Holiday homework for X IGCSE/O level-0620

1 The following techniques are used to separate mixtures.
A simple distillation B fractional distillation C evaporation
D chromatography E fi ltration F diffusion
From this list, choose the most suitable technique to separate the following.
(a) methane from a mixture of the gases, methane and ethane .................. [1]
(b) water from aqueous magnesium sulfate .................. [1]
(c) glycine from a mixture of the amino acids, glycine and lysine ................... [1]
(d) iron fi lings from a mixture of iron fi lings and water .................. [1]
(e) zinc sulfate crystals from aqueous zinc sulfate .................. [1]
(f) hexane from a mixture of the liquids, hexane and octane .................. [1]
[Total: 6]

2 Selenium and sulfur are in Group VI. They have similar properties.
(a) One of the main uses of selenium is in photoelectric cells. These cells can change light
into electrical energy.
(i) Name a process which can change light into chemical energy.
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(ii) Name a device which can change chemical energy into electrical energy.
.............................................................................................................................. [2]
(b) The electron distribution of a selenium atom is 2 + 8 + 18 + 6.
(i) Selenium forms an ionic compound with potassium. Draw a diagram which shows
the formula of this ionic compound, the charges on the ions and the arrangement of
the valency electrons around the negative ion.
Use o to represent an electron from an atom of potassium.
Use x to represent an electron from an atom of selenium.

(ii) Draw a diagram showing the arrangement of the valency electrons in one molecule
of the covalent compound selenium chloride.





Use x to represent an electron from an atom of selenium.
Use o to represent an electron from an atom of chlorine.                          (3)

(iii) Predict two differences in the physical properties of these two compounds.
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.............................................................................................................................. [2]
(c) The selenide ion reacts with water.
Se2– + H2O-->HSe– + OH–

What type of reagent is the selenide ion in this reaction? Give a reason for your choice.
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.................................................................................................................................... [3]
[Total: 13]

3 Iron from the blast furnace is impure. It contains about 4 % carbon and 0.5 % silicon. Most
of this impure iron is used to make mild steel, an alloy of iron containing less then 0.25 %
carbon.

(a) A jet of oxygen is blown through the molten iron in the presence of a base, usually
calcium oxide. Explain how the percentage of carbon is reduced and how the silicon is
removed.
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.................................................................................................................................... [4]

(b) (i) Why are steel alloys used in preference to iron?
.............................................................................................................................. [1]
(ii) State a use of the following alloys.
mild steel ...................................................................................................................
stainless steel ...................................................................................................... [2]

(c) Both iron and steel have typical metallic structures - a lattice of positive ions and a sea
of electrons.
(i) Suggest an explanation for why they have high melting points.
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.............................................................................................................................. [2]

(ii) Explain why, when a force is applied to a piece of steel, it does not break but just
changes its shape.
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.............................................................................................................................. [2]
[Total: 11]

4 A major ore of zinc is zinc blende, ZnS. A by-product of the extraction of zinc from this ore is
sulfur dioxide which is used to make sulfuric acid.
(a) (i) Zinc blende is heated in air. Zinc oxide and sulfur dioxide are formed. Write the
balanced equation for this reaction.
.............................................................................................................................. [2]
(ii) Zinc oxide is reduced to zinc by heating with carbon. Name two other reagents
which could reduce zinc oxide.
.............................................................................................................................. [2]

(iii) The zinc obtained is impure. It is a mixture of metals. Explain how fractional distillation
could separate this mixture.
zinc bp = 908 °C, cadmium bp = 765 °C, lead bp = 1751 °C

...................................................................................................................................
.............................................................................................................................. [2]
(b) Sulfur dioxide is used to make sulfur trioxide in the Contact Process.
2SO2(g) + O2(g)-->2SO3(g)
The forward reaction is exothermic. The conditions used are:
temperature:450 °C
pressure:2 atmospheres
catalyst:vanadium(V) oxide











Explain, mentioning both position of equilibrium and rate, why these conditions give the
most economic yield.

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.................................................................................................................................... [4]
[Total: 10]
to be continued..