Methods of Separating & Purifying Substances (Edexcel GCSE Chemistry: Combined Science)

Exam Questions

1 hour13 questions
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

2c
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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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