OCR Gateway GCSE Chemistry

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GCSEChemistryOCR GatewayTopic Questions2. Elements, compounds & mixtures2.1 Purity & Separating Mixtures

2.1 Purity & Separating Mixtures

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1a6 marks

A student is separating a mixture of three substances, A, B and C.

Look at the table. It gives information about these substances.

Substance State at room temperature Melting point (°C) Boiling point (°C) Solubility in water
A liquid 0 100 soluble
B liquid –117 78 soluble
C solid 1535 2750 insoluble

A and B mix together completely.

Suggest how the student can separate the mixture to get pure samples of substances A, B and C.

Explain in detail how each method works.

How did you do?

1b2 marks

The student has separated a pure sample of substance B from the mixture.

Suggest how the student can check that the sample of substance B is pure.

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1a3 marks

The particle model shows how particles are arranged and how they move in the three states of matter.

i) 
Draw a line to match each diagram with the correct state of matter.

q16ai-paper-1-nov-2020-ocr-gcse-chem

[2]

ii)

When a liquid is heated to its boiling point, it will turn into a gas.

Use the particle model to explain why this is a physical change.

[1]

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1b3 marks

The table shows the properties of different substances.

Substance Melting
point (°C)		 Boiling
point (°C)		 Soluble in
water?		 Conducts
electricity in
solid state?		 Conducts
electricity in
molten state?
A −210 −196 No No No
B 1084 2562 No Yes Yes
C 605 1137 Yes No Yes
D −78 −34 Yes No No

i)
Which two substances are gases at room temperature?

Tick (✓) two boxes.

A   square
B  square
C   square
D   square

[1]

ii)
Substance C is an ionic substance.

Use the information in the table to explain why.

[2]

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1c5 marks

You are provided with a mixture of substances B and C.

Substance B is insoluble in water. Substance C is soluble in water.

i)
Describe how you could separate substance B from the mixture.

[3]

ii)
Describe how you would then obtain substance C after separating substance B.
[2]

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26 marks

A student is separating a mixture of three solid substances, A, B and C.

Look at the table. It gives information about these substances.

Substance Colour Melting point (°C) Is it magnetic? Is it soluble in water?
A grey 1535 yes no
B white 801 no yes
C yellow 1427 no no


Suggest how the student can separate the mixture to get pure, dry samples of substances A, B and C.

Explain why your methods work.

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3a2 marks

A student is separating some mixtures. She wants to make pure water from a solution of salt water. She filters the mixture.

Her method does not work.

Explain why her method does not work and describe the method she should use.

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3b2 marks

The student wants to separate a mixture of two liquids.

The liquids are:

Liquid Boiling point (°C)
Water 100
Ethanol 78

Which separation technique should she use?

Explain how the method works.

How did you do?

3c4 marks

The student separates two solid substances A and B. She wants to check that they are pure.

i)
What is meant by a pure solid?
[1]
ii)
The student measures the melting points of four samples of solid A.

Look at her results.

Sample Melting point
(°C)
1 115
2 119
3 114 – 118
4 120 – 122

She knows that a pure sample of solid A has a melting point of 120 °C.

She concludes that sample 4 is the purest sample of solid A.

Do the results support her conclusion?

Explain your answer using evidence from the table.

[3]

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4a
Sme Calculator6 marks

A painting has arrived at a local museum for a new exhibit.

To make sure the painting is genuine, a sample of the paints from the painting is sent to a laboratory for analysis.

The laboratory uses thin-layer chromatography to compare the sample of the paints from the painting with some paints used in 1849 and 1973.

The chromatograms are shown below.

q18-paper-1-june-2019-ocr-gcse-chem

i)
Look at the chromatogram for the sample of the paints from the painting.

Which paint colour(s) is/are pure?
[1]
ii)
The red, blue and yellow paint from the painting are dissolved in different solvents, X, Y and Z.

The results are shown below.

  Solvent used
X Y Z
Paint
colour		 Red Red solid left in tube Clear red solution Clear red solution
Blue Blue solid left in tube Clear blue solution Clear blue solution
Yellow Yellow solid left in tube Clear yellow solution Yellow solution with some yellow solid left in tube

Which solvent, X, Y or Z, should the laboratory use for the thin-layer chromatography?

Explain why this would be better than the other solvents.

[2]

iii)
Scientists use Rf values to compare the different spots on the chromatogram.

Calculate the Rf value for the red paint in the sample of paints from the painting.

Use the equation: Rf = fraction numerator distance space travelled space by space the space substance over denominator distance space travelled space by space the space solvent end fraction

Give your answer to 2 significant figures.

Rf = .......................................................

[3]

How did you do?

4b1 mark

The laboratory think that the painting was completed in 1849.

Is the laboratory correct?

Explain your answer.

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5a1 mark

A scientist analyses a sample of tomato sauce using chromatography.

The tomato sauce is compared to four known food additives, A, B, C and D, as shown in the chromatogram.

q20-paper-1-nov-2020-ocr-gcse-chem

The start line is not drawn in ink.

Explain why

How did you do?

5b1 mark

Food additive D is insoluble in the solvent used.


Draw on the diagram the spot for food additive D at the end of the experiment.

How did you do?

5c1 mark

Which additive is in the tomato sauce?

Tick (✓) one box.

A   square

B   square

C   square

How did you do?

5d
Sme Calculator3 marks

Calculate the Rf value for additive C.

Use the equation: Rf = fraction numerator distance space travelled space by space the space substance over denominator distance space travelled space by space the space solvent end fraction

Give your answer to 2 significant figures.

Rf value = ........................................................... [3]

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6a
Sme Calculator5 marks

A student analyses a sample of tomato sauce using thin-layer chromatography.

The tomato sauce was compared to four known food additives, A, B, C and D, as shown in the chromatogram.

q20-paper-3-nov-2020-ocr-gcse-chem

i)
Calculate the Rf value for C.

Give your answer to 2 significant figures.

Rf value = ........................................................... [3]

ii)
Which additives are shown to be in the tomato sauce?

Tick (✓) all the correct boxes.

   A   square

   B   square

   C   square

   D   square

[1]

iii)
Suggest why D has not travelled as far up the plate as C.

[1]

How did you do?

6b3 marks

Gas chromatography is another type of chromatography.

The diagram shows the equipment used for gas chromatography.

q20b-paper-3-nov-2020-ocr-gcse-chem

i)
State one similarity and one difference between gas chromatography and thin-layer chromatography.

Similarity .........................................................

Difference .....................................................

[2]

ii)

Explain why thin-layer chromatography is used instead of gas chromatography to analyse the tomato sauce.

[1]

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7a4 marks

A student has a mixture of three substances.

Look at some information about these substances.

Substance Melting point (°C) Boiling point (°C) Solubility in water
Sand 1710 2230 Insoluble
Sodium chloride 801 1413 Soluble
Water 0 100  

Describe how the student can separate the mixture to get pure samples of all three substances.


Explain why each method of separation works.

How did you do?

7b3 marks

The student separates two solid substances A and B.

She wants to check that they are pure.

She measures the melting points of four samples of solid B.

Look at her results.

Sample Melting point (°C)
1 109
2 105
3 104–108
4 110–112

The student knows that a pure sample of solid B has a melting point of 110 °C.

She concludes that sample 4 is the purest sample of solid B.

Do the results support her conclusion?

Explain your answer using evidence from the table.

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8a4 marks

A student is making perfume using violet flowers.

The student does an experiment to extract the perfume from the flowers using the apparatus shown in Fig. 21.1.

fig-21-1-paper-1-nov-2020-ocr-gcse-chem

The teacher says the apparatus is not set-up correctly.

i)
What is the name of the piece of apparatus labelled X?
[1]
ii)
Name the process shown in Fig. 21.1.
[1]
iii)
Suggest two changes that the student should make to set-up the apparatus correctly.


[2]

How did you do?

8b
Sme Calculator3 marks

The student finds out that phosphate fertilisers are used to help to grow flowers.

Compound A is a fertiliser of a metal M with the formula M3PO4.

The relative formula mass of compound A is 164.0.

Do a calculation to identify metal M.

Use the relative atomic masses in the Periodic Table.

Metal M = ..........................................................

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