METHODS OF SEPARATING MIXTURES AND PURIFYING SUBSTANCES (Distillation)

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METHODS OF SEPARATING MIXTURES AND PURIFYING SUBSTANCES  (Distillation)

Distillation involves 2 stages and both are
physical state changes.
(1)  The liquid or solution mixture is boiled to vaporise the most volatile
component in the mixture
(liquid ==> gas). The ant-bumping
granules give a smoother boiling action.
      (2)
      The vapour is cooled by cold water
in the condenser to condense (gas ==> liquid) it back to a liquid
(the distillate) which is collected.

This can be used to purify water because the dissolved
solids have a much higher boiling point and will not evaporate with the steam,
BUT it is too simple a method to separate a mixture of liquids especially if
the boiling points are relatively close.
  Fractional Distillation
Fractional
distillation involves 2 main stages and both are
physical state changes. It can only
work with liquids with different boiling points. However, this method
only works if all the liquids in the mixture are miscible (e.g. alcohol/water,
crude oil etc.) and do NOT separate out into layers like oil/water.
(1) The liquid or solution mixture
is boiled to vaporise the most
volatile component in the mixture (liquid
==> gas). The
ant-bumping granules give a smoother boiling action. 
(2) The vapour passes up through a fractionating column, where the
separation takes place (theory at the end). This column is not used in the
simple distillation described above.
(3) The vapour is cooled by cold
water in the condenser to condense
(gas ==> liquid) it back to a liquid (the distillate)
which is collected.
This can be used to separate alcohol from a fermented sugar
solution.
It is used on a large scale to separate the components of crude oil,
because the different hydrocarbons have different boiling and condensation
points (see oil).
FRACTIONAL DISTILLATION THEORY:
Imagine green liquid is a mixture of
a blue liquid (boiling point 80oC) and a yellow liquid (boiling
point 100oC), so we have a coloured diagram simulation of a
colourless alcohol and water mixture! As the vapour from the boiling mixture
enters the fractionating column it begins to cool and condense. The highest
boiling or least volatile liquid tends to condense more i.e. the yellow liquid
(water). The lower boiling more volatile blue liquid gets further up the
column. Gradually up the column the blue and yellow separate from each other so
that yellow condenses back into the flask and pure blue distils over to be
collected. The 1st liquid, the lowest boiling point, is called the 1st fraction
and each liquid distils over when the top of the column reaches its particular
boiling point to give the 2nd, 3rd fraction etc.
To
increase the separation efficiency of the tall fractionating column, it is usually packed with glass beads, short glass tubes
or glass rings etc. which greatly increase the surface area for evaporation and
condensation. 
In the distillation of crude oil the different fractions are condensed
out at different points in a huge fractionating column. At the top are the very
low boiling fuel gases like butane and at the bottom are the high boiling big molecules of waxes and tar.

Filtration
use a filter paper or
fine porous ceramic to separate a solid from a liquid. It works because the
tiny dissolved particles are too small to be filtered BUT any insoluble ‘non-dissolved’ solid
particles are too big to go through!
Evaporation means a liquid changing to a gas or
vapour. In separation, its removing the liquid from a solution, usually to
leave a solid. It can be done quickly with gentle heating or left out to ‘dry
up’ slowly. The solid will almost certainly be less volatile than the solvent
and will remain as a crystalline residue.
Crystallisation can mean a liquid substance changing
to its solid form. However, the term
usually means what happens when the liquid from a solution has evaporated to a
point beyond the solubility limit. Then solid crystals will ‘grow’ out
of the solution because the solution is too concentrated for all the solid to
remain dissolved at that temperature. Crystallisation is often done from a hot
concentrated solution, because most substance are more soluble the hotter the
liquid. Consequently on cooling a hot concentrated solution, crystals form as
the solubility gets less and less.

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These separation methods are
involved in
 (1)
separation of a sand and salt mixture
 (2)
salt preparations

(1)
The sand/salt mixture is stirred with water to dissolve the salt. The sand is
filtered off and washed with pure water to remove remaining traces of salt
solution. The salt solution (filtrate) is carefully heated in a dish to
evaporate the water and eventually the salt crystals form. Here the solvent is
water, but other mixtures can be separated using the same sequence of
procedures using a different solvent. e.g. copper and sulphur can be separated
using an organic solvent like tetrachloromethane which will dissolve the
sulphur (hazardous chemical solvent).
(2a)
When the water insoluble base (e.g. a metal oxide) is dissolved in an acid, the
excess solid base is filtered off and the filtrate solution heated to evaporate
the water to produce the salt crystals.
(2b)
Two solutions of soluble substances are mixed and react to form an insoluble
salt. The insoluble salt is filtered off to separate it from the solution,
washed with pure water to remove any residual salt solution. The solid is then
removed from the filter paper and dried to give the pure dry insoluble salt.

Definition
of terms.

A
solvent is a liquid that dissolves things.
The
solute is the solid that dissolves in a solvent.
A
solution is a mixture of a liquid with something dissolved in it.
The
technique of solvent extraction involves using a liquid to dissolve a solid to
separate it from a mixture (e.g. in purifying salt in the experiment described
above.
A
saturated solution is one in which no more substance will dissolve in the
liquid.
Soluble
means the substance (gas, liquid or solid) dissolves in a liquid to form a
solution.
Insoluble
a substance won’t dissolve in a particular liquid.
Remember,
a solid may dissolve in one liquid but not in another.

Separating funnel

If two liquids do NOT mix, they form two separate layers and
are known as immiscible liquids (e.g. oil/water). This is illustrated in
the diagram on the left, where the lower grey liquid will be more dense than
the upper layer of the yellow liquid and shows how you can separate these two
liquids using a separating funnel

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1. The mixture is put in the separating
funnel with the stopper on and the tap closed and the layers left to settle
out.
 2. The
stopper is removed, and the tap is opened so that you can carefully run the
lower grey layer off first into a beaker.
3. This leaves behind the upper yellow
layer liquid, so separating the two immiscible liquids.

Decantation
Decanting is the simplest possible way of
separating a liquid (pure or a solution) from an insoluble solid which has a
density greater than water (i.e. > 1.0 g/cm3). The solid-liquid mixture is
allowed to stand e.g. in a beaker, until all the solid settles out to the
bottom of the container. Then the liquid is carefully poured off to leave the
insoluble solid behind. However it is inefficient e.g. a small amount of liquid
is always left in the solid residue and very fine solid particles take some
time to settle out and any disturbance of the liquid can mix them in with the
liquid being poured off. Wine may be served in a decanter to leave the
undesirable solids behind – no good for bits of cork though, they float!

Centrifuges and
centrifuging

Centrifuges
are devices or apparatus that can be used to separate insoluble materials (usually
a solid) from a liquid, where normal filtration does not work well e.g.
a suspension of very fine (tiny) solid particles. The centrifuge
consists of carriage or glass tube holder, mounted on an electrically motor
driven vertical axle. The carriage holds the balanced glass tubes of equal
amounts
of the solid-liquid mixture in each tube, all tubes initially in a
horizontal position before the motor is switched on.  The tube carriage is
rotated at high speed safely in a fully enclosed container. Unbalanced
tubes can break with the extra vibration and this situation has a ‘knock on’
effect, quite literally, as other tubes are likely to shatter with the erratic
high speed unbalanced motion. High velocity glass fragments are not good for
you! On rapid rotation of the carriage the tubes whirl round
horizontally and the centrifugal force causes the more dense insoluble
material particles to move outwards, separating from the liquid. When rotation
ceases the solid particles end up at the ‘bottom’ of the glass tubes with the
liquid above. After the centrifuging operation the liquid can be decanted
off and the solid is left at the bottom of the glass tube. You might be
interested in the solid, liquid or both products depending on the context.
Centrifuges come in all sizes and centrifuge technology has many applications
in the separation of mixtures and the purification of materials.

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Uses of applications: In biology cells can be separated
from fluids. A waste ‘sludge’ can be treated e.g. removing toxic solids from
contaminated water from an industrial process. Milk can be separated from whey.
Edible oils, wines and spirits can be cleaned or ‘clarified’ of solid
impurities. Expensive oils and other fluids used as lubricants in machining
metal parts in industry become contaminated with tiny metal fragments. The
larger pieces of metal are easily removed by filtration or sedimentation
(allowing to settle out) but the very fine metal particles can only be removed
by using a centrifuge.

 This is likely to be a cheaper option than buying more
machine fluid AND reducing pollution since the fluid is recycled leaving less
waste to dispose of.

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1 COMMENT

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