What is a Mixture ... in terms of Chemistry ?

Matter can be classified in various ways according to its structure, behaviours and physical and chemical properties.

The main classifications of matter include the categories element and compound, either of which may also be called a substance (which is a less specific term), and mixture, of which there are also many sub-categories.

Definition of a (Chemical) Mixture:

Mixtures are formed by just mixing or 'intermingling' (another word which means the same as 'mixing') together two or more substances. Those substances may also be referred to as 'constituents' or 'components' - and may be either elements or compounds, and be composed of either atoms or molecules. There are no chemical reactions between the constituents of mixtures, which can therefore also be seperated without any chemical reactions taking place.

List of facts about Mixtures:

  1. Mixtures consist of two or more different elements and / or compounds - physically intermingled, so
  2. Mixtures can include:
    • at least two different types of atoms, or
    • at least two different types of molecules, or
    • at least one type of atom and at least one type of molecule.
  3. Mixtures can be formed from the intermingling of their constituents in any ratios or proportions (unlike compounds, which are formed from specific ratios of amounts of other elements).
  4. Mixtures can be separated into their constituents by physical means.
  5. Mixtures have many of the properties of their constituents (e.g. the element 'oxygen' is part of the mixture 'air' and some of the properties of air are due to the oxygen part - but those properties are generally less strong / intense than those of pure oxygen because of the presence of the other constituents of the mixture called 'air', e.g. many substances, such as wood, burn in air and burn even more vigourously in pure oxygen).
  6. Mixtures do not have fixed precise melting and boiling points (which elements and compounds do have), but instead mixtures melt / freeze and boil / condense over a range of temperatures according to the physical properties of the different constituents of the mixture.
  7. Because no chemical reaction takes place when a mixture is formed, no energy (e.g. heat, light or electrical energy) is released or absorbed during the formation of a mixture.

Do mixtures consist of atoms or molecules ?

Mixtures can consist of either atoms or molecules - but must include at least two different atoms or molecules.

In the following diagrams:

Atoms are represented by single spheres.
Spheres of the same size and colour represent atoms of the same element.

Molecules are represented by two or more spheres joined together.

Molecules of Elements are represented by two or more spheres of the same size and colour joined together.

Molecules of Compounds are represented by two or more spheres of different sizes and colours joined together.

See also the page about Elements Mixtures and Compounds, and Atoms and Molecules.

Different Types of Mixtures:

There are many different types of mixtures, some of which have special names.

These include:

  • Homogeneous Mixtures - the (two or more) substances that form the mixture are evenly distributed throughout the mixture, e.g. vinegar is a homogeneous mixture of ethanoic acid and water.
  • Heterogeneous Mixtures - the (two or more) substances that form the mixture are not evenly distributed throughout the mixture, e.g. oil and water.
  • Solutions - special type of homogeneous mixtures in which one substance (called the 'solute') is dissolved in another substance (called the 'solvent'), e.g. salt water is salt dissolved in water - in such a way that the salt no-longer exists as solid particles within the water.
  • Suspensions - heterogeneous fluid mixtures containing solid particles large enough for sedimentation, which means that the particles (compared with the solute part of a solution) will eventually settle to the bottom of the container (unlike in the case of colloids, below), e.g. particles of sand in water.
  • Colloids - heterogeneous mixtures in which one is substance microscopically dispersed evenly throughout another substance (for comparison, the size of the particles of 'solute' are greater than in the case of a solution, but much smaller than in the case of a suspension). There are many naturally occuring colloids, e.g. milk. Colloids are very important in biology and medicine.
  • Alloys - mixtures in which the main element (or, usually, elements) are metal(s). A more technical definition of an alloy is 'a partial or complete solid solution of one or more elements in a metallic matrix'. Common examples of alloys include bronze, brass and steels. When alloys are specified, the specification often includes the ratios of the different metals in that particular alloy.

These different types of mixtures are interesting but might not be essential knowledge for GCSE Chemistry.

The descriptions are non-technical (omitting details, e.g. of particle sizes), but sufficient for general comparison.

The substances of which a mixture is formed can sometimes be identified visually within the mixture. That is, that you can see individual particles of different substances with your eyes alone, e.g. grains of sand in a glass of water.

In many cases the components of a mixture can be separated quite easily, e.g. using a magnet to separate iron filings from a mixture of iron filings (dark grey colour) and sulphur (yellow colour).

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