Separation Methods for Chemical Mixtures

Heterogeneous Separation Techniques

Decantation Using a Separating Funnel

Two immiscible liquids which have different densities can be separated by decanting using a separating funnel. The process follows these steps:

• First, pour the mixture into the separating funnel.
• Wait several minutes until a clear separation boundary is formed.
• Finally, drain the heavier phase into a beaker or Erlenmeyer flask.

Decanting is an alternative separation technique for an insoluble solid and liquid mixture, but it is generally less suitable than filtering.

Liquid Extraction

This method is used for separating a substance which has different solubility in two immiscible liquids, usually water and an organic solvent. When both solvents are mixed, the substance transfers from one liquid to the other, and then they must be decanted with a separating funnel.

Centrifugation

This technique is a form of sedimentation produced by centrifugal force in a centrifuge. The densest components of the mixture move away along the axis of the centrifuge, while the lightest components remain at the top of the solution.

Understanding Homogeneous Mixtures

A homogeneous mixture is one which has the same appearance everywhere; in other words, it consists of just one phase. Consequently, it is impossible to recognize the components of the mixture with the naked eye. Wine, air, and steel alloys are common examples of homogeneous mixtures.

The prefix "homo" means sameness. A homogeneous mixture has the same composition and physical properties throughout. It possesses specific values for different physical properties which depend solely on its composition. For instance, the hardness of steel is quite different from that of iron and depends entirely on the concentration of carbon in the alloy. Homogeneous mixtures are not as easy to separate as heterogeneous ones, so we usually need to change some features of the sample, such as its state of matter.

Distillation

Distillation is a separation process for the components of a mixture that have different boiling points. The mixture is heated until it reaches the lower boiling point. Then, one of the substances becomes vapor and can be separated. This vapor is cooled until it condenses and is collected in a vessel.

Crystallization

Crystallization is the process of forming solid crystals in a liquid solution as the solvent is evaporated. When all the liquid has evaporated, the solid crystals remain at the bottom of the vessel.

Paper Chromatography

This is used to separate several organic substances which travel through chromatography paper at different speeds. A small amount of the mixture is placed at the bottom of the paper strip in contact with an organic solvent. The polar substances contained in the mixture bind to the paper more tightly, so they travel a shorter distance through the paper strip.