Hwa Chong Institution
Sec 1 Lower Sec Science
Science
INtegrrated
Programme
Separation Techniques Notes
Name: ________________________(
) Class:_____ Date: ______
1) Mixtures and Pure Substances
A pure substance is a single substance not mixed with anything else. White sugar is a
pure substance. The label on a packet of white sugar says `pure refined cane sugar'. This
means no other substance is present. A crystal is a pure substance. White sugar is
made up of crystals with the same shape. This shows that it is made up of the same
single substance: it is pure.
< Pills used as medicine must be
made from very pure substances
Crystals of sugar with a coin to show the scale. >
All the crystals have the same 90° angles, although
many of them have been broken.
In nature, very few substances are found pure. Most substances are mixtures. A
mixture contains two or more substances. Sea water is a mixture. It contains water, salt
and other dissolved gases. Air is a mixture of oxygen, nitrogen and other gases.
Some solid mixtures are made up of different crystals. Most rocks are mixtures.
Granite is commonly used for buildings and roads. Mixtures can easily be separated
into pure substances. The process is called purification. This is done by physical
methods such as filtration, crystallisation or distillation.
No chemical reactions are used. Several methods of purification are discussed in
greater detail. The separation of mixtures into pure substances is important. Chemists
need pure substances to study their properties. Pure sub stances are used in industry to
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make useful products such as food and drugs. Impurities in food and drugs can be
dangerous because they can poison people. An example is a substance called
tryptophan which some people consume as a health food. Some years ago, a cheap but
impure quantity of tryptophan was made and sold by a chemical company for
health food. Unfortunately the impurities caused irreversible damage to
peoples' nervous systems and as a result several thousand healthy people became
so sick that they were forced to live out the rest of their lives in wheelchairs.
Pure crystals have important uses. The great accuracy of a quartz watch is due to a
little quartz crystal which vibrates at exactly 32 768 times per second. This controls
the time. Very pure silicon crystals are needed to make the `chips' used in calculators,
computers, watches and compact disc players.
A pure substance has definite properties. It has a fixed melting point and boiling
point. Pure water, for example, melts at 0°C and boils at 100°C. All samples of
pure water melt and boil at these temperatures. So rain water and water from
melting ice both have the same boiling point of 100°C.
A mixture does not have definite properties. It does not have fixed melting and boiling
points. An example of a mixture is coconut oil which is used for cooking. It melts over
a range of temperatures. It starts melting at 14 °C and completes melting at 22°C.
Petrol fuel for motorcars is also a mixture. It has a boiling point range of 35°C to 75°C.
C11194-05
< Granite rock. You know it is a mixture
because it is made up of different black and
white crystals. The coin shows the relative
sizes of the granite crystals and the coin
< Compact disc players depend on
computer chips made from very pure
silicon
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2) Filtration
Filtration is the usual method of separating a solid from a liquid. A simple way of
doing this is shown below.
The mixture is poured through a filter which is usually made of paper. The filter
paper has tiny holes through which the particles of the liquid are able to pass. The
particles of the solid are large. They cannot pass through the holes and are
trapped by the filter paper.
The solid collected in the filter paper is called the residue. The liquid passing
through the filter is called the filtrate.
When a mixture of sand and water is poured into the filter paper, the solid sand is
trapped in the filter paper (the residue) and the water passes through the paper
(the filtrate).
The filtrate is often a solution of, for example, a solid dissolved in water. This
solution can be separated by crystallisation.
Filtration is one of the steps in making drinking water. All solid impurities are
first removed by filtration. The water is then treated to kill any harmful microorganisms.
Do you know???
Some special filters can filter biological cells. The antibiotic penicillin is made from a
yeast. The penicillin is separated from the yeast by filtration. While the small
penicillin molecules pass through the filter the big yeast cells are trap.
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3) Crystallisation
A solid dissolves in a liquid to produce a solution. The solid that dissolves is
called a solute and the liquid is called a solvent.
When the solution is heated, most of the solvent is evaporated off. The hot solution is
then allowed to cool. The solution is now said to be saturated and as it cools, the
dissolved solid (solute) appears as pure crystals. This process is called
crystallisation. The impurities remain dissolved in the solution.
The main steps in purifying a solid by crystallisation are shown
Impure solids often contain impurities which are insoluble in the solvent. These
insoluble impurities are removed by filtering the mixture before evaporating the
solvent.
Question: How do you get pure salt from impure salt mixture?
The impure salt is a mixture of salt and sand - which one might obtain by evaporating
seawater obtained from a beach. Salt is unusual in that the water solvent must be
completely evaporated from the solution to obtain the crystals of salt.
Filtration followed by crystallisation is often used to make other salts, such as
copper(II) sulphate
Crystallisation from solution is the most common method used by chemists to
purify solids. Pure sugar is obtained this way. Some solids can be purified by
melting them. The hot liquid is then cooled slowly. Crystals form when the liquid
freezes. The first crystals to form are very pure. These are collected before the rest
of the substance freezes completely. This is crystallisation without a sol vent.
This method is used to make the very pure silicon used in computer chips. A
pure silicon crystal is obtained by freezing molten silicon at 1410oC.
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4.1) Distillation
Distillation is used to purify liquids. In this process, the liquid is changed into a
gas by boiling. The gas is pure as other substances are left behind. This gas is then
cooled. It condenses to a pure liquid which is called the distillate.
Distillation can be used to obtain a pure solvent from a solution of a solute. An
example of a simple distillation is the distillation of sea water to obtain pure water.
Below shows the experimental set-up of obtaining pure water from sea water by
distillation. The seawater is boiled in the flask and the steam produced is cooled by
the condenser. The pure liquid is then collected in the beaker and the salt remains
in the flask.
The thermometer shows a temperature of 100°C during the distillation. This is the
boiling point of water. Sea water is a solution of solid salts in water. The salts, like
other solids dissolved in water, cannot be distilled and remain in the flask.
4.2) Fractional Distillation
Miscible liquids are completely soluble in each other. This means two liquids mix to
form one liquid. A mixture of miscible liquids can be separated by fractional
distillation. This is done using a fractionating column.
A fractionating column separates liquids in order of boiling points. The liquid with
the lowest boiling point is distilled first. The liquid with the highest boiling point is
distilled last.
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Ethanol is an important solvent. It can be made by fermenting cane sugar. A mixture
of ethanol and water is obtained from the fermentation. Ethanol is then obtained from
this mixture by fractional distillation. The experiment in the next page shows how
this can be done in the laboratory.
The boiling points of ethanol and water are 78C and 100°C respectively. In the
experimental set-up (top right) the vapour from the boiling mixture of ethanol and
water in the flask contains a larger percentage of the lower boiling point ethanol.
As the vapour moves up the fractionating column, it repeatedly condenses and boils
inside the column. Each time the mixture boils, the percentage of the lower
boiling point ethanol increases. By the time the vapour reaches the top of the
fractionating column, it has become almost pure ethanol.
This vapour then passes into the condenser where it is cooled, and condenses into
liquid ethanol. The thermometer shows a constant temperature of 78C when the
ethanol is being distilled. The water, which has a higher boiling point, remains in
the flask until almost all the ethanol has distilled. (See graph on top left)
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In this experiment, a complete separation is not effected. The distillate contains a
very large percentage of ethanol and a little water.
Fractional distillation is used to obtain pure
oxygen and pure nitrogen from air. The air is first
cooled to about -200C. At this temperature air is
a liquid.
The liquid air is then distilled by allowing the liquid
to warm up. Nitrogen has a lower boiling point
than oxygen, so it distils first. Then the oxygen
(with a higher boiling point) distils. This produces
nitrogen and oxygen separately.
Fractional distillation is also used in the petroleum industry. Petroleum is a
mixture of liquids called hydrocarbons. Fractional distillation is used to separate
the hydrocarbons into, for example, lower boiling point petrol and higher boiling
point kerosene.
Large amounts of liquid chemicals are manufactured in the petroleum industry, and
fractional distillation is commonly used to separate the chemicals into pure liquids.
5) Separating Liquids which are not Miscible
Many liquids do not mix together. An
example is petrol and water. In a
mixture of petrol and water, the
lighter liquid (petrol) forms a
separate layer on top of the heavier
liquid (water). Such a mixture can
be separated with a separating
funnel. The tap is opened so that the
lower water layer runs out first. The
tap is closed as the last drop of
water leaves the funnel. The tap is
then opened again to run the petrol
into another beaker.
Separating petrol and water with a separating funnel
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6) Chromatography
Chromatography is a method of separating and identifying mixtures. There are
several types of chromatography, one of which is paper chromatography. An
example of the use of paper chromatography is in the separation of the dyes in black
ink. This is shown in the experimental set-up below.
Paper chromatography of
black ink
A i m : To separate and identify
the dyes in black ink
1 Draw a pencil line on a piece
of
chromatography
paper
(this paper is like filter paper).
2 Place a drop of black ink on
the pencil line. Also place drops
of coloured dyes (red, orange,
blue, green, etc.) on the pencil
line. These dyes are the ones
you think might be in the black
ink.
3 Roll the paper into a cylinder and secure it. Then place
the paper in a beaker with a
suitable solvent.
4 The solvent will travel up the
paper.
When
the
solvent
reaches the top of the paper,
take the paper out of the
beaker and allow it to dry.
A suitable solvent for this separation is a mixture of butanol (an alcohol), ethanoic
acid and a little water. The solvent chosen depends on the type of substances being
separated.
In the above experiment, the solvent travels up the paper. The dyes on the pencil line
dissolve in the solvent and travel up the paper at different speeds. Hence, the dyes are
separated. The result is called a chromatogram. A typical example is shown on the next
page.
.
Chromatogram of dyes in black ink
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Identical dyes travel the same distance up the paper. The unknown dyes in the black
ink can be identified by comparing them with the known dyes. The result of the
experiment shows that - black ink is made up of four different dyes; - three of the
dyes are the known red, blue and green dyes; - there is no orange dye in the black
ink; - black ink contains one dye that is different from the four known dyes.
Chromatography can also be used to separate and identify colourless substances.
The chromatogram is sprayed with a locating agent to show where the substances
are on the paper. The locating agent is a chemical that reacts with the substances to
produce a coloured product.
An example of separating colourless substances is in identifying the sugars in a
fruit juice. The same method is used as for the black ink in Experiment 2.4. A drop
of fruit juice is used in place of the black ink. A different solvent is used. At the end,
the chromatogram is sprayed with silver nitrate solution followed by dilute
sodium hydroxide. These chemicals make the sugars appear as brown spots. A
typical result is shown.
The result of the experiment shows that
-
fruit juice contains three different sugars;- one of the sugars is sucrose;
-
the fruit juice does not contain maltose and glucose;
-
the fruit juice contains one other sugar different from the three known
sugars.
Paper chromatography can also be
carried out with the solvent running
down the paper. The arrangement of
the apparatus is shown on the left.
This descending method (shown on
the left) of paper chromatography
works better for longer pieces of
paper as the solvent does not have to
move against gravity, and thus flows
more quickly. This means that the
solutes which are separated can
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travel further and thus the separation between the spots is greater.
The chromatogram of the sugars present in fruit juice (page 9) shows the presence
of three different sugars. One of the sugars is different from the three known sugars
(sucrose, glucose and maltose) on the chromatogram. This sugar can be identified
from its Rf value.
The Rf value for any substance on a chromatogram is equal to:
The distances are measured from the starting
line. In the figure on the left, the Rf value of the
substance in the `spot' equals x/y.
The Rf value for the unknown sugar in the
earlier chromatogram can be calculated as
follows (shown below).
The sugar is then identified from the R f values
for different sugars that have already been
calculated. The R f values depend on the
solvent and the temperature.
The R f values for some sugars in the solvent
used to produce the chromatogram (pg 9) are
given in the table (right). From the table, the
unknown sugar is fructose.
Chromatography can be used to separate and identify complicated substances such
as dyes and drugs. It is used to identify artificial dyes in food. Only a few dyes are safe for
food. Many are poisonous. Government laboratories use chromatography to check that
only approved dyes are used in food.
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Pesticides are sprayed on green vegetables to kill insects while herbicides are used to
destroy weeds. These chemicals can be harmful to people. That is why green vegetables
must be tested to check that they only contain tiny amounts of these chemicals when they
are sold in markets. This testing is done by chromatography which can identify the
chemicals present and their amounts.
Paper chromatography is only one type of chromatography. Another important
type is gas chromatography. In gas chromatography, a gas is used in place of a
liquid to separate the substances being analysed. Gas chromatography is used to
find out if athletes are using illegal drugs to improve their performance in
competitions (as in the Olympic Games).
The big advantage of chromatography is that it can be used to identify very tiny
amounts of substances. It can detect less than 0.000 000 000 001 gram or 10-12 g of
the substance.
7) Testing the Purity of Substances
Pure substances have fixed melting and boiling points. These temperatures are given in
most books. So we can easily find out the melting point or boiling point of a pure
substance.
Melting and boiling points are changed by the presence of impurities. This fact can be
used to find out if a substance is pure.
Melting Point
Impure substances are mixtures. Mixtures do not have a fixed melting point. An
example of a mixture is coconut oil which is used for cooking. It melts over a range of
temperature. It starts melting at 14°C and completes melting at 22°C. This fact
can be used to find out if a substance is pure. If it is pure, then all of it should melt at
the same temperature. An impure substance melts over a range of temperature.
Also, impurities lower the melting point. Pure naphthalene melts at 80°C. If the
naphthalene is impure, it melts below 80°C. Impure naphthalene may melt at
78°C or 76°C.
The greater the percentage of impurity, the lower the melting point. Solids must
be very pure to melt exactly at their true melting points. Hence we can find out if a
substance is pure by measuring its melting point.
Boiling Point
We can find out if a liquid is pure by distilling it. If it is pure, all of it distils at the
same temperature. This temperature is the boiling point.
If a liquid is impure (that is, a mixture), it distils over a range of temperature. For
example, petrol which is used as a fuel for motorcars is a mixture. It has a boiling
range of 35°C to 75°C. Similarly, impure ethanol containing water as an impurity,
might start distilling at about 78°C (the boiling point of pure ethanol), but the
distillation may not be complete until the temperature reaches about 100°C.
The boiling range for an impure substance depends on the actual impurities
present and their percentage.
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Chromatography
Chromatography can be used to test the impurity of
complicated substances and those that cannot easily be
melted or distilled. It is a very sensitive test and shows
up very tiny amounts of impurity.
An example is testing the purity of a dye used in food.
The figure of the right shows the result of a
chromatography test. Dye X is pure because it forms
one spot on the chromatogram. Dye Y is impure
because it has separated into several spots on the
chromatogram. The intense spot is pure dye Y; the
three faint spots are impurities.
Did you know??
In 1983, the publisher of a German magazine paid $8
million for diaries which were supposed to have been
written by Adolf Hitler, the German wartime dictator.
Chromatography, however, proved that the diaries were
fakes. The ink used in the writing was shown to be a
mixture of modern inks and not the document ink
normally used between 1941 and 1943, when the diaries
were supposed to have been written. The paper contained
chemicals used to make it appear brighter and whiter.
These chemicals were identified by chromatography and
shown to have been manufactured only after 1955.
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