ANNUAL EXAMINATIONS
Name :
Class :
Class :
Grade 10
Subject :
CHEMISTRY
Date :
18th June 2015
Time :
2 hours
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Instructions
Write your name and class on the exam paper.
This paper is divided into Three sections : Section A , Section B and Section C.
Answer ALL questions in Section A in the space provided.
Answer the question from SectionB.
Answer ONE question from Section C.
Foolscaps are to be attached with this booklet.
Periodic Table will be provided.
This paper consists of eighteen printed pages.
SectionA: Answer all questions in the space provided
Question 1
1(a) Mars is often called the red planet due to the presence of haematite on its surface.
A recent study of the Huygens Crater on Mars has also shown the presence of iron(III)
hydroxide and calcium carbonate.
(i) Calcium carbonate and iron(III) hydroxide undergo thermal decomposition. What is
meant by the term thermal decomposition? [1]
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(ii) Write balanced chemical equations for the thermal decomposition of iron(III)
hydroxide and of calcium carbonate. [4]
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(b) Atmosphere is the term used to describe the collection of gases that surround a
planet. The composition of the atmosphere of Mars is shown in the table below.
Gas
Composition
Carbon dioxide
95.0%
Nitrogen
3.0%
Noble gases
1.6%
Oxygen
Trace
Methane
Trace
1|Page
Compare the composition of the Earth’s atmosphere today, with that of the planet Mars.
[3]
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(c) A spacecraft landed on Mars in 2008. Probes on the spacecraft analysed some soil.
The probes found a compound in the soil that scientists named compound X.
Compound X had the following percentage composition by mass:
10.8% magnesium, 31.8% chlorine and 57.4% oxygen.
Calculate the empirical formula of compound X.
You must show all of your working to get full marks. [3]
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TOTAL 11 marks
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Question 2
2 (a) In an experiment a sample of human saliva was removed from the mouth every
five minutes after a meal and the pH values were determined. The graph below shows
how the pH values of the saliva changed.
(i) How were the pH values of the saliva determined in this experiment? [1]
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(ii) A sample of saliva was tested with phenolphthalein indicator 30 minutes after the
meal. State the colour observed when the saliva was tested with phenolphthalein. [1]
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(iii) When the pH in the mouth is 5.5 or less tooth decay occurs. Use the graph to find
the time after the meal at which teeth would start to decay. [1]
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(b) In the stomach food is digested by the action of hydrochloric acid. Hydrochloric acid
has a pH value between 0 and 2. Excess stomach acid can cause a burning sensation
called indigestion which is treated using antacid tablets to neutralise some of the acid.
3|Page
(i) Explain why hydrochloric acid is a strong acid. [2]
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(ii) Antacid tablets are often prescribed by doctors in case of acid indigestion. Which of
these chemicals is likely to be found as an ingredient in antacid tablets? Explain your
answer including any relevant chemical equation/s.
Magnesium hydroxide, sodium hydroxide, or potassium chloride [4]
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(iii) How does the pH in the stomach change after taking these tablets? [2]
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(c) The digested food from the stomach passes out of the stomach to the intestine where
it is absorbed. Sodium chloride is a salt absorbed across the intestine’s wall. Some tests
were carried out on a sample of sodium chloride to confirm the identity of the cation
and anion present in it. Describe the tests which could be carried out to confirm the
identity of the cation and anion in a sample of sodium chloride. Describe the
observations that will be expected.
4|Page
Test for cation:
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Expected observation/s:
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Test for anion:
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Expected observation/s:
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(d) Hydrochloric acid reacts with calcium hydroxide solution and with solid calcium.
Compare and contrast the reaction of hydrochloric acid and calcium hydroxide
solution with the reaction of hydrochloric acid and solid calcium. In your answer you
must include
● the observations for each reaction
● a balanced chemical equation
In this question you will be assessed on your written communication skills
including the use of specialist scientific terms. [8]
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TOTAL 27 marks
Question 3
3. When hydrochloric acid reacts with sodium thiosulphate, the solution turns
cloudy because of the formation of sulphur. After a time you can no longer see
through it. Rebecca did some experiments using this reaction and timed how long it
took for a cross under the flask to be no longer visible. Her results are in the table
below.
Experiment
Temperature (C)
Volume of sodium thiosulphate
Volume of hydrochloric acid
Volume of water (cm3)
3)
Time
for cross to (cm
disappear
(s)
thththiosulphate
3
(cm )
A
20
50
5
5
80
a) Which experiment was the fastest?
B
20
50
10
0
40
C
20
25
5
30
160
D
30
50
10
0
20
E
40
50
10
0
10
(1)
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b) i) Which experiments can be used to see the effect of changing the
concentration of the sodium thiosulphate?
(1)
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ii) What is its effect?
(1)
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c) i) Which experiments can be used to see the effect of changing the
concentration of the hydrochloric acid?
(1)
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ii) What is its effect?
(1)
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d) i) Which experiments can be used to see the effect of changing the
temperature?
(1)
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6|Page
ii) What is its effect?
(1)
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e) Why was water added in experiments A and C?
(1)
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TOTAL 8 marks
Question 4
4. (a)THREE metals labelled X, Y and Z need to be identified based on some
characteristic physical and chemical properties.
The possible metals are:
iron, magnesium, sodium, copper, zinc
Identify metals X, Y and Z and justify your answer using the reactions described in the
table opposite. Include the products of any reactions in your answer and give balanced
chemical reactions.
7|Page
Metal X is: ______________________________
Because:
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Metal Y is: _________________________________________
Because:
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Metal Z is: _________________________________________
Because:
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[15]
TOTAL 15 marks
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Question 5
5 (a) Copper can be converted into copper(II) nitate .
Describe how you would carry out this conversion ( HINT: This conversion may require
more than one step). A pure dry crystalline of sample of copper(II) nitrate is required.
Explain the need for each step and include any relevant chemical equations. (13)
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TOTAL 13 marks
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Question 6
6a. Which of the following types of chemical reactions can also be classified as redox
reactions?
i. neutralisation
ii. double displacement
iii. synthesis
iv. single displacement
v. decomposition
b. Write half equations to show what is being reduced and what is being oxidised in the
following reactions.
a. reaction between lead and silver nitrate solution
half equation representing reduction: _________________________________________________________
half equation representing oxidation: _________________________________________________________
b. the synthesis of magnesium iodide
half equation representing reduction: _________________________________________________________
half equation representing oxidation: _________________________________________________________
Total 10 marks
10 | P a g e
Question 7
a. Explain why molten ionic compounds conduct electricity but solid ionic compounds
do not. [2]
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b. Draw a labelled diagram of a cell which can be used to electrolyse molten
lead(II)bromide. [3]
c. Draw in the above diagram the direction in which anions and cations move during
electrolysis. [2]
d. State in which direction electrons move in the wires joined to the electrodes in
electrolysis. [2]
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e. State the ions present , name the products , give the electrodes reactions and any
changes to the electrolyte in the electrolysis of the following compounds:
Ions
Products
present
Reaction at
Reaction at
Change to
cathode
anode
electrolyte if
any
Molten
At cathode:
sodium
chloride
At anode:
using inert
electrodes.
Concentrated
At cathode:
aqueous
sodium
chloride,
At anode:
using inert
electrodes
Dilute
At cathode:
sulphuric acid
using inert
At anode:
electrode
Aqueous
At cathode:
copper(II)
sulphate
using carbon
At anode:
electrodes
Aqueous
At cathode:
copper (II)
sulphate
using copper
At anode:
electrode
[16]
12 | P a g e
f. A current of 2 Amps flowed through molten lead(II)bromide for 30 minutes. Calculate
the mass of the product that will be cvollected at the cathode.
INFO: 1Faraday = 965000C
[5]
TOTAL 30 marks
PLEASE TURN OVER FOR SECTION B
13 | P a g e
SECTION B:
Answer this question.
1. The following are the results of experiments carried out in order to determine the
identity of various inorganic solids.
A is a green powder which on heating turns into a black solid B and a gas C which turns
limewater milky. When dilute nitric acid is added to B a blue solution forms. If sodium
hydroxide solution is added to this aqueous blue solution, a blue gelatinous precipitate
D is formed which is insoluble in excess sodium hydroxide.
E is a white solid that sublimes on heating. When sodium hydroxide is added to an
aqueous solution of E and the mixture is heated, gas F is given off which turns red litmus
blue. When dilute nitric acid is added to the aqueous solution of E, followed by silver
nitrate solution, a white precipitate G is formed.
Substance H is a white crystalline solid which decomposes on heating to give a brown
gas I and a colourless gas J and a solid which was red when hot but yellow when cold.
An aqueous solution of H reacts with sodium hydroxide to give white precipitate which
dissolves in excess sodium hydroxide. A solution of H also forms a yellow precipitate on
addition of potassium iodide.
When sodium hydroxide solution is added to an aqueous solution of K, a green
gelatinous precipitate L is formed which is insoluble in excess sodium hydroxide
solution but on standing, turns brown on the surface. When barium nitrate was added
to solution K, a white precipitate M was formed. M dissolved in nitric acid and a gas N
which turned acidified potassium dichromate from orange to green was released
(a) Give the chemical formula of each of the substances A-N and explain the
observations made.(HINT: a table would be the best method to explain the
observations) (14)
(b) Give a balanced equation for
(i) the reaction that occurs when E is warmed with NaOH (2)
(ii) the decomposition of H on heating (2)
(iii) the reaction of M and nitric acid solution solution (2)
TOTAL 20 x1.5=30 marks
14 | P a g e
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PLEASE TURN OVER FOR SECTION C
15 | P a g e
Section C: Answer any ONE question.
1. Explain the chemical concepts behind the following statements as fully as possible.
Give a chemical equation where appropriate.
i) Aluminium is high in the reactivity series and yet it is used to make items such as
cooking pans, which are often used with substances such as dilute acids and salt, which
often cause corrosion of metals. (4)
ii) Calcium chloride can be prepared in the lab by reacting calcium carbonate with
dilutehydrochloric acid but calcium sulfate cannot be prepared by reacting calcium
carbonate with dilute sulfuric acid. (6)
iii) Iron ore is reduced in the blast furnace. (4)
iv) Sulfuric acid is a strong acid but ethanoic acid is a weak acid. (3)
v) Powdered magnesium is used in fireworks rather than magnesium ribbon. (3)
OR
2. Explain how air pollution can cause:
(a) the greenhouse effect
(b) the thinning of the ozone layer
(c) acid rain.
Your answer should include:
•
•
•
•
•
which gases are responsible,
sources,
how the gases cause pollution
the effects and
ways how to reduce pollution.
TOTAL 20 marks
16 | P a g e
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18 | P a g e