Methods of Separating & Purifying Substances (Edexcel GCSE Chemistry)

Which of the following is not a pure substance? 

Substances can be pure or they can be mixtures.

Which of these is a mixture?

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.

Figure 9

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.

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Figure 11

Figure 12

R_f value =

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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.

Figure 13

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. 

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

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

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

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

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

Figure 10

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

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

The changes shown are physical changes.

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

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

What are the stationary and mobile phases for this experiment? 

Mixtures of substances can be separated using different techniques.

Which of the following is a mixture of substances?

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.  

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. 

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. 

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A student making copper sulfate crystals used the method below. 

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

Which method could be used to separate this mixture? 

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.

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

Figure 5

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

The distance moved by the solvent front is 5.8 cm. 

The distance moved by the yellow dye is 4.5 cm.

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

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 sample of potable water contains impurities.

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

Waste water can be used to produce drinking water.

The processes used include sedimentation, filtration and chlorination.

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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.

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.

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.

Figure 11

Higher Only

Waste water may contain phosphate ions, .

Aluminium ions react with phosphate ions to form aluminium phosphate.

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

.................. +  → ............

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

	☐	A
	☐	B
	☐	C
	☐	D

This question is about water.

Tap water is not pure.
Explain why.

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

Figure 1

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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.

Chemistry Only

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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)

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

Copper oxide is an insoluble solid.

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

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

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

Both substances are a solid before heating.

Figure 1

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

Give a reason for your answer.

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

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.

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

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.

Figure 1

They made one mistake setting up the apparatus.

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The student corrects their mistake and continues with the experiment.

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

Figure 2

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

The other inks have moved up the chromatogram.

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

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

Calculate the R_f value for this substance.

Give your answer to two significant figures.

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

Define potable water.

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

 Figure 1

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

Show that the water is below the maximum accepted limit.

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.

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.