3 e1 separation and purification (students' copy)

Separation Techniques

Secondary 3 ExpressChemistry

What is a Pure Substance?A pure substance is one that is not mixed with anything else. (It can be either an element or a compound)

A pure substance always has a fixed and exact melting and boiling point. This means that a pure solid will melt completely at one temperature.

For example, pure ice melts at 0oC and pure water boils at 100oC.

What is a Pure Substance?

Impure substances melt and boil over a range of temperatures.

For example, salt water boils from 104 - 112°C.

Impact of impuritiesImpurities will result in the substances wanting to remain in liquid state.

Therefore, if ice melts at 0°C, will the new melting point likely to be:

a)-5 °C or b) 5 °C ?

Purity of SubstanceImpurities lower the melting point of a solid. The greater the amount of impurities, the lower the melting point.

Impurities increase the boiling point of a liquid. The greater the amount of impurities, the higher the boiling point.

Impact of impuritiesImpurities will result in the substances wanting to remain in liquid state.

Therefore, if ice melts at 0°C, will the new melting point likely to be:

a)-5 °C or b) 5 °C ?

Answer: a (because the lower the melting point, the easier for ice to melt and become a liquid!)

Why add salt to icy roads?In some temperate countries, people throw salts on the roads to prevent ice forming on the surface of the road.

Salt is an impurity and decreases the melting point of water. Hence, freezing takes place at a lower temperature.

A salt spreader working on an icy

road

Purity of SubstanceIn nature, there are very few substances that are found pure. Most substances are found impure as mixtures.

A mixture is made up of two or more substances that are not chemically combined together.

Mixtures can be separated physically (without chemical reaction) into pure substances through the process of purification.

• Impurities in drugs must be detected as they may cause undesirable side effects.

• Chemicals are often added to food and beverages. It is important to ensure that our food contain only chemicals that are safe for consumption.

Importance of Purity

3.7 Determining Purity

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By doing one of the following:

Determination of a Pure Substance

•Checking for exact and constant (or fixed)

melting point of a solid

•Checking the exact and constant (or fixed)

boiling point of a liquid

•Performing chromatography

3.7 Determining Purity

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Separation TechniquesIn this topic, you will learn about the different methods of purifications also known as separation techniques.

1. Filtration2. Evaporation to Dryness3. Crystallisation4. Using a Suitable Solution5. Sublimation6. Using a Magnet7. Simple Distillation8. Fractional Distillation9. Separating Funnel10.Chromatography

Put on your thinking cap! An impure sample of X melts at about 90oC. What is most likely the melting point of X?

(a) between 90oC to 110oC (b) between 80oC to 90oC (c) below 80oC (d) cannot be determined

Put on your thinking cap! An impure sample of X melts at about 90oC. What is most likely the melting point of X?

(a) between 90oC to 110oC (b) between 80oC to 90oC (c) below 80oC (d) cannot be determined

Reason: Impurities lower the melting point of a solid!

FiltrationFiltration is used to separate an insoluble solid from a liquid.

A filter funnel and a filter paper are usually used.

Example: removing sand from water

The solid that remains on the filter paper is called the residue.

The liquid that passes through the filter paper is called the filtrate.

FiltrationSeparation of chalk from water

Evaporation to DrynessThis is a process of obtaining a soluble solid from a solution by heating the solution until all the water has boiled off. This is provided that the solid does not decompose on heating.

Example: heating salt solution to obtain salt crystals

The solution is heated until all the water has evaporated/ been removed. The solid that is dissolved in the solution will remain in the evaporating dish.

Evaporation to Dryness

Separation of Salt from Water

Water in the solution is lost to the atmosphere.

Salt remains as a white residue in the evaporating dish.

salt solution

evaporating dish


URL

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Not all soluble substances can be obtained by evaporation to dryness.

Example: Sugar decomposes on heating


The solid obtained by evaporation to dryness is not always pure. Any soluble impurities will be left together with the solid after heating.

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The process of obtaining a pure solid sample (soluble solid) from its solution.

Examples:

Crystallisation

hydrated copper(II) sulfate crystals

sodium carbonate crystals

Crystallisation

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Crystallisation

If the solid that dissolves in the solution decomposes on heating, crystallisation is used.

Examples: obtain sugar crystals from sugar solution, obtaining blue copper (II) sulfate crystals from copper (II) sulfate solution

heat

Crystallisation

The solution is heated to evaporate most of the solvent (water) until a saturated solution is formed. Note that not all water is evaporated.

A saturated solution is one that contains the maximum amount of solute at a given temperature and cannot dissolve anymore solute.

Preparation of Pure Copper(II) Sulfate Crystals by Crystallisation

impure copper(II) sulfate solution

Step 1

Step 2

• The dissolved copper(II) sulfate appears as pure crystals.

• The hot, saturated solution is allowed to cool to room temperature.

a solution that contains as much dissolved solute as it can at a given temperature

Rapid cooling produces small crystals while slow cooling produces large crystals.

3.2 Separating a Solid from a Liquid

copper(II) sulfate crystals


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• The crystals are then dried by pressing them between pieces of filter paper.

• The cold solution is removed by filtration.

3.2 Separating a Solid from a Liquid

pure copper(II) sulfate crystals

filter paper

Step 3


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Step 4

Step 5

• The residue of pure crystals is washed with cold distilled water.

Crystallisation

1. Heat to saturation point.

2. Allow saturated solution to cool for crystals to form.

3. Filter the mixture and collect crystals as residue.

4. Wash the crystals.

5. Dry the crystals using the filter paper.

Using a Suitable SolventTo separate a mixture of two solids, we use a solvent in which one solid is soluble. (The other solid is insoluble in that solvent.)

Different solids dissolve in different solvents. Some common solvents are water and ethanol.

Example: separating a mixture of sugar (soluble in water) and sand (insoluble in water) using water as the solvent

Using a Suitable Solvent

Sugar crystals

Filter the cooled saturated solution and obtain sugar crystals as

residue

Evaporate to saturation point and allow saturated solution to cool for sugar crystals to form

Using a Suitable Solvent1. Distilled water is added to the mixture of solids and stirred.

2. Sugar will dissolve in water but sand will not dissolve in water.

3. The mixture is filtered and the insoluble sand remains on the filter paper as the residue whereas the sugar solution passes through the filter paper as the filtrate.

4. Wash the residue with distilled water.

5. The filtrate is then evaporated to saturation and left to cool for sugar crystals to form.

6. The sugar crystals is then filtered and dried between sheets of filter paper. Note: In this experiment, there are 3 methods of purification taking place. They are using

a suitable solvent , filtration and crystallisation.

SublimationSublimation is the phenomenon whereby some solids, when heating, changes directly to the gaseous state without passing through the liquid state. On cooling, the reverse process (gas solid) occurs.

SublimationSolids that sublime include iodine, ammonium chloride, dry ice (solid CO2) and napthalene (mothball).

Sublimation can be used to separate a solid that sublimes from one that does not.Example: separating iodine from sand by sublimation

A magnet can be used to separate a magnetic substance from a non-magnetic substance.

Examples of magnetic materials:• Iron

• Steel• Nickel• Cobalt

Using a Magnet

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Some metals are magnetic.

Using Magnets to Separate Two Solids

We can use this property to separate these metals (e.g. iron, nickel, cobalt, steel) from mixtures.

Using a Magnet

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Obtaining the Solvent from a Solution

SOLUTION

SOLUTE SOLVENT

EvaporationCrystallisati

on

Simple distillation

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Simple DistillationIt is used to separate a pure solvent (liquid) from a solution.

Example: obtaining water from sea water

In this process, the liquid is boiled in the distillation flask and its vapour condensed in the condenser to form back liquid. (liquid gas liquid)

The solvent (liquid) is collected as the distillate in beaker.

Simple Distillation

Note: a conical flask is often used instead of a beaker as it has a smaller neck which prevent the loss of distillate from splashing when it drops to the flask.

Simple Distillation

Setting Up the Distillation Apparatus

Thermometer bulb should be just beside the side arm leading to the condenser.

Water enters the condenser from the bottom and leaves from the top.Condenser must be sloping downwards.

Boiling chips are placed in the flask to ensure smooth boiling.

Volatile liquids can be kept in the liquid state by placing receiver on ice.

Simple Distillation

water in

water out

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Simple distillation of salt solution solution 2. Water boils and becomes

vapour. Thermometer measures temperature of the vapour.

3. Water vapour is cooled and condenses into pure liquid (distillate).

5. Pure water is collected as distillate.

4. The salt solution becomes more concentrated as distillation continues. Salt will be collected as residue.

1. Boiling chips are added to ensure smooth boiling. Water vaporises, rises and enters the condenser.

Simple Distillation

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Simple Distillation

Simple DistillationThe condenser (Liebig condenser) is used to condense the vapour from the distillation flask to liquid by cooling the vapour.

Boiling stones/ marble chips/ porcelain chips can be added to the distillation flask to ensure smooth boiling (prevents bumping of liquid).

Simple DistillationThe thermometer bulb must be placed beside the side arm of the distillation flask. This is to measure the boiling point of the liquid.

Cold water will enter the condenser from the bottom and come out from the top.

Reason:1. This ensures that the entire interior of the condenser is filled with water.

2. This ensures the effective cooling of vapour as the flow of water is such that the coldest part of the condenser is at the end just before the vapour escapes from the condenser

Fractional DistillationIt is used to separate two or more miscible (soluble in each other) liquids with different boiling points.

Miscible liquids are liquids that mix with one another to form one layer/liquid.

Eg: ethanol and water are miscible liquids oil and water are immiscible

Fractional DistillationExamples:

separating ethanol and water (both soluble in one another but ethanol has a boiling point of 78oC and water 100oC),

separating liquid air into its component gases (nitrogen, argon, oxygen and etc)

separating petroleum into different fractions (petrol, diesel, kerosene, bitumen and etc)

Fractional Distillation

Comparing Simple Distillation with Fractional Distillation

A fractionating column is attached to the round-bottomed flask and the condenser for fractional distillation.

Glass beads/plates/spiral in the fractionating column provide a large surface area for vapour to condense on.


simple distillation

fractional distillation

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Fractional DistillationThe process is almost similar to simple distillation except that a fractionating column is used.

The purpose of the glass beads in the fractionating column is to provide a large surface area for the repeated condensation of vapour and re-boiling of liquids. This ensures a more complete separation of the mixture.

Other than glass beads, a fractionating column may be filled with plates or a spiral.


1. Ethanol vapour and water vapour rise up the column as the solution is heated.

2. The water vapour condenses in the fractionating column and falls back into the flask.

3. Ethanol, which has a lower boiling point than water, reaches the upper part of the column and is distilled over.


4. At this stage, the thermometer shows a constant temperature of 78oC, which is the boiling point of ethanol. 47


5. In this condenser,• hot ethanol vapour condenses as running water cools it;• liquid ethanol flows down the inner tube of the condenser and into the receiver.

6. Ethanol is collected as the distillate in the receiver.

7. When all the ethanol has distilled over, the temperature rises rapidly to 100oC, which is the boiling point of water. At this temperature, water distils over and can be collected separately.


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Fractional DistillationThe liquids are distilled out in an ascending order of boiling points. The liquid with the lowest boiling point is distilled and collected first. This is followed by the liquid with the next higher boiling point. The liquid with the highest boiling point will be collected last.

Graph showing how temperature changes as a solution of ethanol and water is fractionally distilled

Separating funnelA separating funnel is used to separate a mixture of immiscible liquids. (eg. oil and water)

Immiscible liquids are liquids that do not mix with each other. For example, oil and water are immisicible as they do not mix with each other

Separating funnelThe mixture of oil and water is added into a separating funnel and shaken. The separating funnel is then allowed to stand for the two liquid layers (oil and water) to form.

The less dense liquid (oil) is the top layer while the more dense liquid (water) is in the bottom layer.

ChromatographyChromatography is the method of separating two or more components that dissolve in the same solvent.Paper chromatography is one type of chromatography.

A solvent is a liquid, which a solute (substance) dissolves in.

The most commonly used solvents in paper chromatography are ethanol (an alcohol) and water.

Examples:

Pigments in plants

Colouring in food

Dyes in paints

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Chromatography

The chromatography paper with the separated components is called a chromatogram.

chromatogram

lid

glass tank

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Chromatography

Separating Dyes Found in Green Food Colouring

Chromatography paper

Green food colouring

Ethanol or water as solvent

1. Place a spot of green food colouring on the paper.

2. Dip the paper into a solvent such as ethanol or water. Ensure that the coloured spot is above the solvent level.

3. The components will separate as the solvent travels up the paper.

Pencil line

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Chromatography

Paper ChromatographyExample: Different coloured dyes in black ink can be separated

Paper Chromatography

The most soluble solute will travel the furthest distance in the chromatogram. (It also travels the fastest .)

The most insoluble solute will travel the least distance or even not at all in the chromatogram. (It also travels the slowest) .

A pure substance gives only one spot on a chromatogram.

How does Paper Chromatography Work?


Chromatography can be used to identify substances because identical substances travel up the same distance in the same solvent.


From the results, it can be deduced that:

1.Black ink is not a pure substance but a mixture of substances containing red, green and blue dyes.


From the results, it can be deduced that:

2.Blue dye is the most soluble as it travelled the furthest distance.

3.Red dye is the least soluble as it travelled the least distance.

Interpretation of a Chromatogram

• This food colouring is not pure.

• It consists of 2 component dyes.

• This is a pure substance. Why?

Chromatography

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Chromatography was performed on a sample of food colouring (‘X’) and 4 banned dyes (‘A’, ‘B’, ‘C’ and ‘D’).

Example: Comparison with ‘known’ samples

• If X contains any of the 4 banned dyes, it is not safe to be consumed.

X A B C D

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Chromatography

Conclusions that can be drawn from the chromatogram:


• Identical dyes produce spots at the same height. • Sample X does not contain the banned dyes A, B and D.• However, X contains the banned dye C. Therefore, it must not be consumed.

X A B C D

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Chromatography

Other conclusions that can be drawn from the chromatogram:• Dyes A and D are pure.

• Both dye B and dye C are mixtures of two different dyes.

• Sample X is a mixture of three dyes.

X A B C D


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Chromatography

Rules of Paper Chromatography

The starting line must be drawn using a pencil but not an ink pen.

Reason:Ink is a mixture of dyes that will dissolve in the solvent and interfere with the chromatogram. The solvent does not dissolve the pencil line.


The starting line must be drawn above the solvent level.

Reason:If the starting line is below the solvent level, the ink and dyes will dissolve in the solvent below and not travel up the chromatogram and be separated.


The dyes spotted on the starting line should be small.

Reason: This is to prevent the dyes from spreading sideway and overlap each other.

The beaker/container must be covered with a lid.

Reason: This is to prevent the evaporation of the solvent.

The positions of the solvent front (position reached by solvent) and spot on a chromatogram depend on how long the experiment was allowed to run.

Chromatogram after a period of time.

Chromatogram after a longer period of time.

What is R

f value?

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Chromatography

This ratio is called the

retention factor or Rf value

of the substance.

the distance travelled by

the solvent is a

constant.

The ratio between the

distance travelled by

the substance

and

What is Rf value?

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Chromatography

How to calculate Rf value?

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Chromatography

3.5 cm

4.5 cm

Chromatogram after a period of time.

Chromatogram after a longer period of time.

Rf value of a substance is the same regardless of distance travelled on chromatogram.

Rf =

= 0.67

cm 4.5

cm 3Rf = = 0.67

cm 5.2

cm 3.5

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Chromatography

3 cm 3.5 cm

4.5 cm

5.2 cm

• The Rf value of a substance does not change

as long as chromatography is carried out under the same conditions (i.e. same solvent and same temperature).

Rf Value

• This property allows us to easily identify a substance on a chromatogram.

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Chromatography

• Chromatography can also be used for colourless substances such as amino acids.

How do we identify colourless substances?

• To separate and analyse colourless substances, we apply a locating agent on a chromatogram.

• The locating agent reacts with the colourless substances to form coloured spots.

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Chromatography

Uses of Chromatography

Separation of dyes in inks

Identifying banned drugs in urine sample (for sports competitions)

Identifying banned substances in food samples

Identifying amino acids in proteins

3 e1 separation and purification (students' copy)

Science

melting point of x

fixed melting point

melting point of water

new melting point

purity of substance

fixed boiling point

pure ice melts

exact melting