Separating Mixtures

What is a mixture?

Where different substances (elements or compounds) occupy the same space, but are not chemically bonded together, they are called mixtures. Each of the components of a mixture retains its chemical and physical properties. For example, it is possible to use a magnet to remove iron filings from a mixture of iron and sulfur, as the iron filings remain magnetic in the mixture. Mixtures can be separated into their constituent parts by simple, physical separation techniques.

Some useful terms when discussing mixtures:

• Solution: A mixture of a liquid (solvent) and one or more dissolved solids (solutes).
• Solvent: A liquid that dissolves certain solutes to make a solution.
• Solute: A solid that can be dissolved in solvent to make a solution.
• Soluble: Can be dissolved in a given solvent.
• Insoluble: Can not be dissolved in a given solution.

Separations

Separations are techniques used to split up mixtures into their different components. The types of separation used to accomplish this will depend on the states of matter in which we find the mixtures components. For mixtures containing liquids and insoluble solids components, filtration is often used. For mixtures containing liquids (solvents) and soluble (solutes), techniques such as evaporation, crystallization, or distillation may be used. For liquid-liquid mixtures containing multiple different liquids, a range of techniques, such as Fractional distillation,  can be used depending on the properties of the liquids involved. Described below are some of the most common techniques used to separate mixtures.

Filtration

Distillation

Distillation is used to separate out the solvent from a solution (e.g., to separate water from salt water solution).

To perform distillation, the solution is poured into a round-bottomed flask, connected to a water-cooled condenser all in a closed system to prevent the loss of gasses. The solution is heated causing the liquid solvent to boil and turn from liquid to gas. The gas passes through the condenser, is cooled, and condenses back into a liquid (called the distillate), which is collected. The solid solute remains as residue on the inside of the round-bottomed flask.

Evaporation

Crystallisation

Crystallisation is also used to separate out a solute from a solution, e.g. to separate salt from salt water solution, but specifically when the solute will thermally decompose (break down at high temperatures).

To perform crystallisation, the solution is poured into an evaporating basin and gently heated, causing liquid solvent to evaporate, until crystals start to form. The evaporating basin is then removed from the heat and allowed to slowly cool, encouraging the formation of crystals of the solute in residual solution. Once cool, any remaining solution can be removed by filtration, and the crystals patted dry.

Fractional Distillation 

Fractional distillation is used to separate a mixture of two or more liquids with different boiling points, e.g. the separation of a mixture of different hydrocarbons in crude oil.

To perform fractional distillation the mixture of liquids is poured into a round-bottomed flask, connected to a water-cooled condenser. A thermometer is also included to allow measurement of temperature of the gases as they pass through the glassware. The mixture is heated, and the temperature of the mixture reaches a point where it equals the lowest boiling point in the mixture. The liquid with that boiling point in the mixture turns into a gas and passes through the condenser, where it is cooled and condenses back into a liquid. The temperature is kept constant during this process until no more liquid boils off. This liquid (referred to as a “fraction”) can then be collected. Once all of the first fraction has been collected, the temperature is raised until it reaches the next lowest boiling point, and the process is repeated to collect the second fraction. This is repeated until all of the liquids in the mixture have been separated by their boiling point.

Chromatography

Chromatography can be used to separate mixtures according to the solubility of the components. One common technique is paper chromatography (discussed in more detail here). Chromatography makes use of the attraction of a substance to a stationary phase and to a solvent to separate mixtures along a chromatography medium. There are a wider range of chromatography techniques, including column, gas, and thin layer chromatography.