Topic 4
Unit 14
Suggested answers to in-text activities and unit-end exercises
In-text activities
Checkpoint (page 10)
1 a) • Gas bubbles were gives off.
• A pale green solution formed.
b) Fe(s) + H2SO4(aq)
+
c) Fe(s) + 2H (aq)
FeSO4(aq) + H2(g)
Fe2+(aq) + H2(g)
Unit 14
Topic 4
2 a) • Effervescence occurred.
• A blue solution formed.
b) CuCO3(s) + 2HNO3(aq)
c) CuCO3(s) + 2H+(aq)
Cu(NO3)2(aq) + H2O(l) + CO2(g)
Cu2+(aq) + H2O(l) + CO2(g)
Checkpoint (page 13)
When dissolved in water, citric acid gives hydrogen ions which react with hydrogencarbonate ions to give carbon
dioxide gas.
H+(aq) + HCO3–(aq)
H2O(l) + CO2(g)
Checkpoint (page 24)
1 a)
i) Iron(II) hydroxide
ii) Green
iii) Fe2+(aq) + 2OH–(aq)
b) i) Copper(II) hydroxide
ii) Pale blue
iii) Cu2+(aq) + 2OH–(aq)
Fe(OH)2(s)
Cu(OH)2(s)
2 a) X — calcium Y — sodium
b) i) Carbon dioxide
ii) Calcium carbonate
iii) CaCO3(s) + 2HCl(aq)
c) Y was sodium chloride.
CaCl2(aq) + H2O(l) + CO2(g)
It gave a white precipitate with silver nitrate solution. It was a chloride.
Problem Solving (page 25)
• First add sodium carbonate solid to the reagents. Effervescence occurs for dilute sulphuric acid.
• Then add copper(II) chloride solution to the remaining three reagents. Both dilute aqueous ammonia and
dilute sodium hydroxide solution give a pale blue precipitate. However, the pale blue precipitate dissolves in
excess dilute aqueous ammonia.
• The remaining reagent is distilled water.
Topic 4
Checkpoint (page 28)
1 a) Insoluble calcium sulphate forms when calcium carbonate reacts with dilute sulphuric acid. The calcium
sulphate covers the surface of calcium carbonate and prevents further reaction.
Unit 14
b) Add calcium carbonate to dilute hydrochloric acid / dilute nitric acid.
2 a) Sodium hydroxide is very corrosive.
b) Use magnesium hydroxide / aluminium hydroxide as the active ingredient.
3 a) When water is added to concentrated sulphuric acid, a lot of heat is produced. The heat can cause the
acid droplets to spurt out from the container.
b) Add concentrated sulphuric acid to a large amount of water while stirring.
Internet Search & Presentation (page 28)
Hazard warning symbol
Concentrated
hydrochloric acid
Risk descriptions
• Causes burns
• Irritating to respiratory system
Safety precautions
• In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
• In case of accident and if you feel unwell, seek medical advice immediately.
Hazard warning symbols
Concentrated
nitric acid
Risk descriptions
• Causes severe burns
• Contact with combustible material may cause fire
Safety precautions
• Do not inhale fumes / vapour / spray.
• In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
• In case of accident and if you feel unwell, seek medical advice immediately.
Hazard warning symbol
Concentrated
sulphuric acid
Topic 4
Risk descriptions
• Reacts violently with water
• Causes severe burns
• Irritating to respiratory system
Safety precautions
• Never add water to concentrated sulphuric acid.
• In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
• In case of accident and if you feel unwell, seek medical advice immediately.
Unit 14
Hazard warning symbol
Concentrated sodium
hydroxide solution
Risk description
• Causes severe burns
Safety precautions
• Wear protective gloves and eye / face protection.
• In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
• In case of accident and if you feel unwell, seek medical advice immediately.
Hazard warning symbol
Concentrated potassium
hydroxide solution
Risk descriptions
• Harmful if swallowed
• Causes severe burns
Safety precautions
• Wear protective gloves and eye / face protection.
• In case of contact with eyes, rinse immediately with plenty of water and seek
medical advice.
• In case of accident and if you feel unwell, seek medical advice immediately.
Unit-end exercises (pages 34 – 41)
1
gives 1 hydrogen
hydrochloric
acid
sulphuric acid
ion per molecule
commonly
used
acid
in the
laboratory
gives 2 hydrogen
in water
ions per molecule
Topic 4
gives 3 hydrogen
nitric acid
ions per molecule
properties
Unit 14
has a
when
dissolved
when litmus
solution is
added to it, a
reacts with
reactive
metals to
give
reacts with
carbonates /
hydrogencarbonates
to give
sour
red
hydrogen
carbon dioxide
taste
colour
appears
gas
gas
dibasic acid
tribasic acid
reacts with
hydroxides /
oxides of metals
to give
salt
and
water
monobasic
acid
conducts
electricity
to make a light
bulb glow
2
base
potassium
hydroxide solution
soluble
in water
sodium hydroxide
solution
commonly used in
alkali
the laboratory
calcium hydroxide
solution
has a
when litmus
solution is
added to it,
a
bitter
slippery
blue
taste
feel
colour
appears
reacts with
some metal
ions to give
insoluble
metal
hydroxide
precipitates
reacts with
ammonium
compounds
to give
ammonia
reacts with
an acid to
give
salt
gas
and
Unit 14
has a
Topic 4
aqueous
ammonia
properties
conducts
electricity
to make a light
bulb glow
water
3
Colour in
Indicator
4 a)
dilute hydrochloric acid
dilute sodium hydroxide solution
Litmus solution
red
blue
Methyl orange
red
yellow
Phenolphthalein
colourless
red
Adding
NaOH(aq) to
solution
containing
Colour of
precipitate
formed
Precipitate
dissolves in
excess NaOH(aq)?
(Yes or No)
Ca2+(aq)
white
No
Ca2+(aq) + 2OH–(aq)
Mg2+(aq)
white
No
Mg2+(aq) + 2OH–(aq)
Mg(OH)2(s)
Al3+(aq)
white
Yes
Al3+(aq) + 3OH–(aq)
Al(OH)3(s) + OH–(aq)
Al(OH)3(s)
[Al(OH)4]–(aq)
Ionic equation(s)
Ca(OH)2(s)
Continued on next page
Topic 4
Unit 14
b)
5
Adding
NaOH(aq) to
solution
containing
Colour of
precipitate
formed
Precipitate
dissolves in
excess NaOH(aq)?
(Yes or No)
Pb2+(aq)
white
Yes
Pb2+(aq) + 2OH–(aq)
Pb(OH)2(s) + 2OH–(aq)
Pb(OH)2(s)
[Pb(OH)4]2–(aq)
Zn2+(aq)
white
Yes
Zn2+(aq) + 2OH–(aq)
Zn(OH)2(s) + 2OH–(aq)
Zn(OH)2(s)
[Zn(OH)4]2–(aq)
Fe2+(aq)
green
No
Fe2+(aq) + 2OH–(aq)
Fe(OH)2(s)
Fe3+(aq)
reddish
brown
No
Fe3+(aq) + 3OH–(aq)
Fe(OH)3(s)
Cu2+(aq)
pale blue
No
Cu2+(aq) + 2OH–(aq)
Cu(OH)2(s)
Adding
NH3(aq) to
solution
containing
Colour of
precipitate
formed
Precipitate
dissolves in
excess NH3(aq)?
(Yes or No)
Mg2+(aq)
white
No
Mg2+(aq) + 2OH–(aq)
Al3+(aq)
white
No
Al3+(aq) + 3OH–(aq)
Al(OH)3(s)
Pb2+(aq)
white
No
Pb2+(aq) + 2OH–(aq)
Pb(OH)2(s)
Zn2+(aq)
white
Yes
Zn2+(aq) + 2OH–(aq)
Zn(OH)2(s) + 4NH3(aq)
Zn(OH)2(s)
[Zn(NH3)4]2+(aq) + 2OH–(aq)
Fe2+(aq)
green
No
Fe2+(aq) + 2OH–(aq)
Fe(OH)2(s)
Fe3+(aq)
reddish
brown
No
Fe3+(aq) + 3OH–(aq)
Fe(OH)3(s)
Cu2+(aq)
pale blue
Yes
Cu2+(aq) + 2OH–(aq)
Cu(OH)2(s) + 4NH3(aq)
Cu(OH)2(s)
[Cu(NH3)4]2+(aq) + 2OH–(aq)
Chemical
Concentrated hydrochloric acid
Concentrated nitric acid
Ionic equation(s)
Ionic equation(s)
Mg(OH)2(s)
Hazard warning label(s)
B
B, D
Concentrated sulphuric acid
B
Dilute hydrochloric acid
C
Dilute sulphuric acid
C
Dilute sodium hydroxide solution
B or C
6 C Option A — Sulphuric acid is used in car batteries.
Option B — Hydrochloric acid is produced in human stomach.
Option D — Concentrated hydrochloric acid is not an oxidizing agent. Therefore the following
hazard warning label should NOT be displayed on its bottle.
Option D — The following hazard warning labels should be displayed on a bottle of concentrated nitric
acid.
Unit 14
Topic 4
7 C Option A — Nitric acid is used to make fertilizers, but it is NOT used as a fertilizer.
8 C
9 D Drain cleansers usually contain sodium hydroxide.
10 B
Option
Solution
Observation upon the
addition of dilute sodium
hydroxide solution
A
ammonium
sulphate
no precipitate
B
iron(III) sulphate
brown precipitate
C
lead(II) nitrate
white precipitate
D
magnesium
chloride
white precipitate
11 A
12 A
13 A (3) Glass cleansers usually contain ammonia.
14 C
15 D (1) Heating ammonium chloride solution with dilute sodium hydroxide solution liberates ammonia gas.
NH4Cl(aq) + NaOH(aq)
NH3(g) + NaCl(aq) + H2O(l)
(2) When mixed with dilute sodium hydroxide solution, zinc sulphate solution gives a white precipitate
which dissolves in excess alkali (due to the formation of a soluble complex salt).
Zn2+(aq) + 2OH–(aq)
Zn(OH)2(s) + 2OH–(aq)
(3) Carbonic acid and dilute sodium hydroxide solution undergo a neutralization reaction.
Zn(OH)2(s)
[Zn(OH)4]2–(aq)
16 a) Solid citric acid has no effect on magnesium because it does not contain hydrogen ions.
When citric acid dissolves in water, the molecules dissociate to give hydrogen ions. These hydrogen
ions are responsible for all the acidic properties.
b) When dilute sulphuric acid reacts with calcium carbonate, insoluble calcium sulphate forms.
c) Ammonia gas contains no hydroxide ions.
The calcium sulphate covers the surface of calcium carbonate and prevents further reaction.
However, it reacts with water to give an alkaline solution.
Topic 4
17 a) Gas bubbles are given off. / The zinc granules dissolve in the acid.
Zn(s) + H2SO4(aq)
Unit 14
+
or Zn(s) + 2H (aq)
b) Gas bubbles are given off. / The magnesium ribbon dissolves.
Mg(s) + 2HCl(aq)
+
or Mg(s) + 2H (aq)
c) Effervescence occurs. / Solid copper(II) carbonate dissolves in the acid. / A blue solution forms.
CuCO3(s) + 2HNO3(aq)
Cu(NO3)2(aq) + H2O(l) + CO2(g) +
or CuCO3(s) + 2H (aq)
Cu2+(aq) + H2O(l) + CO2(g)
d) Effervescence occurs. / Solid sodium hydrogencarbonate dissolves in the acid.
NaHCO3(s) + HCl(aq)
+
or NaHCO3(s) + H (aq)
ZnSO4(aq) + H2(g) Zn2+(aq) + H2(g)
MgCl2(aq) + H2(g) Mg2+(aq) + H2(g)
NaCl(aq) + H2O(l) + CO2(g)
Na+(aq) + H2O(l) + CO2(g)
18 a) A blue precipitate forms;
the precipitate dissolves in excess dilute aqueous ammonia to give a deep blue solution.
Cu2+(aq) + 2OH–(aq)
Cu(OH)2(s) + 4NH3(aq)
b) A white precipitate forms.
c) A white precipitate forms;
the precipitate dissolves in excess dilute sodium hydroxide solution to give a colourless solution.
Zn2+(aq) + 2OH–(aq)
Zn(OH)2(s) + 2OH–(aq)
d) A green precipitate forms.
Al3+(aq) + 3OH–(aq)
Fe2+(aq) + 2OH–(aq)
Cu(OH)2(s)
[Cu(NH3)4]2+(aq) + 2OH–(aq)
Al(OH)3(s)
Zn(OH)2(s)
[Zn(OH4)]2–(aq)
Fe(OH)2(s)
e) Ammonia gas is given off.
2NH4Cl(aq) + Ca(OH)2(aq)
2NH3(g) + CaCl2(aq) + 2H2O(l)
19 a) The chicken foot becomes smaller. It turns somewhat ‘translucent’.
b) The chicken foot breaks up.
c) This shows the corrosive nature of concentrated sodium hydroxide solution.
d) Wash the affected area with plenty of water immediately.
20 a) Reagent in bottle B or D — dilute hydrochloric acid or dilute sulphuric acid
+
Mg(s) + 2H (aq)
The colourless gas which burns with a ‘pop’ sound is hydrogen.
b) Reagent in bottle B — dilute hydrochloric acid
c) Reagent in bottle A or C — dilute sodium hydroxide solution or calcium hydroxide solution
d) Reagent in bottle C — calcium hydroxide solution
Ca(OH)2(aq) + CO2(g)
The white precipitate is insoluble calcium carbonate.
K2CO3(s) + 2HCl(aq)
Cu2+(aq) + 2OH–(aq)
Mg2+(aq) + H2(g)
Unit 14
Topic 4
2KCl(aq) + H2O(l) + CO2(g)
Cu(OH)2(s)
CaCO3(s) + H2O(l)
21 a) A — Iron
B — Iron(II,III) oxide
C — Iron(II) sulphate solution
D — Hydrogen
E — Iron(II) hydroxide
b) i) Fe(s) + H2SO4(aq)
ii) Fe2+(aq) + 2OH–(aq)
FeSO4(aq) + H2(g)
Fe(OH)2(s)
22 Carry out flame test on each solid.
Both sodium carbonate and sodium nitrate give a golden yellow flame. The two ammonium compounds do
not give a characteristic flame colour.
Distinguish the two sodium compounds by adding dilute hydrochloric acid to each separately.
Sodium carbonate gives gas bubbles while there is no observable change for sodium nitrate.
Dissolve the two ammonium compounds in water. Take a small amount of each solution, add an equal
amount of dilute nitric acid and a few drops of silver nitrate solution to each separately.
The solution of ammonium chloride gives a white precipitate while there is no observable change for the
solution of ammonium nitrate.