Homogeneous and heterogeneous mixtures

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 in which proportions vary throughout the sample. "Homogeneous" and "heterogeneous" are not absolute terms, but are dependent on context and the size of the sample. In chemistry, if the volume of a homogeneous suspension is divided in half, the same amount of material is suspended in both halves of the substance. 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.

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 where the ratio of solute to solvent remains the same throughout the solution and the particles are not visible with the naked eye, even if homogenized with multiple sources. In solutions, solutes will not settle out after any period of time and they can't be removed by physical methods, such as a filter or centrifuge. 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

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 trivial solutions. In part of the literature, they are not even classified as solutions. In gas, intermolecular space is the greatest—and intermolecular force of attraction is least. Some examples can be oxygen, hydrogen, or nitrogen.

Mixtures

In chemistry, a mixture is a substance containing two or more elements or compounds that are not covalently bound to each other and 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, therefore ratios are not necessarily considered. Solid homogeneous mixtures, like gaseous and liquid homogeneous mixtures, contain different elements mixed uniformly and cannot be separated easily. An example of a solid homogeneous mixture is bronze, which is a mixture of copper and tin.

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, centrifugation 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:

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

where

$h_{i}$ is the heterogeneity of the $i$ particle of the population,

$c_{i}$ is the mass concentration of the property of interest in the $i$ particle of the population,

$c_{\text{batch}}$ is the mass concentration of the property of interest in the population,

$m_{i}$ is the mass of the $i$ particle in the population, and

$m_{\text{aver}}$ is the average mass of a particle in the population.

Homogenization

Main articles: Homogenization (chemistry) and Mixing (process engineering)

See also

• Chemical substance