaystyle m_{i}}$ is the mass of the ${\displaystyle i}$th particle in the population, and
${\displaystyle m_{\text{aver}}}$ is the average mass of a particle in the population.