Methods of Purification and Analysis

By the end of the lesson, you should be able to:

□ Describe chromatography and interpret a chromatogram.□ Describe in general the use of locating agents.□ Describe filtration.□ Describe crystallisation.□ Describe distillation.□ Describe fractional distillation, examples: crude oil; ethanol from fermentation.□ Describe how the purity of substances can be tested using boiling point, melting point and chromatography.□ Explain the need for pure substances in everyday life.□ Suggest a suitable method for separating a mixture to obtain pure componenets.□ Use Rf values to identify substances on chromatograms.

Please tick in the box if you can do any of the above.

The Need for Pure Substances

Many industries need pure substances to make products such as foods, steel, computer chips and medicines.

What is pure substance?

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A pure substance is a single substance not mixed with anything else. E.g. white sugar or copper(II) sulphate crystals.

In nature, very few substances are pure. Most substances are impure and in the form of mixtures.

What is a mixture? Give an example of a mixture.A mixture contains two or more substances. E.g. seawater as it contains water, salt and other dissolved solids.

Obtaining Pure Substances

Mixtures can be easily separated into pure substances. This process is called purification.

It is done by using physical methods WITHOUT chemical reactions.

Filtration

What is it used for?This is used to separate a liquid from a solid.

What happens in the process? - The method is shown in the diagram above.- The solid is trapped by the filter paper. What do you call this

solid? residue- The liquid goes through the filter paper. What do you call this

liquid? Filtrate

How does the process work?

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- The filter paper has holes in it called pores. The particles of liquid are small enough to go through the pores, but the bits of solid are too large to go through the pores.

What are the main uses for filtration? Used to purify drinking water by removing insoluble solids.

Quick Questions:1. The dyes in ink go through filter paper. Why?Because the dye molecules are smaller than the pores in the paper and so go straight through them.

2, Salt in sea water cannot be separated from the water by filtration. Why?Because the sodium and chloride ions are much smaller than the pores in the paper.

Crystallisation

What is it used for? Use to separate a dissolved solid from a solution as well-formed crystals.

What happens in the process? - The solution is put into an evaporating basin and heated.- The liquid is boiled until much of the water has been

evaporated. - The hot solution is allowed to cool.- Crystals form on cooling.- These can be filtered off and dried by pressing them between

sheets of filter paper.

How does the process work?

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- The water is evaporated to make the hot solution saturated. This means it can no longer dissolve any extra solid.

- A hot solution can dissolve more solid than a cold one. - So when the solution cools, a lot of solid must come out of the

solution as crystals.

What are the main uses for crystallisation? To obtain pure sugar from sugar cane water.

NOTES: Some water must usually be left when aqueous solutions of salts are being crystallized as the salt crystals need water of crystallization.

Sublimation

What is it used for? Separates a mixture of solids, one of which sublimes.

What happens in the process? - If a mixture of iodine and sand is heated in a beaker, the

iodine changes from solid to vapour directly.- The vapour changes back to solid directly on a cold surface.- The sand is not affected by the heat and remains in the

beaker.

NOTES: As only a few solids sublime, this method of separation is limited.

Simple Distillation

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What is it used for? Use to separate a liquid from a solution of a solid. The solid is called the solute and the liquid is called the solvent.

What happens in the process? - The solution is heated in a flask as shown in the diagram.- The solution boils. The solvent turns into a vapour and leaves

the flask.- It is then cooled by a condenser, which changes it back into

liquid, called the distillate.- How does the process work? - The solvent has a low boiling point and so is easily changed

into a gas on heating.- The solute has a high melting and boiling point and so does

not boil and remains in the flask. What are the main uses for simple distillation? Use to extract sodium chloride from sea water.

Quick Questions:1. Where is the bulb of the thermometer placed?It is placed beside the side arm of the distillation flask.

2. Why is it important that the thermometer does not dipped into the solution?This ensures that the thermometer measures the boiling point of the substance that is being distilled.

3. Where does the water enter and leave the condenser?Enter from the bottom of the condenser and leave from the top.

4. Why do we not let it flow in the opposite direction?

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This is to ensure that the whole condenser is cold enough to collect the distillate.

5. Why do you think the condenser slopes downwards?So that the pure solvent can run into the receiver.

Fractional Distillation

What is it used for? To separate a mixture of two or more liquids which mix completely with one another. (They are said to be miscible)

What happens in the process? - The mixture of liquids is heated in the flask as shown in the

diagram,- The mixture boils. The liquid with the lowest boiling point

comes out of the top of the fractionating column.- It is then cooled by a condenser, which changes back into

liquid. What do you call this liquid? distillate- After most of the first liquid has distilled, the liquid with the

next lowest boiling point is then distilled.

How does the process work? - The liquids must have different boiling points. - When the mixture of the liquids boil, the fractionating column

makes the high boiling point liquids condense and return to the flask, allowing the lowest boiling point liquid to remain as a gas and escape.

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What are the main uses for fractional distillation? - To separate substances in crude oil.- To separate gases from liquid air.

Quick Questions:

The thermometer shows a constant temperature during the distillation as pure liquid is being distilled as can be seen from the graph above.

1. When a mixture of ethanol and water is being distilled, which one will be distilled first? Ethanol

2. What would be temperature while ethanol is distilled?78°C

3. When will the temperature increases to 100°C?When all the ethanol has been distilled.

Use of a Separating Funnel

-Liquids that do not mix with each other are said to be immiscible liquids, such as oil

and water, can be separated by using a

separating funnel.

- The mixture is placed in a separating funnel

and allowed to stand.

- The oil and water form two separate layers,with the less dense liquid being on top.

Determining Purity

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There are 3 methods to determine the purity of a substance:a) By chromatographyb) By boiling pointc) By melting point

a) By chromatography

What is the main use of chromatography?Use to separate and identify mixtures of substances, such as coloured dyes in inks and food. It is also use to analyse whether a substance is pure or not.

How does chromatography work?The substance in the mixture dissolves in the solvent and move up the paper. They become spread out as each substance moves at a different speed.

Why do certain spots move further away from the others?- Different substances have different solubilities in the same solvent.- The more soluble the substance the faster it will dissolve in the solvent.- The more soluble substance will get carried along faster by the solvent and move further ahead of the less soluble solutes.

What is a chromatogram?It is the chromatography paper with the separated components .

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Chromatogram 1 chromatogram 2

How would you interpret chromatogram 1 and chromatogram 2?Chromatogram 1: It shows that the spot of green food colouring is a mixture of two dyes because there are two spots on the chromatogram.

Chromatogram 2:It shows that the substance is pure because it is only made up of only one dye.

Rf values

Substances on chromatograms can be identified by their Rf value.

Rf value = distance moved by substance on the paper Distance moved by the solvent

A B

Calculate the Rf value for chromatogram A and B.

Each substance has a particular Rf value for a given solvent, chromatography paper and temperature.

A substance can be identified by- looking up its Rf value in a reference book.- Finding a known spot on the chromatogram at exactly the

same height as it must also have the same Rf value.

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Most of the time, chromatography is used to separate coloured substances. However, chromatography can also be use to analyse colourless substances.

How do we perform chromatography on colourless substances?

By applying a locating agent on the chromatogram.

Experiment on Chromatography

Aim of experiment:To separate and identify the different compounds in green ink using paper chromatography.

Procedure: a)Draw a starting line in the chromatography paper about 2 cm

using pencil. b)Place a drop of fine ink on the pencil line. c) Pour about 1 cm of ethanol in the beaker. d) Lower the paper into the beaker containing the ethanol until the spots of ink are just above the surface of the solvent. Cover the beaker with a lid. e) Leave it to develop. f) Remove the paper when the solvent has almost reached the top of the paper. g) Allow the paper to dry and paste it in the space above.

Result: List the colours seen in the paper_______________________________________________________

Conclusion: What can you conclude from this experiment? ______________________________________________________________________________________________________________

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Follow-up Questions: In preparing the chromatogram, the following instructions were given. Suggest a reason for each instruction.

1) The starting line should be drawn with a pencil rather than with ink.Because ink is a mixture of dyes and therefore if used for drawing the starting line, it will interfere with the chromatogram.

2) At the end of the experiment, the solvent front should be near the top of the paper.To ensure that all the mixture are separated.

3) The spots of the solutions and dyes on the starting line should be small. This is to avoid smudging of the dyes that can affect the separation of the mixtures.

4) The beaker is closed with a lid. To avoid the evaporation of the solvent.

5) The starting line must be above the solventTo prevent the chemicals in the spot from dissolving into the main solvent solution.

b) By boiling point

Experiment on boiling point

Aim of experiment:To study the effect of an impurity on the boiling point of propanone.

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The setup use for the experiment to determine the boiling point of a substance.

Procedure:1. Pour 10 cm3 of propanone into a small distillation flask. Add two or three boiling chips into the liquid to ensure smooth boiling.2. Clamp the small distillation flask in a beaker half-filled with water and fix the thermometer so that the bulb is situated at the side outlet tube.3.Heat the water gently while stirring continuously.4. Record the temperature when the liquid boils.5. Repeat the experiment with a mixture containing propanone and alcohol

Results:Plot two graphs of time against temperature by showing clearly the main difference between the boiling point of pure propanone and impure propanone.

Follow-up questions:1. What is the boiling point of a) pure propanone 56°C

b) propanone containing alcohol?

2. What are the effects of the alcohol on the boiling point of propanone?The boiling point will increase.The liquid will boil over a range of temperature.The greater the amount of impurities, the higher the boiling point.

3.Why must the water not heated too quickly?By heating it quickly, the boiling point might be missed or overlooked.

4. Ethylbenzene has a boiling point of 136°C. Can its boiling point be determined by using the apparatus in this experiment? State and

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explain TWO changes that would be necessary in Figure 2 in order to determine the boiling point of ethylbenzene.1) The water bath should be change using a liquid which is higher than the boiling point of ethylbenzene.2) The thermometer should be change to a thermometer which has a higher temperature range.

By melting point

Experiment on melting point

Aim of experiment:To study the effect of an impurity on the melting point of benzoic acid

The setup use for the experiment to determine the melting point of a substance.

Procedure:

1. Put a small amount of pure solid benzoic acid into a melting point.2. Secure the tube to the thermometer with a rubber band and place them in a small beaker containing paraffin oil.3. Heat the oil slowly while stirring continuously. Record the temperature when the solid melts.4. Repeat the experiment using a mixture containing benzoic acid and a small amount of naphthalene as the impurity.

Results:Plot two graphs of time against temperature by showing clearly the main difference between the melting point of pure benzoic acid and impure benzoic acid.

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Follow-up questions:1. What is the melting point ofa) pure benzoic acid 122°Cb) benzoic acid contaminated with naphthalene? 118-121°C

2. State the effect of the impurity on the melting point of benzoic acid.The melting point will decrease.The substance will melt over a range of temperature.The greater the amount of impurities, the lower the melting point

3. Why must a stirrer be used?To ensure equal distribution of heat throughout the liquid.

4. Suggest why paraffin is used instead of water in this experiment.By using water, the boiling point of benzoic acid will not be able to be reach since water will boil at 100°C.

The importance of purity in substances Foodstuffs that are impure i.e. contaminated with harmful

substances can cause adverse side-effects in the human body and in extreme cases, may even lead to death.

Drugs that are impure can cause adverse allergic reactions that may lead to death.

Pure silicon is used to make silicon chips for making microprocessors.

Pure copper is needed to make copper wire used in electrical wiring.

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