Methods of Separating & Purifying Substances (Edexcel GCSE Chemistry)

Exam Questions

2 hours23 questions
1a3 marks

The boxes below show separation techniques and materials that could be separated.

Draw one straight line from each separation technique to a material that could be separated by that technique. 

2-2-q1e-separation-techniques

1b1 mark
Which of the following is a mixture?

 
  A Salt
  B Air
  C Water 
  D Ammonia
1c3 marks

Here are five statements about how paper chromatography can be used.

Place ticks in the boxes by three statements that are correct.

  To distinguish between pure and impure substances  
  To identify substances by calculation and use of Rf values  
  To determine the amount of each substance in a mixture  
  To identify substances by comparison with known substances  
  To predict the solubility of a substance in water  

 

1d1 mark

Which separation technique is this equipment used for? 

q1-paper-1-nov-2020-ocr-gcse-chem

  A Crystallisation
  B Evaporation
  C Distillation
  D Filtration

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2a4 marks

Substances can be classified as elements, compounds or mixtures.

Each of the boxes in the diagram represents either an element, a compound or a mixture.

2-2-methods-of-separating-and-purifying-substances
i)
Explain which two boxes represent an element.
 (2)
ii)
Explain which two boxes represent a mixture.
(2)
2b3 marks

The list gives the names of some methods used in the separation of mixtures:

  • chromatography
  • crystallisation
  • distillation
  • filtration

Use names from the list to choose a suitable method for each separation. Each name may be used once, more than once or not at all.

i)
Separating water from sodium chloride solution.
(1) 
iii)
Separating the blue dye from a mixture of blue and red dyes.
(1) 
iii)
Separating potassium nitrate from potassium nitrate solution.
(1) 
2c2 marks

A student decided to investigated the colours in three different dyes, A, B & C, using paper chromatography.

The dyes are soluble in propanone but not soluble in water.

The apparatus used is shown in Figure 1.

aqa-gcse-8-1e-tq3b-chromatography-equipment


Figure 1 

The student made two mistakes when setting up the equipment.

Place ticks in boxes by the two statements that are correct. 

The dyes are all on the same piece of chromatography paper  
The solvent used is water  
A lid is on the beaker  
The water is not level with the dyes  
The start line is drawn in ink  

2d3 marks

The student corrected the mistakes and obtained the chromatogram shown in Figure 2.

aqa-gcse-8-1e-tq3c-chromatogram-of-dyes
Figure 2 



Put ticks (✔ ) in the boxes to show which statements are true and which statements are false. 

  True False
 Dye A is the most soluble     
 Dye C is a mixture    
 Dye C contains dye B     
 Dye C contains dye A    
 Dye B is a pure substance    

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3a1 mark

A student is given a mixture of sodium chloride (NaCl) and sand.

Use the correct words from the box to complete the sentences:

an element                   a mixture           a pure substance    

Sodium chloride is …………………………

Sand is …………………….

3b1 mark

Sodium chloride is soluble in water. Sand is insoluble.

The student adds water to the mixture of sodium chloride and sand.

Complete the sentence to describe what happens.

The sodium chloride …………… to form a solution.

3c1 mark

What separating technique should they use to separate the sodium chloride solution and the sand?

A Chromatography
B Crystallisation
C Filtration
D Fractional distillation
3d2 marks

The student left the beaker of sodium chloride solution out uncovered in the classroom.

The following week there was just a white solid at the bottom of the beaker.

Explain what happened.

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4a3 marks

Substances can be elements, compounds or mixtures.

Draw one straight line from each substance to show if it is an element, compound or mixture.

edx-2-2-easy-q4a-table

4b1 mark

A student sets up the apparatus shown in Figure 1.

edx-2-2e-q4b-distillation-equipment-old

 Figure 1

What separating technique can be carried out using this apparatus?

A Chromatography
B Crystallisation
C Distillation
D Filtration
4c1 mark

There is one error in how they set up the apparatus in Figure 1.

State what this is.

4d3 marks

The student corrects their error.
They use the apparatus to separate water from a salt solution.
They put the salt solution in the round-bottomed flask.
They then heat the solution to 100°C.

Explain why pure water collects in the beaker.

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5a1 mark

Water from rivers needs to be treated before it is potable.

State what is meant by potable water.

5b1 mark

Sedimentation, filtration and chlorination are used to treat river water. 

What happens during sedimentation?

A Chlorine is added to the water
B Large insoluble particles in the water sink to the bottom of tank
C Water is passed through a wire mesh
D Water is passed through sand and gravel
5c1 mark

State why river water needs to be filtered.

5d2 marks

Explain why chlorination is used in water treatment.

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11 mark

Which of the following is not a pure substance? 

  A Seawater
  B Calcium carbonate
  C Distilled water 
  D Ammonia

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2a1 mark

Substances can be pure or they can be mixtures.

Which of these is a mixture?

  A chlorine
  B sodium
  C sodium chloride
  D sodium chloride solution
2b3 marks

Figure 9 shows some mixtures to be separated and possible methods of separation.

Place a tick (✓) in one box in each row of the table to show the best method to separate the first named substance from each of the mixtures.

method of separation
substance to separate crystallisation filtration simple
distillation
fractional
distillation
sand from a mixture
of sand and sodium
chloride solution
       
copper sulfate crystals from copper sulfate solution        
useful liquids from
crude oil
       

   

Figure 9

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4 marks

Paper chromatography was used to separate a mixture of blue and red inks. A spot of the mixture was placed on chromatography paper as shown in Figure 10.

fig-10-q7c-1cho-1f-specimen-2018
Figure 10

i)
Give a reason why the start line is drawn in pencil rather than in ink.
(1)
ii)
The chromatography paper, with the spot of mixture on it, was placed in a beaker with the bottom of the paper in solvent.
On Figure 11, complete the diagram showing the position of the chromatography paper with the spot of mixture at the start of the experiment.

(1)

fig-11-q7cii-1cho-1f-specimen-2018

Figure 11

iii)
The chromatography was carried out and the result is shown in Figure 12.
fig-12-q7ciii-1cho-1f-specimen-2018

Figure 12

The blue spot had moved 14.5 cm and the solvent front had moved 15.3 cm.
Calculate the Rf value of the substance in the blue spot, giving your answer to 2 significant figures.

Rf value = fraction numerator distance space travelled space by space straight a space dye over denominator distance space travelled space by space solvent space front end fraction

(2)

2d4 marks

P, Q, R and S are mixtures of food colourings.
They are investigated using paper chromatography.
Figure 13 shows the chromatogram at the end of the experiment.

fig-13-q7d-1cho-1f-specimen-2018

Figure 13

i)
Which mixture contains an insoluble food colouring?
(1)
  A mixture P
  B mixture Q
  C mixture R
  D mixture S

ii)
Give a change that could be made to the experiment to obtain an Rvalue for the insoluble colouring.
(1)
iii)
Explain, by referring to Figure 13, which mixture is separated into the greatest number of soluble food colourings by this chromatography experiment.
(2)

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31 mark

A student was investigating the colours in black food colouring. 

Chromatography was carried out on samples A-E and compared to the black food coloring. 

The following chromatogram was produced. 

q2-2-2-chromatogram-of-substances-a-e

Which of the following conclusions is not true for this chromatogram?

 
  A Sample B is insoluble in the solvent used 
  B Samples C and D are pure substances 
  C Sample D is the least soluble of the samples
  D The black ink is a mixture of  A, E and an unknown

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4a1 mark

An ink is a mixture of coloured substances dissolved in water.

Which method is used to separate the coloured substances in the ink?

 
  A chromatography
  B crystallisation
  C filtration
  D fractional distillation

4b4 marks

The apparatus shown in Figure 9 can be used to separate water from ink.

fig-9-q6b-1cho-1f-paper-1-june-2018
Figure 9

i)
Cold water flows through the condenser.

On Figure 9 use arrows to show where the water should flow in and where it should flow out.

(1)

ii)
Explain why a condenser is used.

(2)

iii)
The flask was heated with a Bunsen burner.

Give the name of an alternative piece of apparatus that could be used to heat the flask.

(1)

4c2 marks

The particles in the ink in the flask can be shown as in Figure 10.

fig-10-q6c-1cho-1f-paper-1-june-2018

Figure 10

In the boxes below, draw the arrangement of particles that would be expected at A and B shown in Figure 9.

fig-q6c-1cho-1f-paper-1-june-2018

4d2 marks

Changes of state between the three states of matter are shown in Figure 11.

SOLID space space space leftwards arrow over rightwards arrow from melting to freezing of space space LIQUID space space leftwards arrow over rightwards arrow from evaporating to condensing of space space space GAS

Figure 11


The changes shown are physical changes.

Explain why these changes are called physical changes rather than chemical changes.

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51 mark

A student set up the following apparatus to investigate the colours in three different food colourings.  

1-2-q4-edexcel-igcse-chemistry

What are the stationary and mobile phases for this experiment? 

 
      Stationary  Mobile
  A chromatography paper  water
  B beaker start line
  C water food colourings
  D start line chromatography paper

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6a1 mark

Mixtures of substances can be separated using different techniques.

Which of the following is a mixture of substances?

  A air
  B carbon dioxide
  C gold
  D titanium
6b2 marks

Figure 5 shows the apparatus that a student set up to obtain pure water from ink.

There are three mistakes in the way the apparatus has been set up.  

fig-5-q4b-1cho-1f-paper-1-june-2019
Figure 5
i)
One mistake is that the bulb of the thermometer is too low.

The bulb of the thermometer should be level with the side arm.

Give a reason why the bulb of the thermometer should be level with the side arm. 
(1)
ii)
State one other mistake in Figure 5.
(1)
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6 marks

Paper chromatography is used to separate the substances in five different food colourings, P, Q, R, S and T.

Figure 6 shows the chromatogram at the end of the experiment. 

fig-6-q4c-1cho-1f-paper-1-june-2019
Figure 6
i)
The steps needed to carry out the chromatography experiment are listed below.
They are not in the correct order.
  1. leave the solvent to rise up the paper
  2. put solvent in the beaker
  3. draw a start line on the piece of paper 3
  4. place the paper in the beaker
  5. remove the paper when the solvent is near the top
  6. put small spots of the food colourings on the start line

List the steps in the correct order.

The first two steps have been done for you. 

(2)

fig-q4ci-1cho-1f-paper-1-june-2019    

ii)
Explain, using Figure 6, which food colouring contains the greatest number of coloured substances. 
(2)

iii)
During chromatography of the food colourings, the solvent front moved 8.00 cm and the food colouring R moved 2.30 cm.

Calculate the Rf value for food colouring R.
Give your answer to two significant figures. 
(2)

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71 mark

A student making copper sulfate crystals used the method below. 

q4-2-2-practical-unreacted-copper-carbonate

Unreacted copper carbonate was left over as it had been added in excess.

Which separation technique could be used to separate the unreacted copper carbonate and solution into different containers?

 
  A Filtration
  B Crystallisation
  C Simple distillation
  D Fractional distillation

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81 mark

A mixture contains methanol and ethanol. They have different boiling points. 

Which method could be used to separate this mixture? 

  A Filtration
  B Simple distillation
  C Crystallisation
  D Evaporation

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9a5 marks

A sample of rock salt contains a mixture of sodium chloride and some insoluble substances.

The rock salt is added to water and the mixture stirred.
The mixture is then filtered to obtain a filtrate of sodium chloride solution.

i)
Draw a labelled diagram of the apparatus used to filter the mixture and collect the sodium chloride solution.
(2)
ii)
Describe how a sample of pure, dry sodium chloride crystals can be obtained from the filtrate.
(3)
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4 marks

Inks contain coloured dyes.
Samples of four inks, W, X, Y and Z, were separated using paper chromatography.
Figure 5 shows the chromatogram obtained.

fig-5-q5b-1cho-1h-paper-1-nov-2020

Figure 5

i)
In the experiment, the solvent front moved 6.5 cm.
Calculate the Rf value of the dye that is present in both inks X and Z.
(1)
Rf = ...................................................
ii)
State what could be changed in the experiment to make the Rf value more accurate.
(1)
iii)
In this experiment, ink sample Y did not move from the start line.
Explain a change to the experiment that would be needed to separate the dyes in ink sample Y.
(2)

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10
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1 mark

A student carried out chromatography on a sample of green ink. 

q6-2-2-chromatogram-with-inks

The distance moved by the solvent front is 5.8 cm. 

The distance moved by the yellow dye is 4.5 cm.

Calculate the Rf value of the yellow dye to 2 significant figures. 

 
  A 0.78
  B 0.80
  C 1.3
  D 26

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111 mark

During the treatment of ground water, large insoluble particles are allowed to settle to the bottom of a tank.

What is this process called? 

  A Sedimentation
  B Anaerobic digestion
  C Screening
  D Chlorination

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12a1 mark

A sample of potable water contains impurities.

Why is this sample of water potable even though it contains impurities?

  A the impurities have no smell
  B the impurities are colourless 
  C the impurities are harmless
  D the impurities are soluble 

12b3 marks

Waste water can be used to produce drinking water.

The processes used include sedimentation, filtration and chlorination.

i)
What is sedimentation?

  A the waste water is heated so the impurities evaporate
  B the waste water has an acid added to remove impurities 
  C the impurities in the waste water settle to the bottom of their container
  D the impurities in the waste water are bleached 

(1)

ii)
State why the waste water is filtered.

(1)

iii)
State the reason for chlorination.

(1)

12c3 marks

Some salts can be added to waste water to remove impurities.

In an experiment, different masses of salt A were added to 1000 cm3 samples of waste water.

The experiment was repeated with salt B.
The percentages of impurities removed from the waste water are shown in Figure 10.

fig-4-q3c-1cho-1h-paper-1-nov-2021

Figure 10

It was concluded that the best way to purify 1000 cm3 of the waste water is to add 100 mg of salt B.

Use the information about salt A and salt B in Figure 10 to evaluate this conclusion.

12d6 marks

A sample of water was contaminated with a dissolved solid.


Devise a plan to separate pure water from this mixture, including a test to show that the water obtained is neutral.

You may use some or all of the apparatus shown in Figure 11 and any other laboratory apparatus.

fig-11-q9d-1cho-1f-paper-1-nov-2021

Figure 11

12e2 marks

Higher Only

Waste water may contain phosphate ions, begin mathsize 14px style P O subscript 4 to the power of 3 minus sign end style.

Aluminium ions react with phosphate ions to form aluminium phosphate.

Complete the ionic equation for the formation of aluminium phosphate in this reaction

.................. + PO subscript 4 superscript 3 minus end superscript → ............

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131 mark

What is the correct expression for calculating the Rf value in chromatography?

  A straight R subscript straight f space equals space fraction numerator distance space the space solute space travels over denominator distance space the space solvent space travels end fraction


B R subscript f space equals space fraction numerator distance space the space solvent space travels over denominator distance space the space solute space travels end fraction


C R subscript f space equals space fraction numerator distance space the space mobile space phase space travels over denominator distance space the space stationary space phase space travels end fraction


D R subscript f space equals space fraction numerator distance space the space stationary space phase space travels over denominator distance space the space mobile space phase space travels end fraction

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1a1 mark

This question is about water.

Tap water is not pure.
Explain why.

1b4 marks

Figure 1 shows some apparatus that can be used to make pure water from tap water.

edx-2-2e-q4b-distillation-equipment

Figure 1

i)
Name the separating technique used to make pure water from tap water.

[1]

ii)
Describe how you would use the apparatus to make pure water from tap water.
Include what you would see happen.

[3]

1c2 marks

A scientist wanted to identify an unknown chloride salt.
They first dissolved the salt in pure water to form a solution.
They then add it to sodium hydroxide solution.
A green precipitate formed.

Explain why it is important that the water used to dissolve the salt was pure.

1d2 marks

Chemistry Only

i)
Name the salt.

[1]

ii)
State its formula.

[1]

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2a3 marks

Higher Tier Only

A student was asked to make pure copper sulfate crystals by reacting copper oxide with sulfuric acid.

The symbol equation for the reaction is:

CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)

They used 0.005 mol of sulfuric acid and 4.0 g of copper oxide.

Show that the copper oxide is in excess.

(relative atomic masses: Cu = 63.5, O = 16)

2b2 marks

Explain why it is important that the copper oxide is in excess.

2c1 mark

Copper oxide is an insoluble solid.

State the separating technique the student should use to remove the excess copper oxide.

2d4 marks

Describe how the student can make pure, dry copper sulfate crystals from the copper sulfate solution.

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3a2 marks

Figure 1 shows a heating curve for two substances - A and B.

Both substances are a solid before heating.

edx-2-2h-q3a-heatign-curve

Figure 1

Identify which substance - A or B is a pure substance and which is a mixture.

Give a reason for your answer.

3b2 marks

Suggest the reason for the horizontal part of substance B’s heating curve, labelled X  on Figure 1.

3c1 mark

Combining the alkanes heptane and hexane makes a mixture.
Hexane has a boiling point of 69°C. Heptane has a boiling point of 98°C.

State why hexane and heptane are saturated.

3d3 marks

Describe the process used to separate a mixture of these two liquids.

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4a2 marks

Some inks are a mixture of soluble coloured substances.

A student wanted to use paper chromatography to separate the coloured substances in five different inks: A, B, C, D and E.

Figure 1 shows how they set up the apparatus.

edx-2-2h-q4a-chromatography-set-up

Figure 1

They made one mistake setting up the apparatus.

i)
State what this is.

[1]

ii)
Explain why it is important they correct the mistake.

[1]

4b1 mark

The student corrects their mistake and continues with the experiment.

Figure 2 shows the chromatogram at the end of the experiment.

edx-2-2h-q4b-chromatography-results

Figure 2

Give the reason why the spot of food colouring B has not moved.

4c3 marks

The other inks have moved up the chromatogram.

Give three conclusions that can be made about the inks from the chromatogram. 

4d2 marks

One coloured substance in food colouring C moved 4.2 cm when the solvent front moved 8.6 cm.

Calculate the Rf value for this substance.

Give your answer to two significant figures.

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5a1 mark

Water taken from rivers has to be cleaned through a series of processes to form potable water.

Define potable water.

5b2 marks

Figure 1 shows the maximum accepted limits of substances in potable water.

Substance Maximum accepted limit
Calcium 200 mg dm-3
Nitrate 1.0 mg dm-3
Harmful bacteria 0 per dm3
pH 6.5 - 8.5

 Figure 1

A 250 cm3 sample of water contains 47 mg of calcium.

Show that the water is below the maximum accepted limit.

5c2 marks

At the end of the treatment process the water is found to have a pH of 6.0.

Suggest what could be done to change it to an acceptable pH.

5d2 marks

Water is tested at each stage of treatment.
A sample is found to contain 10 000 harmful bacteria per 100 cm3.

State what process the water needs to go through before it is potable.

Give a reason for your answer.

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