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Decantation is a process for the separation of mixtures of immiscible liquids or of a liquid and a solid mixture such as a suspension. The layer closer to the top of the container—the less dense of the two liquids, or the liquid from which the precipitate or sediment has settled out—is poured off, leaving the other component or the more dense liquid of the mixture behind. An incomplete separation is witnessed during the separation of two immiscible liquids.

Immiscible liquid separation
Decantation can be used to separate immiscible liquids that have different densities. For example, when a mixture of water and oil are present in a beaker, a distinct layer between the two consistency is formed, with the oil layer floating on top of the water layer. This separation can be done by pouring oil out of the container, leaving water behind. Generally, this technique gives an incomplete separation as it is difficult to pour off all of the top layer without pouring out some parts of the bottom layer.

A separatory funnel is an alternative apparatus for separating liquid layers. It has a valve at the bottom to allow draining off the bottom layer. It can give a better separation between the two liquids.

Liquid-solid separation
Decantation can also separate solid and liquid mixtures by allowing gravity to pull the solid fragments to settle at the bottom of the container. In laboratory situations, decantation of mixtures containing solids and liquids occur in test tubes. To enhance productivity, test tubes should be placed at a 45° angle to allow sediments to settle at the bottom of the apparatus.

A centrifuge may also be used in decantation as the natural process of settling down is time consuming and tedious. A centrifuge forces the precipitate to the bottom of the container; if the force is high enough, solids can aggregate to form pellets, making it easier to separate the mixtures. Then the liquid can be more easily poured away, as the precipitate will tend to remain in its compressed form.

Examples
Decantation is frequently used to purify a liquid by separating it from a suspension of insoluble particles (e.g. in red wine, where the wine is decanted from the potassium bitartrate crystals to avoid unsavory taste). This makes the wine more tonic and astringent.

Cream accelerates to the top of milk, allowing the separation of milk and cream. Fat is determined in butter by decantation.

To obtain a sample of clear water from muddy water, muddy water is left in a container until the mud settles, and then the clear water is poured into another container.