Mixture

In chemistry, a mixture is a material consisting of two or more different substances that are physically combined. Normally, in nature, it is very unlikely that matter exists in the form of a pure substance; in fact, most of the time it exists in the form of a mixture. The components of a mixture may be solid, liquid, or gaseous.

Mixtures may be either homogeneous or heterogeneous. A mixture is called homogeneous when it consists of a single phase and its components are indistinguishable by direct observation. A homogeneous mixture is called a solution. An example of homogeneous mixtures are alloys. Homogeneous mixtures can only be separated by state transitions that involve the components in different ways. For example, salt dissolved in water is a homogeneous mixture and can be separated by heating the mixture: the water evaporates while the salt precipitates as a solid (this process occurs in salt pans).

A mixture is heterogeneous when it consists of two or more phases and its components are easily distinguishable. A suspension is an example of a heterogeneous mixture. Heterogeneous mixtures are easier to separate than homogeneous mixtures, even by mechanical methods. When two or more substances are combined to form a mixture, they do not change their intimate chemical structure, as happens in chemical reactions.

Classification of mixtures

Homogeneous mixtures: occur in a single phase (particle diameter < 1 nm)

  • solutions
    • alloys
    • gaseous mixtures

Heterogeneous mixtures: they occur in several phases (particle diameter > 1 nm)

  • dispersions (particle diameter > 1 µm)
  • foams (dispersed phase: gas; continuous phase: liquid)
  • emulsions (dispersed phase: liquid; continuous phase: liquid)
  • suspensions (dispersed phase: solid; continuous phase: liquid)
  • mists (dispersed phase: liquid; continuous phase: gas)
  • fumes (dispersed phase: solid; continuous phase: gas)
  • colloids (particle diameter < 1 µm)
  • colloidal emulsion (dispersed phase: liquid; continuous phase: liquid)
  • liquid aerosol (dispersed phase: liquid; continuous phase: gas)
  • solid aerosol (dispersed phase: solid; continuous phase: gas)
  • colloidal foam (dispersed phase: gas; continuous phase: liquid)
  • sol (dispersed phase: solid; continuous phase: liquid)
  • solid foam (dispersed phase: gas; continuous phase: solid)
  • gel (dispersed phase: liquid; continuous phase: solid)
  • colloidal solid suspension (dispersed phase: solid; continuous phase: solid)
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