Common Mistakes in Chemistry

1
Common Mistakes in Chemistry
2

1. Read the question (5 - 10 s)
2. Think (10 s)
3. Pair-up and discuss your answer (10 15 s)
4. One group will be invited to share the answer
   with whole class (10 15 s)

3
Whats wrong with the following statement?
Oxygen relights a growing splint.
4
Correct statement
Oxygen relights a glowing splint.
5
Whats wrong with the following statement?
Nitrogen dioxide is an acidic oxide. It dissolves
in water to form nitric acid.
6
Correct statement
Nitrogen dioxide is an acidic oxide. It dissolves
in water to form nitric acid and nitrous acid.
2NO2 H2O ?? HNO3 HNO2
7
Whats wrong with the following statement?
Ethanol turns green acidified potassium
dichromate solution to orange.
8
Correct statement
Ethanol turns orange acidified potassium
dichromate solution to green.
9
Whats wrong with the following statement?
Ammonia gas turns red litmus paper blue.
10
Correct statement
Ammonia gas turns moist red litmus paper blue.
11
Whats wrong with the following statement?
Water vapour turns cobalt chloride to pink.
12
Correct statement
Water vapour turns dry/ anhydrous/ blue cobalt
chloride to pink.
13
Whats wrong with the following statement?
Nitrogen is an inert gas.
14
Correct statement
Nitrogen is an inactive gas. N2(g) 3H2(g)
2NH3(g) Nitrogen acts as oxidising
agent. N2(g) O2(g) ?? 2NO(g) Nitrogen acts
as reducing agent.
15
Whats wrong with the following statement?
Chlorine gas turns moist blue litmus paper red.
16
Correct statement
Chlorine gas turns moist blue litmus paper red
and then white.
17
Whats wrong with the following statement?
Sulphur dioxide turns potassium dichromate from
orange to green.
18
Correct statement
Sulphur dioxide turns acidified potassium
dichromate from orange to green.
19
Whats wrong with the following statement?
Ethene turns acidified potassium dichromate from
orange to green.
20
Correct statement
Ethene turns acidified potassium permanganate
from purple to colourless.
21
Whats wrong with the following statement?
Carbon dioxide turns limewater milky. The white
precipitate is not soluble in excess carbon
dioxide.
22
Correct statement
Carbon dioxide turns limewater milky. Ca(OH)2(aq)
CO2(aq) ?? CaCO3(s) H2O(l) Excess carbon
dioxide turns the milky solution
colourless. CaCO3(s) CO2(aq) H2O(l) ??
Ca(HCO3)2(aq)
23
Whats wrong with the following statement?
Hydrogen burns with a pop sound.
24
Correct statement
Pure hydrogen burns with a blue flame. A mixture
of hydrogen and air/ oxygen burns with a pop
sound. We can use a burning splint to test for
hydrogen gas collected in a test tube because
hydrogen mixes up with air when we remove the
stopper of the test tube.
25
Whats wrong with the following statement?
Seperating funnel is used to seperate two
immiscible liquids.
26
Correct statement
Separating funnel is used to separate two
immiscible liquids.
27
Whats wrong with the following statement?
We use mortar to grind the calcium carbonate into
powder.
28
Whats wrong with the following statement?
We use mortar and pestle to grind the calcium
carbonate into powder.
29
Whats wrong with the following statement?
We use spoon to transfer chemicals from reagent
bottle to the test-tube.
30
Whats wrong with the following statement?
We use spatula to transfer chemicals from reagent
bottle to the test-tube.
31
Whats wrong with the following statement?
Tripod stand, wire gauze, Bunsen burner, crucible
heat-proof mat are used to convert magnesium to
magnesium oxide.
32
Whats wrong with the following statement?
Tripod stand, pipe-clay triangle, Bunsen burner,
crucible heat-proof mat are used to convert
magnesium to magnesium oxide.
33
Whats wrong with the following statement?
pH of 1M hydrochloric acid is higher than that of
1M ethanoic acid.
34
Correct statement
pH of 1M hydrochloric acid is lower than that of
1M ethanoic acid.
35
Whats wrong with the following statement?
The oxidation number of silver in Ag2O is 1.
36
Correct statement
The oxidation number of silver in Ag2O is 1.
37
Whats wrong with the following statement?
Acid is a hydrogen-containing compound which
dissolves in water to give H(aq).
38
Correct statement
Acid is a hydrogen-containing compound which
dissolves in water to give H(aq) as the only
cation.
39
The following answer is imprecise, amend it.
Base is a substance which reacts with acid to
give salt and water only. Base comprises metal
oxide and metal hydroxide.
40
Precise answer
Base is a substance which reacts with acid to
give salt and water only. Base comprises metal
oxide, metal hydroxide and ammonia.
41
The following answer is imprecise, amend it.
Salt is a compound derived from acid. The
hydrogen ion in acid is replaced by metal ion.
42
Precise answer
Salt is a compound derived from acid. The
hydrogen ion in acid is replaced by metal ion or
ammonium ion.
43
Whats wrong with the following statement?
In the electrolysis of dilute sodium chloride
solution, hydrogen ion discharges instead of
sodium ion because hydrogen ion is above sodium
ion in the electrochemical series.
44
Correct statement
In the electrolysis of dilute sodium chloride
solution, hydrogen ion discharges instead of
sodium ion because hydrogen ion is below sodium
ion in the electrochemical series.
45
Whats wrong with the following statement?
In the electrolysis of dilute sodium chloride
solution, hydroxide ion discharges instead of
chloride ion because hydroxide ion is below
chloride ion in the electrochemical series.
46
Correct statement
In the electrolysis of dilute sodium chloride
solution, hydroxide ion discharges instead of
chloride ion because hydroxide ion is above
chloride ion in the electrochemical series.
47
Whats wrong with the following structure?
48
Correct structure
49
Whats wrong with the following statement?
The functional group of ethanol is HO.
50
Correct statement
The functional group of ethanol is OH.
51
Whats wrong with the following structure?
I
II
52
Correct structure
53
Whats wrong with the following statement?
Helium has an octet structure.
54
Correct statement
Helium has a duplet structure.
55
Whats wrong with the following statement?
Na is a stronger oxidising agent than Mg because
Na loses electron more readily than Mg.
56
Correct statement
Na is a stronger reducing agent than Mg because
Na loses electron more readily than Mg.
57
Whats wrong with the following statement?
During the electrolysis of dilute sulphuric acid,
hydrogen discharges at the cathode.
58
Correct statement
During the electrolysis of dilute sulphuric acid,
hydrogen ion discharges at the cathode.
59
Whats wrong with the following statement?
During the electrolysis of dilute zinc sulphate,
hydroxide ion discharges preferentially at the
anode because hydroxide ion is below sulphate ion
in the electrochemical series.
60
Correct statement
During the electrolysis of dilute zinc sulphate,
hydroxide ion discharges preferentially at the
anode because hydroxide ion is above sulphate ion
in the electrochemical series.
61
Whats wrong with the following statement?
All sulphates are soluble in water except calcium
sulphate.
62
Correct statement
All sulphates are soluble in water except calcium
sulphate, barium sulphate, strontium sulphate and
lead sulphate.
63
Whats wrong with the following statement?
All carbonates are insoluble in water except
sodium and potassium carbonate.
64
Correct statement
All carbonates are insoluble in water except
sodium, potassium and ammonium carbonate.
65
Whats wrong with the following statement?
Nylon is a biodegradable condensation polymer.
66
Correct statement
Nylon is a non-biodegradable condensation polymer.
67
Whats wrong with the following statement?
Urea methanol is a thermosetting polymer.
68
Correct statement
Urea methanal is a thermosetting polymer.
69
Whats wrong with the following statement?
Isotopes have similar chemical properties.
70
Correct statement
Isotopes have same chemical properties.
71
Whats wrong with the following statement?
Electrolyte is a compound which in aqueous
solution or when molten, conducts electricity.
72
Correct statement
Electrolyte is a compound which in aqueous
solution or when molten, conducts electricity and
is decomposed in the process.
73
Whats wrong with the following statement?
Members of a homologous series have the same
chemical properties but different physical
properties.
74
Correct statement
Members of a homologous series have similar
chemical properties but graduation in physical
properties.
75
Whats wrong with the following statement?
An advantage of using iron over PVC in making
water pipe is that iron is harder than PVC.
76
Correct statement
An advantage of using iron over PVC in making
water pipe is that iron has a higher tensile
strength than PVC.
77
Whats wrong with the following formula?

• NaSO4
• Zn(Cl2)
• KCr2O7
• (NH3)2SO4

78
Correct formula

• Na2SO4
• ZnCl2
• K2Cr2O7
• (NH4)2SO4

79
Whats wrong with the following structure?
repeating unit of poly(propene)
80
Correct structure
repeating unit of poly(propene)
81
Whats wrong with the following electronic
structure?
nitrogen
82
Correct electronic structure
Nitrogen octet structure for each nitrogen atom
83
Whats wrong with the following ionic equation?
Ca2(aq) Na2CO3(aq) ??
CaCO3(s) 2Na(aq)
84
Correct ionic equation
Ca2(aq) CO32-(aq) ?? CaCO3(s)
Na2CO3(aq) is an electrolyte.
85
Whats wrong with the following ionic equation?
CaCO3(aq) 2H(aq) ??
Ca2(aq) H2O(l) CO2(g)
86
Correct equation
CaCO3(s) 2H(aq) ??
Ca2(aq) H2O(l) CO2(g) Calcium carbonate is
insoluble in water and hence no mobile carbonate
ions are available for the reaction.
87
Whats wrong with the following equation?
CH3COOH(aq) ?? CH3COO-(aq) H(aq)
88
Correct equation
CH3COOH(aq) CH3COO-(aq) H(aq)
Ethanoic acid is a weak acid. Ionization is
incomplete.
89
Whats wrong with the following equation?
NH3(aq) H2O(l) ?? NH4(aq) -OH(aq)
90
Correct equation
NH3(aq) H2O(l) NH4(aq) -OH(aq)
Aqueous ammonia is a weak base. Ionization is
incomplete.
91
Whats wrong with the following statement?
Solid iodine is purple.
92
Correct statement
I2(s) is black I2 in KI is brown (iodine is
slightly soluble in water, but highly soluble in
KI solution.) I2 vapour is purple
93
Whats wrong with the following equation?
NH3(g) HCl(g) ?? NH4Cl(g)
94
Correct equation
NH3(g) HCl(g) ?? NH4Cl(s)
Ammonium chloride is an ionic compound. It is a
solid at room temperature and pressure.
95
Whats wrong with the following equation?
Cu(s) 2HCl(aq) ?? CuCl2(aq) H2(g)
96
Correct equation
Copper is below hydrogen in the electrochemical
series, it does not react with dilute
hydrochloric acid.
97
Whats wrong with the following statement?
Table sugar (cube sugar) undergoes dehydration.
conc. H2SO4 C6H12O6(s)
?? 6C(s) 6H2O(l)
98
Correct statement
The main constituent of cube sugar/ table sugar
is sucrose.

• conc. H2SO4
• C12H22O11(s) ?? 12C(s) 11H2O(l)

99
Whats wrong with the following diagram?
100
Correct diagram
101
The following answer is imprecise, amend it.
Ester can be formed by heating alkanoic acid with
an alkanol.
102
Precise answer is needed
Ester can be formed by heating alkanoic acid with
an alkanol under reflux in the presence of
concentrated sulphuric acid.
H2SO4 e.g.
CH3COOH C2H5OH CH3COOC2H5
H2O
103
The following answer is imprecise, amend it.
Copper(II) ion is blue in colour.
104
Precise answer is needed
Aqueous copper(II) ion is blue in colour.
105
The following answer is imprecise, amend it.
Aluminium foil is corrosion resistant because it
has an oxide layer on its surface.
106
Precise answer is needed
Oxide layer on the surface of aluminium foil is
impervious/ impermeable to oxygen or water
107
The following answer is imprecise, amend it.
Electrical conductivity of copper(II) sulphate
solution is due to free ions
108
Precise answer is needed
Electrical conductivity of copper(II) sulphate
solution is due to mobile ions not free ions
109
The following answer is imprecise, amend it.
Mixing of toilet cleaner and bleaching solution
is dangerous.
110
Precise answer is needed
Toxic chlorine gas is produced on mixing toilet
cleaner and bleaching solution.
Cl- ClO- 2H ?? Cl2 H2O
111
The following answer is imprecise, amend it.
Both chlorine bleach and chlorine water are
alkaline.
112
Precise answer is needed
Chlorine bleach is alkaline and chlorine water is
acidic.
Chlorine bleach is a solution of sodium
hypochlorie, which is a salt of strong base and
weak acid.
Chlorine water is a solution of hydrochloric acid
and hypochlorous acid.
Cl2 H2O HCl HOCl
113
The following answer is incorrect, amend it.
1st 2nd 3rd 4th
15.0 15.7 15.8 15.9
Mean volume of titrant used (15.0 15.7
15.8 15.9)/4 15.4 cm3
114
Correct answer
1st 2nd 3rd 4th
15.0 15.7 15.8 15.9

• Mean volume of titrant used
• (15.7 15.8 15.9)/3 15.8 cm3
• the datum 15.0 should be discarded

115
The following answer is incorrect, amend it.
Formula mass of methanol is 30 g
116
Correct answer

• Formula mass of methanol is 30

117
The following answer is imprecise, amend it.
In sacrificial protection, a more reactive metal
(e.g. Zn) is put near a less reactive metal (Fe)
118
Precise answer is needed
In sacrificial protection, a more reactive metal
(e.g. Zn) is connected to a less reactive metal
(Fe)
119
The following answer is imprecise, amend it.
PVC is formed by additional polymerisation of
chloroethene
120
Correct answer
PVC is formed by addition polymerisation of
chloroethene
121
Common Mistakes in Chemistry(Part 2)
122

1. Read the question (5 - 10 s)
2. Think (10 s)
3. Pair-up and discuss your answer (10 15 s)
4. One group will be invited to share the answer
   with whole class (10 15 s)

123
The following answer is imprecise, amend it.
Metal reacts with dilute hydrochloric acid to
give hydrogen.
124
Precise answer is needed
Metals below hydrogen in the electrochemical
series (Cu, Ag, Au) cannot displace hydrogen from
dilute acids. Mg(s) 2H(aq) ?? Mg2(aq)
H2(g)
125
Precise answer is needed
126
The following answer is incorrect, amend it.
Copper does not react with acid.
127
Precise answer is needed
Copper does not react with dilute acid. Cu
reacts with concentrated sulphuric acid and
concentrated nitric acid. Cu(s) ?? Cu2(aq)
2e- SO42-(aq) 2e- 4H(aq) ?? SO2(g)
2H2O(l)
128
The following answer is incorrect, amend it.
Rusting of iron is a redox reaction. Iron is
oxidised in the presence of water.
129
Precise answer is needed
Rusting of iron is a redox reaction. Iron is
oxidised in the presence of water and
oxygen. Fe(s) ?? Fe2(aq) 2e- O2(g)
2H2O(l) 4e- ?? 4-OH(aq)
130
The following answer is incorrect, amend it.
After scratching, galvanized iron rusts faster
than pure iron.
131
Precise answer is needed
After scratching, tin-plated iron rusts faster
than pure iron because tin is below iron in the
ECS. Zinc is above iron in the ECS. By
sacrificial protection, galvanized iron does not
rust after scratching.
132
The following answer is imprecise, amend it.
On the addition of excess iron(II) sulphate, the
acidified potassium permanganate turns from
purple to colourless.
133
Precise answer is needed
On the addition of excess iron(II) sulphate, the
acidified potassium permanganate turns from
purple to yellow (aqueous iron(III) ion is
yellow).
134
The following answer is imprecise, amend it.
Calcium granules react with excess dilute
sulphuric acid readily and completely. The
resulting solution is colourless.
135
Precise answer is needed
Calcium granules react with dilute sulphuric acid
readily and the reaction stops after a short
while. This is because CaSO4 is insoluble in
water. It forms a layer on the surface of the
calcium granules and stops further reaction.
136
The following answer is incorrect, amend it.
Excess calcium is added to dilute hydrochloric
acid. The resulting solution is calcium chloride.
137
Precise answer is needed
Excess calcium is added to dilute hydrochloric
acid. The resulting solution is a mixture of
calcium chloride and calcium hydroxide.
138
The following answer is incorrect, amend it.
The following reactions are reversible
esterification ionization of ethanoic acid
SO2 combines with O2 to form SO3
neutralization
139
Precise answer is needed
The following reactions are reversible
esterification ionization of ethanoic acid
SO2 combines with O2 to form SO3 except
neutralization
140
The following answer is incorrect, amend it.
Electricical conductivity of 1M ethanoic acid is
lower than that of 1M ethanol because the
concentration of mobile ions in 1M ethanoic acid
is lower than that in 1M ethanol.
141
Precise answer is needed
Electricical conductivity of 1M ethanoic acid is
higher than that of 1M ethanol because the
concentration of mobile ions in 1M ethanoic acid
is higher than that in 1M ethanol.
142
The following answer is incorrect, amend it.
Aqueous solution of ammonium chloride is alkaline.
143
Precise answer is needed
Aqueous solution of ammonium chloride is acidic.
H2O H -OH NH4Cl ?? NH4
Cl- NH3 is a weak alkali, NH4 combines with
OH. NH4 -OH NH3 H2O HCl is a strong
acid and Cl- does not combine with H. -OH is
removed, but not H, so the solution is acidic.
144
The following answer is incorrect, amend it.
Aqueous solution of sodium ethanoate is acidic.
145
Precise answer is needed
Aqueous solution of sodium ethanoate is
alkaline. H2O H -OH CH3COONa ??
Na CH3COO- CH3COOH is a weak acidic, CH3COO-
combines with H. CH3COO- H
CH3COOH NaOH is a strong alkali Na does not
combine with -OH. H is removed, but not -OH, so
the solution is alkaline.
146
The following answer is incorrect, amend it.
A metal which is lower in the ECS can displace
another metal higher in the ECS from its salt
solution.
147
Correct answer is needed
A metal which is higher in the ECS can displace
another metal lower in the ECS from its salt
solution. Cu(s) 2Ag(aq) ?? Cu2(aq)
2Ag(s)
148
The following answer is incorrect, amend it.
Flame colour of sodium compounds is intense
yellow while that of calcium is bluish-green.
149
Precise answer is needed
Flame colour of sodium compounds is intense
yellow while that of calcium is brick-red. Flame
colour of copper(II) compounds is bluish-green.
150
The following answer is incorrect, amend it.
Metal hydroxide is precipitated out when sodium
hydroxide is added to an aqueous solution of a
salt.
151
Precise answer is needed
Insoluble metal hydroxides (e.g. Cu(OH)2,
Fe(OH)2, Fe(OH)3, Zn(OH)2) are precipitated out
when sodium hydroxide is added to Cu2(aq),
Fe2(aq), Fe3(aq), Zn2(aq).
152
The following answer is incorrect, amend it.
Carbon monoxide can be used to reduce metal oxide
to metal.
153
Precise answer is needed
Carbon monoxide can be used to reduce oxides of
moderately active metals (e.g. iron, lead, tin,
copper) to metal. Electrolysis is used to extract
active metals (e.g. Al, Mg, Na) from their molten
oxides or chlorides.
154
The following answer is incorrect, amend it.
Alkanols are miscible with water.
155
Precise answer is needed
Simple alkanols (methanol, ethanol and
propan-1-ol) are miscible with water. Miscibility
decreases with the size of alkanol.
156
The following answer is incorrect, amend it.
Soapy detergent acts as a wetting agent because
it can increase the surface tension of water.
157
Precise answer is needed
Soapy detergent acts as a wetting agent because
it can decrease the surface tension of water.
158
The following answer is incorrect, amend it.
Detergent particles have hydrophilic heads which
are soluble in oil or grease.
159
Precise answer is needed
Detergent particles have hydrophobic tails which
are soluble in oil or grease.
160
The following answer is incorrect, amend it.
Detergent forms scum in hard water.
161
Precise answer is needed
Soapy detergent forms scum in hard water.
162
The following answer is incorrect, amend it.
The melting point range of petrol is narrow
because petrol is a mixture.
163
Precise answer is needed
The melting point range of petrol is wide because
petrol is a mixture.
164
The following answer is incorrect, amend it.
The viscosity of lubricating oil is lower than
that of petrol because the intermolecular forces
between hydrocarbon molecules in lubricating oil
are stronger than that in petrol.
165
Precise answer is needed
The viscosity of lubricating oil is higher than
that of petrol because the intermolecular forces
between hydrocarbon molecules in lubricating oil
are stronger than that in petrol.
166
The following answer is incorrect, amend it.
Incinerator is commonly used to dispose of
plastic wastes in Hong Kong.
167
Precise answer is needed
Landfilling is commonly used to dispose of
plastic wastes in Hong Kong.
168
The following answer is incorrect, amend it.
Injection moulding is used to manufacture
thermosetting polymer.
169
Precise answer is needed
Injection moulding is used to manufacture
thermoplastic polymer. OR Compression moulding
is used to manufacture thermosetting polymer.
170
The following answer is imprecise, amend it.
Catalytic converter in car is used to reduce the
emission of carbon monoxide and nitrogen oxides
171
Precise answer is needed
Catalytic converter in car is used to reduce the
emission of carbon monoxide, nitrogen oxides and
unburnt hydrocarbons.
172
The following answer is imprecise, amend it.
In the electrolysis of very dilute sodium
chloride solution, hydrogen gas is formed at the
cathode because hydrogen accepts electrons more
readily than sodium.
173
Precise answer is needed
Hydrogen gas is formed at the cathode because
hydrogen ion accepts electrons more readily than
sodium ion.
174
The following answer is imprecise, amend it.
Sodium hydroxide, because it is alkaline, can
emulsify oil.
175
Precise answer is needed
Oil undergoes hydrolysis (breaks down to form
glycerol and sodium salt of alkanoic acid) in
alkaline medium.
176
The following answer is imprecise, amend it.
Quartz has giant covalent bond.
177
Precise answer is needed
Quartz has giant covalent network/ structure.
178
The following answer is imprecise, amend it.
In a study of the effect of concentration change
on reaction rate, the reaction rate between
hydrochloric acid and solid calcium carbonate is
used. Identical apparatus are used and the
following factors are kept constant except the
concentration of HCl(aq) mass of calcium
carbonate, temperature, volume of
HCl(aq)
179
Precise answer is needed
In a study of the effect of concentration change
on reaction rate, the reaction rate between
hydrochloric acid and solid calcium carbonate is
used. Identical apparatus are used and the
following factors are kept constant except the
concentration of HCl(aq) mass of calcium
carbonate, temperature, particle size of
CaCO3 volume of HCl(aq)
180
The following answer is imprecise, amend it.
In a study of the effect of temperature change
on reaction rate, the reaction rate between
thiosulphate and acid is used. Time taken for the
disappearance of a cross below the conical flask
is compared. Identical apparatus are used and the
following factors are kept constant except the
reaction temperature volume
concentration of thiosulphate, volume
concentration of acid
181
Precise answer is needed
In a study of the effect of temperature change
on reaction rate, the reaction rate between
thiosulphate and acid is used. Time taken for the
disappearance of a cross below the conical flask
is compared. Identical apparatus are used and the
following factors are kept constant except the
reaction temperature volume
concentration of thiosulphate, volume
concentration of acid, same student is
involved in observing the disappearance of cross

182
The following answer is incorrect, amend it.
Reactivity increases down group 7. Reactivity
decreases down group 1.
183
Correct answer is needed
Reactivity decreases down group 7. Reactivity
increases down group 1.
184
The following answer is imprecise, amend it.
The attraction between atoms in gold is ionic
bond.
185
Precise answer is needed
The attraction between atoms in gold is metallic
bond.
186
The following answer is imprecise, amend it.
Relative atomic mass is the weighted average of
isotopic masses.
187
Precise answer is needed

• Relative atomic mass is the weighted average of
  isotopic masses on the carbon-12 scale.

188
The following answer is imprecise, amend it.
Ethyne (C2H2) burns with a more sooty flame than
ethane (C2H6) because ethyne has a smaller
molecular size.
189
Precise answer is needed
Ethyne (C2H2) burns with a more sooty flame than
ethane (C2H6) because ethyne has a higher carbon
content than ethane.
190
The following answer is imprecise, amend it.
Leakage of town gas is dangerous because carbon
monoxide, a constituent of town gas, is toxic.
191
Precise answer is needed

• Leakage of town gas is dangerous because town gas
  forms an explosive mixture with air and carbon
  monoxide, a constituent of town gas, is toxic.

192
The following answer is imprecise, amend it.
Sulphur dioxide is toxic.
193
Precise answer is needed
Sulphur dioxide attacks the respiratory system.
Individuals who suffer from chronic respiratory
diseases such as bronchitis or asthma are much
more susceptible to attack
194
The following answer is imprecise, amend it.
In the titration of sodium hydroxide with
hydrochloric acid. At the end point, the methyl
orange indicator turns orange.
195
Precise answer is needed
Methyl orange changes from yellow to orange.
Initial and final colour should be specified.
196
Name a gas which could be collected by method (a)
197
Examples
Gaseous substances should be heavier than air
and do not react with air, e.g. HCl, Cl2,
SO2. Average molar mass of air is 28.8g mol-1.
198
Name a gas which could be collected by method (b)
199
Examples
Gaseous substances should be lighter than air
and do not react with air, e.g. H2, NH3.
200
Name a gas which could be collected by method (c)
201
Examples
Gaseous substances should be insoluble in water
e.g. H2, N2 .
202
How can the Bunsen value prevent sucking back?
203
Bunsen valve is made of rubber, which is
flexible. It allows gaseous product to move
outward but prevent water to flow in the opposite
direction.
204
Why is an inverted funnel used here?
205

• It provides a larger contact area for the
  dissolution of ammonia, sucking back is prevented.

206
Should you need further assistance, please send
your e-mail to Mr. S.K. Leung (skleung2_at_emb.gov.hk
.)
207
Homework

• Based on the mistakes you have made in your
  assignment, prepare ten similar questions and
  answers.