C8. Separation techniques

Question 1

What do chemical processes do?

Answer 1

Displace constituent elements from their compounds

Question 2

What do physical processes do?

Answer 2

Know that physical processes are required to separate mixtures, including miscible/immiscible liquids and dissolved/insoluble solids

Question 3

What is decanting?

Answer 3

Decanting is a quick and simple way to separate a solid and a liquid or solution. If the mixture is left, the solid will settle to the bottom of the container. All that is required is a steady hand to carefully
pour the liquid or solution away without disturbing the solid.

Question 4

What is filtration?

Answer 4

Filtration is used to separate a liquid (or solution) from an insoluble solid. The solid particles are
larger and are trapped by the mesh of the filter paper. The smaller liquid particles are able to pass
through the filter paper into the collection vessel. This separated liquid is known as the filtrate and
the solid is the residue.

Question 5

When do I use a separating funnel?

Answer 5

A separating funnel is used to separate two immiscible liquids. When two immiscible liquids are
added to a separating funnel, they form two layers. The upper layer has the lower density. When the
tap is opened, the lower layer can be poured out. The narrowing walls of the separating funnel
make it easier to close the tap the moment the last drop of the lower layer has passed through.

Question 6

What is simple distillation and when do I use it?

Answer 6

Simple distillation is used to separate a liquid and soluble solid from a solution (e.g., water from a solution of salt water) or a pure liquid from a mixture of liquids

The solution is heated, and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask

The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker

After all the water is evaporated from the solution, only the solid solute will be left behind

Question 7

When do I use distillation?

Answer 7

Distillation (sometimes called simple distillation) is used to separate two substances with differing boiling points.
Simple distillation is typically used to separate the solvent from a solution, leaving the solute behind.
The mixture is heated to above the boiling point of the solvent. The solvent becomes a vapour and starts to spread out through the apparatus. As the vapour hits the condenser it cools and condenses as droplets which run down the condenser to the collection vessel. The higher boiling point solute remains in the heated flask. If the solute is stable to heating, it can also be collected once the solvent has been removed.

Question 8

What is fractional distillation?

Answer 8

Fractional distillation is used to separate miscible liquids (ones soluble in each other) from a mixture.
Like simple distillation, fractional distillation requires each liquid to have a different boiling point. In the separation of larger mixtures such as the hydrocarbons in crude oil, the separation can be into groups of compounds with similar boiling points, or it can be used to separate individual components.
During the distillation, the mixture is heated. When the temperature of the mixture reaches the boiling point of the component with the lowest boiling point, the temperature no longer rises and the first component boils.
This lowest boiling point component rises up the fractionating column towards the condenser where it is condensed and collected.
The thermometer at the entrance to the condenser monitors the boiling point of the component being distilled.
Once the first component has boiled away, the remaining mixture then rises in temperature until the boiling point of the next lowest component is reached. This then boils away to be condensed and collected.
When separating the two liquids, a thermometer is used to monitor the temperature of the vapour that is being condensed.

Question 9

What is evaporation and crystallisation?

Answer 9

This technique is also used to separate the solvent from a solute and uses the difference in boiling
point and/or volatility to separate the mixture. In this separation method, it is the solute rather than the solvent that is wanted and so a condenser is not used to collect the solvent as it evaporates or boils away.
To separate the solute, the solution is heated gently until crystals just begin to appear.
This is the crystallising point. The remaining concentrated solution is then left to evaporate.
It’s not sensible toheat to dryness because some solids may decompose if heated too strongly.

Question 10

What is chromatography?

Answer 10

Chromatography is used to separate mixtures of different solutes, such as dyes in ink, or it can be
used to separate mixtures of different liquids.
In chromatography, differences in the relative affinity to the stationary phase of the chromatogram to
the mobile phase (the solvent) lead to the separation of the substances.
In paper chromatography, a small sample of the mixture is spotted on to a piece of chromatography
paper. This paper is then placed into a tank or beaker which contains a solvent at the bottom and
the lid is replaced. The solvent then rises up the paper by capillary action and through the mixture.
The different components each have a different tendency to adsorb to the paper. If the component
adsorbs more strongly to the paper (the stationary phase) then it does not travel up as readily.

Question 11

What is an Rf value?

Answer 11

Rf values are used to identify the components of mixtures

The Rf value of a particular compound is always the same

However, it does depend on the solvent used

If the solvent is changed then the Rf value changes

Rf= Distance moved by substance/ Distance moved by solvent

Question 12

What is centrifrugation?

Answer 12

Centrifugation is used to separate heterogeneous mixtures based on their differences in particle
mass. It can be used to separate mixtures of gases but it is more commonly used to separate solids
from liquids or solutions.
If a liquid is mixed with a solid, the solid particles to settle to the bottom of a container as they are
more dense than the liquid. When the solid is small or finely divided, random motion and collisions
of the particles causes them to settle much more slowly and in some cases they do not settle to the
bottom.
Centrifugation encourages settling by spinning the mixture at high speed in a horizontal circle. As
the mixture is spun, the heavier particles are brought to the end of the tube. The heaviest particles
settle out first, becoming a pellet at the bottom of the tube.
The remaining liquid may be pure, a solution or still contain small solid particles. The remainder,
called the supernatant may be decanted into a new tube and spun again at a higher speed to
separate the smaller particles.
Centrifugation is used in medicine for the separation of blood components such as red blood cells
and platelets from the remaining plasma.

Question 13

How to establish purity with a chromatogram?

Answer 13

To establish the purity of a substance using chromatography, perform the separation and examine the chromatogram: a pure substance will produce a single spot, while an impure substance will show multiple distinct spots at different heights, indicating the presence of various components

Question 14

What does miscible mean?

Answer 14

Two liquids that mix completely to form a uniform solution

Question 15

How to test for purity?

Answer 15

In addition to separating mixtures, some techniques allow the purity of samples to be checked.
In chromatography, the Rf value of a substance using a constant stationary phase and solvent is always the same.
Chromatography can check whether a mixture contains that component and, if there are spots present with different Rf values, these would indicate an impurity. However, it should be noted that
an impurity might have the same Rf value as a desired compound by coincidence and/or not be coloured/visible. Therefore it is not possible to be 100% sure of purity after analysis by this type of
chromatography.
Distillation can also be used to indicate whether there is an impurity. During the boiling of a liquid,
the boiling point can be found. If a liquid starts to boil off at a higher temperature than expected at a
particular pressure, this indicates the presence of an impurity.
A similar change occurs at the freezing point when an impurity is present in a liquid. In this case, the freezing point is lowered.
In general, pure substances have definite, sharp melting and boiling points whereas impure substances melt and boil over a range of temperatures.
Evaporation can be used to check for dissolved impurities. If a solution is evaporated to dryness, any solid residue is a dissolved impurity