Activity: Take a glass or beaker and fill it half with water. Add two spoonful of sand in it , stir the mixture and let the sand settle down at the bottom. When water above the sand become clear, pour it carefully into another beaker so that sand is not disturbed.
Decantation is a process to separate mixtures by removing a liquid layer that is free of a precipitate. The purpose may be to obtain a decant (liquid free from particulates) or to recover the precipitate. Decantation relies on gravity to pull precipitate out of solution, so there is always some loss of product, either from precipitate not fully falling out of solution or from leaving some liquid when separating it from the solid portion.
A piece of glassware called a decanter is used to perform decantation. There are several decanter designs. A simple version is a wine decanter, which has a wide body and a narrow neck. When wine is poured, solids stay in the base of the decanter. In the case of wine, the solid is usually potassium bitartrate crystals. For chemistry separations, a decanter may have a stopcock to drain away the precipitate or dense liquid or may have a partition to separate fractions.
How Decanting Works
There are two main decantation methods:
Separating Liquids and Solids
Decanting is done to separate particulates from a liquid by allowing the solids to settle to the bottom of the mixture and pouring off the particle-free part of the liquid.
For example, a mixture (possibly from a precipitation reaction) is allowed to stand so that gravity has time to pull the solid to the bottom of a container. The process is called sedimentation. Using gravity only works when the solid is less dense than the liquid. Clearwater may be obtained from mud by simply allowing time for the solids to separate from the water.
The separation may be enhanced using centrifugation. If a centrifuge is used, the solid may be compacted into a pellet, making it possible to pour off the decant with minimal loss of liquid or solid.
Separating Two or More Liquids
Another method is to allow two immiscible liquids to separate and the lighter liquid is poured or siphoned off. A common example is decantation of oil and vinegar. When a mixture of the two liquids is allowed to settle, the oil will float on top of the water so the two components may be separated. Kerosene and water can also be separating using decantation.
The two forms of decantation may be combined. This is especially useful if it's important to minimize loss of a solid precipitate. In this case, the original mixture may be allowed to settle or may be centrifuged to separate the decant and sediment. Rather than immediately drawing off the liquid, a second immiscible liquid may be added that is denser than the decant and that doesn't react with the sediment. When this mixture is allowed to settle, the decant will float on top of the other liquid and sediment. All of the decant can be removed with minimal loss of precipitate (except a tiny amount that remains floating in the mixture). In an ideal situation, the immiscible liquid that was added has a high enough vapor pressure that it evaporates, leaving all of the sediment.
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