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Separation

Two solids can be separated based on their different properties. Sieving or sublimation are possible ways of separating two different solids in a mixture.

Laboratory sieves are used to separate solids of different sizes.
Laboratory sieves are used to separate solids of different sizes.

When two solids in a mixture have different particle sizes, they can be separated using a sieve.

A sieve has a mesh that contains holes that allow particles of a certain size to pass through.

Pure crystals of benzoquinone (an organic compound) deposited on a watch glass. The compound has recrystallized on the glass after sublimation.
Pure crystals of benzoquinone (an organic compound) deposited on a watch glass. The compound has recrystallized on the glass after sublimation.

Sublimation is used when there is a solid in the mixture that sublimes (i.e. changes directly from a solid to a gas).

The mixture of solids is first heated in a beaker covered by a watch glass. The vapour of the solid that sublimes rises to the cold surface of the watch glass, where it is deposited as pure crystals.

Solids are separated from liquids using filtration, precipitation or evaporation.

Filtration separates solids from a suspension using filter paper.

Sand can be separated from sea water (which is a solution of dissolved salts) through filtration.

The mixture is through a funnel lined with filter paper. The liquid passes through, leaving the solids trapped by the paper.

Precipitation is the solidification of dissolved ions out of solution.

Silver ions can be solidified out of a solution by adding chloride ions. An insoluble salt of silver chloride forms when the ions are mixed.

Increasing the concentration of one of the compounds in solution can cause the compound to precipitate (or solidify) out of solution.

Many solid particles are less soluble at lower temperatures. Solids tend to precipitate out of solution when temperature decreases.

Diagram of precipitation
Diagram of precipitation

Evaporation removes the liquid from solid-liquid mixtures. Heat and exposure to air turns the liquid into gas, which escapes and leaves the solid behind.

Heating of salt water causes the water to evaporate, leaving the salt behind.

Leaves of mangrove plants secrete concentrated salt solutions through their leaves. Crystals form on the leaves when water evaporates from the salt secretions.

Crystallisation is a form of precipitation that results in the formation of well-ordered crystals instead of disordered solids.

The following basic steps are carried out in crystallisation:

  • Dissolve the impure solid in a suitable hot solvent.
  • Allow excess solvent to evaporate until the solution becomes saturated.
  • Gradually cool the solution to allow pure crystals to precipitate out of solution.
  • Filter the mixture to isolate the crystals from the liquid.
  • Dry the crystals.

The saturated solution has to cool slowly for pure crystals to form.

Forcing the solution to cool rapidly will result in crystals of lower purity, as impurities get captured in the crystalline structure of the product and have no time to escape.

A separating funnel is used to separate two immiscible liquids in the laboratory.

If a liquid dissolves in another liquid, then the two liquids are miscible (e.g. ethanol and water are miscible).

If two liquids do not dissolve (the case of oil and water), the two liquids are immiscible.

When a mixture of two immiscible liquids is placed in a separating funnel, the mixture is first shaken by inverting the funnel a few times.

At this stage, the funnel is closed at each end to prevent leakage of the mixture.

Two distinct layers of liquid then form inside the funnel.

The denser liquid settles at the bottom of the funnel and forms the bottom layer, while the less dense liquid layer settles on top.

The layers are separated by draining off the bottom layer into a beaker. This completes the separation.

Two distinct layers formed in the separating funnel. The bottom layer has a higher density than the top layer.
Two distinct layers formed in the separating funnel. The bottom layer has a higher density than the top layer.

Distillation is a technique for separating mixtures based on differences in boiling points. Simple distillation is used to separate liquids that have very different boiling points.

When the temperature of a liquid mixture is increased, the more volatile liquid (the liquid with the lower boiling point) boils first.

The vapour of this liquid rises through a column and passes through a condenser, where it condenses back to liquid form.

The condensed vapour is collected as a liquid called the distillate. The distillate is collected in a conical flask and is separated from the other liquid.

Water in the condenser is made to flow from bottom to top so that no air bubbles form.

These air bubbles will disrupt the condensation process as parts of the condenser are not entirely cooled.

Fractional distillation separates liquids with only small differences in similar boiling points.

A fractionating column is used in fractional distillation.

It is similar in design to a simple distillation column, except that it has glass or metal plates or beads packed inside the column.

These plates or beads provide a large surface area for multiple condensation sites within the column.

The gas condenses at these sites in the column and then vaporises again to move to higher levels. The gas condenses and vaporises multiple times on its way up the column.

The gas becomes purer as it rises through the column because more of the less volatile liquid condenses and falls back down the column.

Fractional distillation is used in industry to separate crude oil into gasoline and other liquids.

Paper chromatography is the separation of a small amount of liquid mixture using a solvent and paper.

A mixture (usually ink or a pigment) is dotted onto a piece of chromatography paper.

The end of the paper is then placed in a pool of a solvent, which travels up through the paper.

Different components in a sample mixture have different solubilities in the solvent. The more soluble the component is, the faster it will travel up the paper with the solvent.

The results of separating components in brown, green, blue and black marker ink.
The results of separating components in brown, green, blue and black marker ink.

In paper chromatography, each of the components being separated is assigned a retention factor ($$R_\text{f}$$) value. This value depends on how far the component travels up the paper.

The $$R_\text{f}$$ value is the ratio of the distance travelled by the component to the distance travelled by the solvent.

$$$R_\text{f}\text{ value} = \displaystyle{\frac{\text{Distance travelled by a component}}{\text{Distance travelled by the solvent}}}$$$
The visual results of paper chromatography (known as a chromatogram).
The visual results of paper chromatography (known as a chromatogram).

The $$R_\text{f}$$ value of component X is given by $$\frac{A}{B}$$.

Markings on the chromatogram must be made in pencil. Pencil lead is made of graphite, which is insoluble in most solvents.

Pencil markings do not interfere with the separation of components of interest.

A locating agent is sprayed on the chromatography paper to react with colourless components and give them colour.

This allows for previously colourless components to be identified. The $$R_\text{f}$$ values of these components can then be determined.

The purity of gold alloys is described using carats.
The purity of gold alloys is described using carats.

The purity of a sample is the percentage of that sample that contains the desired compound or element.

Impurities refer to the unwanted compounds or elements present in the sample.

The purity of a solid substance can be determined from the melting point.

Each compound (or element) has a fixed and narrow melting point range. When a pure substance reaches this temperature, the molecules in the crystalline structure of the solid break apart.

An impure sample has a lower melting point than a pure sample. This is because the impurities distort the crystalline structure of the substance, making the molecules easier to break apart.

Less energy (a lower temperature) is needed to melt the impure sample.