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

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.

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)

Figure 11

iii) The chromatography was carried out and the result is shown in Figure 12.

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.

R_f value =

(2)

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

i) Which mixture contains an insoluble food colouring? (1)

ii) Give a change that could be made to the experiment to obtain an R_f value 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)

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.

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)

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.

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.  

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)

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.

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)

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 R_f value for food colouring R.

Give your answer to two significant figures. 

(2)

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.

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)

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

i) In the experiment, the solvent front moved 6.5 cm. Calculate the R_f value of the dye that is present in both inks X and Z. (1)

R_f = ...................................................

ii) State what could be changed in the experiment to make the R_f 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)

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.

i) What is sedimentation?

(1)

ii) State why the waste water is filtered.

(1)

iii) State the reason for chlorination.

(1)

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?