Separation & Purification (Cambridge (CIE) O Level Chemistry): Exam Questions

Describe how to separate the following. In each example, give a description of the procedure used and explain why this method works.

Copper powder from a mixture containing copper and zinc powders.

procedure..............................................

explanation .........................................

Nitrogen from a mixture of nitrogen and oxygen.

procedure............................................

explanation........................................

Glycine from a mixture of the two amino acids glycine and alanine. Glycine has the lower R_f value.

procedure.........................................

explanation.....................................

Magnesium hydroxide from a mixture of magnesium hydroxide and zinc hydroxide.

procedure..................................................

explanation..............................................

An important aspect of chemistry is purity and methods of purification.

Give an example of a substance used in everyday life which must be pure.

A list of techniques used to separate mixtures is given below.

i) From the list, choose the most suitable technique to separate the following.

 Water from sea-water ........................................................................

 Helium from a mixture of helium and methane .................................

 Ethanol from a mixture of ethanol and propanol ...............................

 Iron filings from a mixture of iron filings and water ..........................

 A mixture of two amino acids, glycine and alanine ...........................

[5]

ii) Describe how you would obtain a pure sample of copper(II) sulfate-5-water crystals from a mixture of copper(II) sulfate-5-water with copper(II) oxide using some of the techniques listed above.

[4]

Petroleum contains hydrocarbons which are separated by fractional distillation.

i) Complete the following definition of a hydrocarbon:

A hydrocarbon is a compound which ............................................

[1]

ii) Explain what is meant by the term fractional distillation.

[1]

Some of the fractions obtained from petroleum are given below. State a use for each fraction.

bitumen ....................................................................

lubricating fraction ..............................................

paraffin fraction ........................................................

gasoline fraction ......................................................

A list of techniques used to separate mixtures is given below.

filtration
diffusion
fractional distillation
simple distillation
crystallisation
chromatography

From this list, choose the most suitable technique to separate the following mixtures.

 A technique may be used once, more than once or not at all.

Butane from a mixture of propane and butane.

Oxygen from liquid air.

Water from aqueous magnesium sulfate.

Potassium chloride from aqueous potassium chloride.

Silver chloride from a mixture of silver chloride and water.

The following techniques are used to separate mixtures.

From this list, choose the most suitable technique to separate the following.

methane from a mixture of the gases, methane and ethane.

Water from aqueous magnesium sulfate.

Iron filings from a mixture of iron filings and water.

Zinc sulfate crystals from aqueous zinc sulfate.

Hexane from a mixture of the liquids, hexane and octane:

Nerol can be extracted from some plants.

Crushed plant leaves containing nerol are mixed with an organic solvent called octane.

Nerol dissolves in octane.

Describe how you would separate the crushed plant leaves from the solution of nerol in octane.

The boiling point of nerol is 224 °C.

The boiling point of octane is 126 °C.

Explain how distillation separates nerol from the octane.

The mixture of coloured compounds in plant leaves can be separated by chromatography.

The apparatus is shown.

 On the diagram:

●   draw an ‘X’ to show where the mixture of coloured compounds is placed at the start of the experiment

●   draw a line to show the level of the solvent at the start of the experiment.

In 1985 the fullerenes were discovered. They are solid forms of the element carbon. The structure of the C₆₀ fullerene is given below.

i) In the C₆₀ fullerene, how many other carbon atoms is each carbon atom bonded to?

[1]

ii) Another fullerene has a relative molecular mass of 840.

How many carbon atoms are there in one molecule of this fullerene?

[1]

Fullerenes are soluble in liquid hydrocarbons such as octane. The other solid forms of carbon are insoluble.

Describe how you could obtain crystals of fullerenes from soot which is a mixture of fullerenes and other solid forms of carbon.

A mixture of a fullerene and potassium is an excellent conductor of electricity.

i) Which other form of solid carbon is a good conductor of electricity?

[1]

ii) Explain why metals, such as potassium, are good conductors of electricity.

[2]

iii) The mixture of fullerene and potassium has to be stored out of contact with air. There are substances in unpolluted air which will react with potassium.

Name two potassium compounds which could be formed when potassium is exposed to air.

[2]

A student is told to produce the maximum amount of copper from a mixture of copper and copper(II) carbonate.

The student adds the mixture to an excess of dilute sulfuric acid in a beaker and stirs the mixture with a glass rod. The copper(II) carbonate reacts with the sulfuric acid, forming a solution of copper(II) sulfate but the copper does not react with the sulfuric acid.

The student then

• removes the unreacted copper from the mixture,

• converts the solution of copper(II) sulfate into copper by a series of reactions.

Describe two things that the student would observe when the mixture is added to the dilute sulfuric acid.

Describe how the student can produce pure dry copper from the mixture of copper and copper(II) sulfate solution.

The student then adds sodium hydroxide solution to the copper(II) sulfate solution to produce copper(II) hydroxide. 

i) Describe what the student would observe.

[1]

ii) Write an ionic equation for this reaction.

[1]

After separating the copper(II) hydroxide from the mixture, the copper(II) hydroxide is heated strongly. The copper(II) hydroxide decomposes into copper(II) oxide and steam.

i) Write an equation for the decomposition of copper(II) hydroxide. Include state symbols.

[2]

ii) Name a non-metallic element that can be used to convert copper(II) oxide into copper.

[1]

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

A sample of ink, K, is known to contain three different coloured substances.

Evaluate the use of the following methods to separate the coloured substances in the sample of ink, K.

• chromatography

• crystalisation

• filtration

• fractional distillation

• simple distillation

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

 i) Cold water flows through the condenser. 

 Add labelled arrows to the diagram to show where the water should flow in and out of the apparatus

[1]

ii) Explain why a condenser is used.

[1]

iii) The flask was heated with a Bunsen burner. 

Name an alternative piece of apparatus that could be used to heat the flask. 

[1]

The arrangement of the particles in the ink inside the flask is shown.

 In the boxes below, draw the arrangement of particles that would be expected at A and B shown in part (b).