criteria of purity

Criteria of purityModel Answers 1

Level IGCSE

Subject Chemistry (0620/0971)

Exam Board Cambridge International Examinations (CIE)

Topic Experimental techniques

Sub-Topic Criteria of purity

Booklet Model Answers 1

Time Allowed: 36 minutes

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1 The diagram shows the paper chromatograms of four substances, W, X, Y and Z.

W X Y Z

Which two substances are pure?

A W and X B W and Y C X and Y D X and Z

Paper chromatography is used to separate substances based on their solubility in the

solvent used.

Different components of the substances will travel different distances, producing several

spots on the chromatogram.

Substances W and Y have 2 spots each, indicating that these substances contain 2

different components.

Substances X and Z have only 1 spot each, indicating that these substances contain

only 1 component, hence they are pure.

2

Model answers are copyright. © 2018 Save My Exams Ltd. All rights reserved.


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2 Chromatography is used to find out if a banned dye, P, is present in foodstuffs.

The results are shown in the diagram.

Which foodstuff contains P?

AP BB C D

Compare the chromatogram for the banned dye P by using a ruler and carefully

identifying the spots which are the same distance from the baseline as P.

Only B matches so B is the one that contains P.

A, C and D do not match and so do not contain the banned dye.

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3 A fruit drink coloured orange contains a dissolved mixture of red and yellow colouring agents.One of these colouring agents is suspected of being illegal.

Which method could be used to show the presence of this illegal colouring agent?

A

B

C

D

chromatography

distillation

evaporation

filtration

Chromatography is the technique used to identify mixtures of coloured compounds.

Inks, dyes, paints and food colouring agents are some examples of the substances

which can be separated and identified using chromatography.

B is incorrect as distillation is used to separate a pure liquid from a solution or a

mixture of liquids.

C is incorrect as evaporation is used to separate a soluble solid from a solution.

D is incorrect as filtration is used to separate an insoluble substance from a liquid.

Study of the composition of ink using paper chromatography

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4 A student was provided with only a thermometer, a stopwatch and a beaker.

What could the student measure?

A 10.5 g solid and 24.8 cm3 liquid

B 10.5 g solid and 25 °C

C 24.8 cm3 liquid and 45 seconds

D 25 °C and 45 seconds

For this question it is best to define what each piece of equipment is used for.

○ Thermometer: used to measure temperature

○ Stopwatch: used to measure time

○ Beaker: used to roughly measure volumes of liquids

A and B are incorrect as the student would require a balance to measure the mass.

C is incorrect as the volume measurement (24.8 cm3) requires greater accuracy than a

beaker can provide. A burette should be used to measure the volume to this accuracy.

D is the only option that matches as the student can measure both temperature and

time accurately.

Burette

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5 An aqueous solution is coloured.

Which method of separation would show that the solution contains ions of different colours?

A chromatography

B crystallisation

C distillation

D filtration

Chromatography is the technique used to separate and / or identify mixtures of

coloured compounds.

Inks, dyes, paints and food colouring agents are some examples of the substances

which can be separated using chromatography.

B is incorrect as crystallisation is used to separate a dissolved solid from a solution.

C is incorrect as distillation is used to separate a pure liquid from a solution or a

mixture of liquids.

D is incorrect as filtration is used to separate an insoluble substance from a liquid.

Study of the composition of ink using paper chromatography

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6 Solid W melts at exactly 54 °C and boils at exactly 302 °C.

Solid X, when dissolved in water and examined using paper chromatography, shows a blue colour and a red colour.

Which row is correct?

Impure substances have melting points and boiling points that fall in a range of

temperatures while pure substances have exact and sharp melting and boiling

points.

Solid W has an exact melting point and an exact boiling point; hence it is a pure

substance.

A, C and D are therefore incorrect.

Paper chromatography is used to separate substances based on their solubility

in the solvent used.

Different components of the substances will travel different distances, producing

several spots on the chromatogram.

Solid X produces two spots, indicating it must contain more than one substance

• it is a mixture.

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7 Diagram 1 shows the paper chromatogram of substance X.

R

Y

X

diagram 1

Diagram 2 shows the cooling curve for substance Y.

0

147

0

temperature / °C

time

diagram 2

Which statement about X and Y is correct?

A X is a mixture and Y is a pure substance.

B X is a pure substance and Y is a mixture.

C X and Y are mixtures.

D X and Y are pure substances.

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diagram 1

Substance X is a mixture as the chromatogram shows two spots at R and Y meaning

that it contains two components.

Y is a pure substance as pure substances have sharp melting points whereas mixtures

melt over a range of temperatures.

This difference is most easily seen when the temperature of a substance is measured as

it cools as the temperature stays constant during the phase change.

This results in there being a horizontal section on a cooling graph for a pure substance

which does not appear on the graph of an impure substance.

B, C and D are incorrect as these options do not match with the properties of X and Y.

Chromatogram of sample X

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8 A student is investigating a coloured mixture using chromatography.

paper

pencil line

solvent

Where should he place the coloured mixture?

A in the solvent

B just above the pencil line

C just below the pencil line

D on the pencil line

In paper chromatography the sample spots should all go onto the base line.

This is always made with a pencil as to use ink would contaminate the sample being analysed.

The distance travelled by the sample from the baseline to the solvent front and from the

baseline to the sample spot is measured using a ruler.

These measurements are used to calculate the retention factor ( Rf value) using the equation:

○ Rf = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 𝑏𝑦 𝑠𝑎𝑚𝑝𝑙𝑒 𝑠𝑝𝑜𝑡𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 𝑏𝑦 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 𝑓𝑟𝑜𝑛𝑡

A is incorrect as if the sample is placed in the solvent it would dissolve.

B and C are incorrect as the sample spot should be placed on the pencil line as it is this line that

is used to measure the distances for the equation of the Rf value.

If the spot is above or below this line then the measurements will be inaccurate.

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9 A sample of a dye is investigated by chromatography.

A line is drawn across a piece of chromatography paper and a spot of the dye is placed on it.

The paper is placed in water.

water

chromatographypaper

linespot of dye

Which row is correct?

what is used to draw the line

position of spot

A ink

B ink

C pencil

D pencil

above the level of the water

below the level of the water

above the level of the water

below the level of the water

Pencil is used to draw the line rather than a pen which contains ink.

If you use a pen, the ink will dissolve and be separated. This could mix with the

samples as they separate and interfere with the results.

The sample spot must be above the level of the water otherwise it would dissolve into

the water.

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10 In the chromatography experiment shown, which label represents the solvent front?

C

D

B

A

The solvent is what carries the sample up the chromatography paper and causing it to

separate into its constituent components.

The run is finished when the solvent is close to the top of the chromatography paper.

As the sample is placed as a concentrated spot on the baseline, it will rise up vertically.

The solvent rises up across the entire paper, forming the line at the top.

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11 Chromatography experiments are carried out on four substances, P, Q, R and S.

The same solvent is used in each experiment.

The resulting chromatograms are shown below.

P Q R S

baseline

Which statement is not correct?

A P and Q are pure substances.

B P and R are different substances.

C R and S are pure substances.

D S is a mixture of substances.

Paper chromatography is used to separate substances based on their solubility in the

solvent used.

Different components of the substances will travel different distances, producing several

spots on the chromatogram.

Statement A is correct as P and Q have produced only one spot each, indicating that

they are pure substances.

Statement B is correct as the distance travelled by the spot on chromatogram P is

different to that travelled on chromatogram R.

Statement C is incorrect as S has two spots, indicating it contains two substances,

hence it cannot be a pure substance.

Statement D is correct as S is a mixture of two substances.

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12 The paper chromatogram below was obtained from four different dyes.

Which dye has an Rf value of 0.7?

A B C D

solvent front

baseline

3. Using a ruler,

Measure the distance from the baseline to each sample spot

Measure the distance from the baseline to the solvent front.

Using the equation for the retention factor:

Rf = distance travelled by sample spot

distance travelled by solvent

Calculate which sample spot (A, B, C or D) produces an answer of 0.7cm.

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13 The diagram shows the apparatus used to separate the different components of a mixture bychromatography.

baseline

solvent

spot

chromatography paper

solvent front

Which statement about this experiment is correct?

A A locating agent is used to find the position of the solvent front.

B The components to be separated must be soluble in the solvent.

C The baseline on which the spot of the mixture is placed is drawn in ink.

D The Rf value is calculated by the distance travelled by the solvent front

the distance travelled by the component

In chromatography, the solvent carries the substances with it as it moves up the paper

and the substance separates into its different components.

Therefore the substance to be separated must be soluble in the chosen solvent.

An insoluble component would remain at the baseline.



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14 A sample of a green food colouring was separated into its component colours using paperchromatography.

The results obtained are shown.

baseline

blue spot

yellow spotsolvent front

What is the Rf value of the blue spot?

A 0.45 B 0.90 C 1.10 D 2.20

Using a ruler:○ Measure the distance between the between the baseline and the blue spot.

○ And the distance between the baseline and the solvent front.

Use the equation for calculating the Rf value:

○ Rf = 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑑𝑑𝑡𝑡𝑑𝑑𝑡𝑡𝑑𝑑𝑡𝑡𝑡𝑡𝑑𝑑𝑑𝑑 𝑏𝑏𝑏𝑏 𝑑𝑑𝑑𝑑𝑠𝑠𝑠𝑠𝑡𝑡𝑑𝑑 𝑑𝑑𝑠𝑠𝑠𝑠𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑑𝑑𝑡𝑡𝑑𝑑𝑡𝑡𝑑𝑑𝑡𝑡𝑡𝑡𝑑𝑑𝑑𝑑 𝑏𝑏𝑏𝑏 𝑑𝑑𝑠𝑠𝑡𝑡𝑡𝑡𝑑𝑑𝑑𝑑𝑑𝑑 𝑓𝑓𝑡𝑡𝑠𝑠𝑑𝑑𝑑𝑑

○4.9𝑑𝑑𝑠𝑠

Rf = 2.2𝑑𝑑𝑠𝑠= 0.448

This value is close to 0.45.



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15 Amino acids are colourless and can be separated and identified by chromatography.

beaker

glass cover

solvent

chromatographypaper

What additional apparatus is required to identify the amino acids present in a mixture?

A a locating agent

B a ruler

C a ruler and a locating agent

D neither a ruler or a locating agent

A ruler is needed to accurately measure the distances moved by the sample spots and

the solvent front.

These distances are required to be able to calculate the retentionfactor. Amino acids are colourless and therefore they cannot be seen as they move up the

paper.

The locatingagent reacts with the samples producing colouredspots.



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16 What is always true for a pure substance?

A It always boils at 100 °C.

B It contains only one type of atom.

C It has a sharp melting point.

D It is solid at room temperature.

Statement A is not true as each pure substance has a different boiling point depending

on its chemical structure.

Statement B is nottrue as pure substances can contain many atoms of the same type.

Statement C is true as pure substances have no impurities and melt at precise and

predictable temperatures.

The melting points of pure substances are therefore sharp and can be seen as

horizontal lines on a heating curve.

Statement D is not true as a pure substance can be a solid, liquid or a gas at room

temperature, depending on its chemical structure.

Heating curve for a pure substance



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17 A sample of liquid X turns blue cobalt(II) chloride paper pink. The sample boils at 102 °C.

Which statements are correct?

1 X contains water.

2 X is impure water. 3

X freezes above 0 °C.

A 1, 2 and 3 B 1 and 2 only C 1 and 3 only D 2 and 3 only

Cobalt (II) chloride turns fromblue to pink in the presence of water.

This test is usually done using cobalt chloride paper and is a standard test for water.

The equation is:

○ CoCl2 + 6H2O ⇌ CoCl2.6H2O

X must therefore contain water as the test is positive, hence statement 1 is correct.

The sample boils at 102ºC but water boils at 100ºC.

Differences in boiling points are caused by the presence of an impurity, hence

statement 2 is also correct.

The freezing point of X is most unlikely to be below 0ºC as impurities lower the

vapourpressure which causes the freezing point of water to decrease, hence

statement 3 is incorrect.

Therefore options A, C and D are incorrect.

Cobalt (II) chloride paper



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18 The results obtained from a chromatogram are shown.

Which row gives the Rf values of substance X and substance Y?

The retention factor (Rf value) is calculated for each sample using the equation:


B is correct as the Rf values as calculated for both substances matches with those given

in option B:


The Rf value for X is3.0𝑑𝑑𝑠𝑠 = 0.6


The Rf value for Y is 2.5𝑑𝑑𝑠𝑠 = 0.5

A, C and D are therefore incorrect as these Rf values do not match with the actual

values for X and Y.



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19 A student carried out paper chromatography on a mixture of amino acids.

The student sprayed the dried chromatogram with a locating agent.

What is the function of the locating agent?

A to dissolve the amino acids

B to form coloured spots with the amino acids

C to preserve the amino acids

D to stop the amino acids reacting

Amino acids are linked together via amide links (peptide bonds) (-CONH-).

These can be broken down and separated by hydrolysis in acidic conditions.

The resulting amino acids can be identified by paper chromatography.

A locating agent is used as amino acids are colourless and cannot be seen on the

paper.

The locating agent reacts with the amino acids to form colouredproducts.

These are visible on the chromatogram and so the Rfvalue can be calculated and used

to identify the amino acids.

A is incorrect as the amino acids are dissolved by the solvent (known as the mobile

phase) as it moves up the paper (known as the stationary phase).

C is incorrect as preservation of the amino acids is not a stepin paper chromatography.

D is incorrect as amino acids are rendered inactive during the process and do not react

with each other or the solvent.



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Paper chromatography



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20 The painkiller paracetamol is synthesised from 4-aminophenol.

Chromatography was carried out on an impure sample of paracetamol. The results are shown (not drawn to scale).

S P

0.35 cm

0.26 cm

0.17 cm

solvent front

baseline

key

PS = impure sample of paracetamol

= pure paracetamol

The sample of paracetamol was contaminated with 4-aminophenol only.

What is the Rf value of 4-aminophenol?

A 0.49 B 0.65 C 0.74 D 1.35

The chromatogram shows the results of both substances.

S and P have a spot at the same height, 0.17cm, which corresponds to the paracetamol

present in both samples.

The impurity in sample S is visible at 0.26cm and this is the value which must be used to

calculate the retention factor for the 4-aminophenol.

The retention factor (Rf value) is calculated using the equation:

○ Rf = 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑙𝑙𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑙𝑙𝑙𝑙𝑑𝑑𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑙𝑙𝑑𝑑𝑑𝑑

C is correct as the Rf value is as calculated for the 4-aminophenol:

○ Rf value for 4-aminophenol is0.260.35

= 0.74

A, B and D are therefore incorrect as these Rfvalues do not match with the valuecalculated for the 4-aminophenol.



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21 Pure water has a boiling point of 100 °C and a freezing point of 0 °C.

What is the boiling point and freezing point of a sample of aqueous sodium chloride?

Sodium chloride in water can be considered an impurity.

The presence of an impurity affects the physical properties of a substance.

Pure water boils at 100ºC and freezes at 0ºC.

In water, sodium chloride causes the boiling point to increase and the freezing point to

decrease.

This is why salt is used to prevent icing on roads as it lowers the freezing point, making it

more difficult for ice to form at temperature close to 0ºC.

Melting Points & Boiling Points

Exam tip: when given data about boiling points and melting points, think about

what you know about water and compare to the data, irrespective of the values

given. You know that water boils at 100ºC, so at any temperature above this it

will be a gas. Water freezes at 0ºC so at any temperature below this it will be a

solid. And any temperature in between 0ºC and 100ºC it will be in the liquid



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Effect of adding an impurity to the boiling point and freezing point of water



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22 Impurities change the melting and boiling points of substances.

Sodium chloride is added to a sample of pure water.

How does the addition of sodium chloride affect the melting point and boiling point of the water?

Sodium chloride in water can be considered an impurity.

The presence of an impurity affects the physical properties of a substance.

Pure water boils at 100ºC and freezes at 0ºC.

In water, sodium chloride causes the boiling point to increase and the freezing point to

decrease.

This is why salt is used to prevent icing on roads as it lowers the freezing point,

making it more difficult for ice to form at temperature close to 0ºC.









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23 A compound, X, has a melting point of 71°C and a boiling point of 375°C.

Which statement about X is correct?

A It is a liquid at 52 °C and a gas at 175°C.

B It is a liquid at 69 °C and a gas at 380 °C.

C It is a liquid at 75 °C and a gas at 350 °C.

D It is a liquid at 80 °C and a gas at 400 °C.

Compound X melts at 71ºC so it will be a liquid at temperatures above this.

It boils at 375ºC so it will be gas above this temperature.

At 80ºC it will be a liquid and at 400ºC it will be a gas.

Heating curve









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24 A chromatogram obtained from the chromatography of four substances is shown.

Which substance has an Rf value of 0.6?

solvent front

baseline

A B C D

5.5 cm 5.2 cm 5.8 cm 6.3 cm

10.5 cm



○ Rf = 6.310.5

= 0.6



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25 The diagram shows a chromatogram of four substances.

Which substance has an Rf value of approximately 0.32?

A B C D

solvent front

baseline

4 cm

6 cm

12 cm

14 cm

19 cm

0 cm

Using a ruler, measure the distance between the between the baseline and each

sample spot and also the solvent front.



○ Rf = 6𝑐𝑚19𝑐𝑚= 0.315

This value is close to 0.32.

Option A is the food dye with an Rf value closest to the result from the analysis.



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26 A student used chromatography to analyse a green food colouring.

The chromatogram obtained is shown.

solvent front

baseline

blue spot

yellow spot

The table lists some yellow food dyes and their Rf values.

Which yellow food dye does the green food colouring contain?

yellow food dye Rf value

A 0.48

B 0.32

C artr

D

Quinolene Yellow

Sunset Yellow

tartrazine

Yellow 2G 0.82

0.69

Using a ruler, measure the distance from the baseline to the solvent front and from the

baseline to the yellow sample spot.



○ Rf = 1.2 𝑐𝑚3.75 𝑐𝑚

= 0.32

This value matches the Sunset Yellow food dye, option B.



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27 How can the amino acids in a protein be separated and identified?

A Add a locating agent to the protein.

B Hydrolyse the protein and then use chromatography.

C Polymerise the protein and then add a locating agent.

D Use chromatography on a solution of the protein.

The amino acids in proteins are linked together via amide links (peptide bonds).

These can be broken down and separated by hydrolysis in acidic conditions.

The resulting amino acids can be identified by paper chromatography.

A locating agent would have to be used as the compounds are colourless.

The Rf value is calculated and used to identify the amino acids.

Hydrolysis of a peptide bond



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28 A student is investigating a coloured mixture using chromatography.

paper

pencil line

solvent

Where should the student place the coloured mixture?

A in the solvent

B just above the pencil line

C just below the pencil line

D on the pencil line

In paper chromatography the sample spots should all go onto the baseline.

This is always made with a pencil as to use ink would contaminate the sample being

analysed.

The distance travelled by the sample from the baseline to the solventfront and from the

baseline to the sample spot is measured using a ruler.

These measurements are used to calculate the retention factor ( Rf value) using the

equation:


A is incorrect as if the sample is placed in the solvent it would dissolve.

B and C are incorrect as the sample spot should be placed on the pencil line as it is this

line that is used to measure the distances for the equation of the Rf value.

If the spot is above or below this line, then the measurements will be inaccurate.



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29 Propanone, C3H6O, is a liquid at room temperature.

What is the boiling point of pure propanone?

A –61 ºC to –51ºC

B –56 ºC

C 51 ºC to 61ºC

D 56ºC

For propanone to be a liquid at room temperature, then its boiling point must be

aboveroomtemperature, which is taken to be 20ºC.

A and B are therefore incorrect as these values are both negative.

C is also incorrect as even though the values are above room temperature, there is

insufficientinformation in the question to be able to establish an exact temperature

range.



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30 Substance Q boils at 445 ºC and is a yellow solid at room temperature.

Which temperature could be the melting point of pure Q?

A –9 ºC

B 72ºC to 78ºC

C 116ºC

D 116ºC to 126 C

Substance Q is a solid at room temperature which is 20ºC.

Its melting point must therefore be above this temperature, so A can be disregarded.

The question states that substance Q boils at 445ºC.

Substances that boil at specific temperatures are pure, meaning that their melting point

would also be a specific temperature.

Therefore, B and D can also be disregarded as these options provide a temperature

range and not a specific one.



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criteria of purity

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