Purity, Formulations & Chromatography (AQA GCSE Chemistry: Combined Science)

Exam Questions

2 hours17 questions
1a1 mark

The term 'pure substance' has a different meaning in chemistry compared to its use in everyday life.

Which is the definition of a 'pure substance' in chemistry?

Tick (✓) one box.

A useful product made by mixing substances  
A substance that has had nothing added to it  
A single element or compound  
A substance made from only one type of atom  
1b1 mark

The meaning of a pure substance in everyday life is different to in chemistry.

Complete the sentence.

In everyday language, a pure substance means a substance that has had ____________________ added to it.

1c2 marks

Which are examples of pure substances in chemistry terms?

Tick (✓) two boxes.

Pure water  
Pure orange juice  
Pure air  
Pure carbon dioxide  
Pure milk  
1d2 marks

Melting points and boiling points can be used to determine if a substance is chemically pure.

Complete the sentences.

Choose the answers from the box.

the same as different to range
                sharp                         high


A pure substance has a ____________________  melting point or boiling point.

An impure substance has a wider range of melting point or boiling point, which is ____________________  the pure sample.

1e1 mark

The melting point of a sample of impure water was found experimentally.

Draw one line from the sample to the most likely melting point of the impure water.

D5yrXpCk_melting-point-of-impure-water

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2a1 mark

Fertilisers are an example of a formulation.

What is a formulation?

2b1 mark

A fertiliser contains 25% by mass of nitrogen; 16% phosphorous and 23% potassium.

How does this information show that it is a formulation?

2c2 marks

Give two other examples of formulations.

2d2 marks

Aspirin tablets contain aspirin as the active ingredient and other inactive ingredients.

Suggest the reasons for adding the inactive ingredients below.

Inactive ingredient: Flavouring

Reason: _____________________________________________________________

Inactive ingredient: Binding agent

Reason: _____________________________________________________________

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3a2 marks

Paper chromatography involves a stationary phase and a mobile phase.

Draw one line from each phase to its identity.

mobile-stationary-phase

3b2 marks

A student 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.

Figure 1

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

The student made two mistakes when setting up the equipment.

What are the two mistakes?

Tick (✓) two boxes

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  
3c2 marks

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

Figure 2

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

What two conclusions can be drawn from this chromatogram?

Tick (✓) two boxes.

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  
3d
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2 marks

The student records some measurements from the chromatogram.

The measurements are:

  • the colour from dye A moves 3.6 cm
  • the solvent moves 8.0 cm

Calculate the Rf value for the colour from dye A.

Use the equation:

straight R subscript straight f equals fraction numerator distance space moved space by space colour over denominator distance space moved space by space solvent end fraction

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4a2 marks

Green ink is made from mixing dyes together.

A student produced the chromatogram in Figure 1 when they investigated green ink using paper chromatography.

The solvent used was water.

Figure 1

aqa-gcse-8-1e-tq4a-green-ink-chromatogram

The student took some measurements from the chromatogram.

The measurements are:

  • the yellow dye moves 4.8 cm
  • the solvent moves 6.0 cm

Calculate the Rf value of the yellow dye.

straight R subscript straight f space equals space fraction numerator distance space moved space by space dye over denominator distance space moved space by space solvent end fraction

4b1 mark

What would the Rf value of the blue dye be?

Tick(✓) one box.

Less than the Rf of the yellow dye  
The same as the Rf of the yellow dye  
Greater than the Rf value of the yellow dye  
There is not enough information to determine the Rf value  
Greater than 1.0  
4c1 mark

The green ink is made up of 25% blue ink and 75% green ink.

Which word correctly describes the green ink?

Tick (✓) one box.

Element  
Mobile phase  
Formulation  
Compound  
4d2 marks

Why could the green ink not be described as a pure substance?

4e2 marks

The student repeated the investigation but used ethanol as the solvent instead of water.

The blue dye is not soluble in ethanol.

The yellow dye is more soluble in ethanol than water.

Which of the two statements about the resulting chromatogram are correct?

Tick (✓) two boxes.

It would have two spots above the start line   
It would have one spot above the start line and one spot on the start line  
The Rf value of the yellow dye would increase  
The Rf value of the blue dye would be 1.0  
The yellow spot wouldn't travel as far up the paper  

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1a1 mark

Methylated spirit is a versatile product made from a mixture of substances.

Table 1 below shows the mass of substances present in a sample of methylated spirit.

Table 1

Substance Mass in grams
Ethanol 260.5
Methanol 25.3
Pyridine 2.5
Methyl violet 1.0


What name is given to a product such as methylated spirit?

1b
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2 marks

Calculate the percentage of ethanol in the sample of methylated spirit.

Use Table 1 above to answer this question.

Give your answer to 2 significant figures. 

1c4 marks

The sample of methylated spirit was analysed and found to contain small amounts of other substances not stated in the formulation.

Comment on how this may affect the performance of the formulation.

1d2 marks

Formulations are used in many different industries. Give two examples of formulations.

1e5 marks

Describe the design and general properties of a formulation. 

Include some examples of common formulations.

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2
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1 mark

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

voCYRTKg_q2

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. 

  • 0.78

  • 0.80

  • 1.3

  • 26

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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. 

Dhhz0WAe_q3

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

  • Sample B is insoluble in the solvent used 

  • Samples C and D are pure substances 

  • Sample D is the least soluble of the samples

  • The black ink is a mixture of  A, E and an unknown

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4a4 marks

This question is about pure substances and mixtures.

Explain what the term ‘pure’ means in chemistry and compare it to the everyday meaning.

4b4 marks

Explain how boiling point and melting point analysis can be used to differentiate between pure and impure substances.

4c3 marks

A student was testing the melting point of a sample of gallium using a melting point apparatus. 

The sample started to melt at 24 oC and melted completely at 27 oC. 

Data tables places the melting point of gallium at 29.76 oC

Is the sample of gallium pure or impure? 

Give reasons for your answer.

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5a4 marks

This question is about chromatography.

A student used paper chromatography to investigate different types of food dye.

This is the method the student used.

  1. Put a spot of food colouring brand X on the start line.
  2. Place spots of four separate dyes labelled A, B, C and D, on the start line.
  3. Place the bottom of the paper in a beaker of water and leave it there for several minutes.

The apparatus the student used is shown in Figure 1.

Figure 1

aqa-gcse-sq-8-1m-q3a-chromatography-experiment-with-errors

Identify two mistakes the student made in setting up the apparatus and explain what problems the mistakes would cause.

5b2 marks

Explain how the student could avoid these mistakes.

5c3 marks

Another student set up the apparatus correctly and obtained a set of results which are shown in Figure 2

The result for sample D is not shown.

Figure 2

chromatography-results-aqa-gcse-sq-8-1m-3c

Calculate the Rf value of dye B, showing clearly your working out.

Give your answer to two significant figures.

5d2 marks

The  Rf value of Dye D = 0.80. 

Use this value to calculate the distance dye D moved on the chromatography paper.

5e4 marks

Explain how chromatography is able to separate the different dyes in sample X.

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6a2 marks

A chemist was asked to analyse several samples of substances used as food colourings. 

He performed paper chromatography on the samples he was given, which he labelled A, B, C, D and E.

The chromatogram in Figure 3 shows the chemists' results.

Figure 3

chromatography-results-of-black-dye-aqa-gcse-sq-8-1m-4a

What can you deduce about the composition of the black food colouring?

6b3 marks

Calculate the Rf value of dye C, showing clearly your working out.

Give your answer to two significant figures.

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7
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1 mark

Paracetamol is a formulation.

One tablet has a mass of 1.30 g and contains 500 mg of the active ingredient, paracetamol. 

What is the percentage by mass of paracetamol in one tablet?

  • 4%

  • 38%

  • 50%

  • 76%

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81 mark

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

Gc9L6B6d_q5

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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    1a
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    3 marks

    Black ink is made from other colour dyes.

    A student investigated black ink using paper chromatography in a beaker with water as the solvent.

    Figure 1 shows the chromatogram that the student obtained. It is not to scale.

    Figure 1

    chromatogram-black-ink

    The Rf value of the blue dye = 0.86

    The distance moved by the solvent = 6.3 cm

    Calculate the distance moved by the blue dye, in cm.

    Give your answer to two significant figures.

    1b2 marks

    The student took measurements from the chromatogram and calculated the Rf value of the red dye to be 1.3.

    Explain why this value is not correct.

    1c2 marks

    What does the Rf value of a dye depend on?

    1d2 marks

    In Figure 1, the yellow dye has not moved from the start line.

    Give a reason why it has not moved and suggest how the method could be altered to obtain a chromatogram in which all three dyes move up the paper.

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    2a2 marks

    Paraffin wax is a flammable substance that is obtained from crude oil.

    A student investigated how the temperature of paraffin wax changed as it cooled using the steps below.

    • Heat the paraffin wax until it melts
    • Continue to heat for a further minute
    • Measure and record the temperature every minute

    The results they obtained are plotted in Figure 1.

    Figure 1

    aqa-gcse-8-1h-tq2a-cooling-curve

    What conclusion can you draw from these results?

    Explain your answer.

    2b1 mark

    Suggest a method that would be most suitable for heating the wax.

    2c4 marks

    The student repeated the experiment using pure stearic acid which has a melting point of 69 oC.

    They heated the sample up to 80 oC and allowed it to cool. After 30 minutes, the temperature of the stearic acid was 40 oC.

    Sketch the expected results in Figure 2.

    Figure 2

    XS199QQV_blank-graph-for-cooling-curve

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    3a1 mark

    A student investigated the food colouring in three drinks using paper chromatography.

    Figure 1 shows the chromatogram showing the student's results.

    Figure 1

    drinks-chromatogram

    Why does the start line need to be drawn in pencil?

    3b4 marks

    What conclusions can the student make from the chromatogram in Figure 1?

    3c2 marks

    Explain how the student can calculate the Rf value of the colours contained in the drinks.

    3d4 marks

    Explain how chromatography separates the different colours in the drinks by paper chromatography.

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    4a4 marks

    Cold and flu medicines are formulations.

    An example of the packaging of some tablets used to treat cold and flu is shown in Figure 1.

    Figure 1

    aqa-gcse-8-1h-tq4a-cold-and-flu-tablet

    State what a formulation is, and explain how the information on the packet of the cold and flu tablets in Figure 1 shows that it is a formulation.

    In your answer, suggest possible reasons for the other, non-active, ingredients.

    4b
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    3 marks

    Using the information in Figure 1, calculate the percentage by mass of phenylephrine hydrochloride in each tablet.

    4c
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    1 mark

    The formula of paracetamol is C8H9NO2.

    Calculate the relative formula mass of paracetamol.

    Relative atomic masses:  H = 1;  C = 12;  N = 14;  O = 16

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    5a
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    5 marks

    New evidence suggests that a food colouring that has a Rf value of 0.7 may cause dangerous side effects.

    The chromatogram of two sweets, X and Y, were collected and the results are shown in Figure 1.

    Figure 1

    sweets-chormatogram

    For each of the sweets, state whether they should or should not be sold until further research into the food colouring has taken place.

    Give reasons for your answer.

    Use the scale on the chromatogram to help you answer this question.

    5b2 marks

    Some food colouring additives are thought to cause hyperactivity in young children.

    Suggest two reasons why manufacturers still use these additives in their products.

    5c4 marks

    One of the additives thought to cause hyperactivity is the additive tartrazine, E102, and it is often added to orange drinks.

    An investigation into the food colouring in an orange drink was carried out using paper chromatography.

    Figure 2 shows the chromatogram produced.

    Figure 2
    wm9xZXxk_chromatogram-orange-drink

    The manufacturer of the drink stated that their drink did not contain any additives that caused hyperactivity.

    Is this claim correct? Explain your answer.

    Use the chromatogram in Figure 2.

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