1 common mistakes in chemistry. 2 1.read the question (5 - 10 s) 2.think (10 s) 3.pair-up and...

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

What’s wrong with the following statement?

Oxygen relights a growing splint.

4

Correct statement

Oxygen relights a glowing splint.

5

What’s 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

What’s 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

What’s wrong with the following statement?

Ammonia gas turns red litmus paper blue.

10

Correct statement

Ammonia gas turns moist red litmus paper blue.

11

What’s 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

What’s 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

What’s 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

What’s 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

What’s 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.

CH2 CH2

H2O +[O]CH2 CH2

OH OH

21

What’s 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

What’s 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

What’s 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

What’s wrong with the following statement?

We use mortar to grind the calcium carbonate into powder.

28

What’s wrong with the following statement?

We use mortar and pestle to grind the calcium carbonate into powder.

29

What’s wrong with the following statement?

We use spoon to transfer chemicals from reagent bottle to the test-tube.

30

What’s wrong with the following statement?

We use spatula to transfer chemicals from reagent bottle to the test-tube.

31

What’s wrong with the following statement?

Tripod stand, wire gauze, Bunsen burner, crucible & heat-proof mat are used to convert magnesium to magnesium oxide.

32

What’s 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

What’s 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

What’s 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

What’s 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

What’s 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

What’s 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

What’s wrong with the following structure?

C C C

HH

H H H

HH

48

Correct structure

C C C

H

H H H

HH

49

What’s wrong with the following statement?

The functional group of ethanol is –HO.

50

Correct statement

The functional group of ethanol is –OH.

51

What’s wrong with the following structure?

C C C

HH

H H H

HH

OH

propan -1- ol

C C C

HH

H H H

H

H

OH

propan -1- ol

I II

52

Correct structure

C C C

HH

H H H

HH

OH

propan -1- ol

53

What’s wrong with the following statement?

Helium has an octet structure.

54

Correct statement

Helium has a duplet structure.

55

What’s 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

What’s 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

What’s 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

What’s 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

What’s 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

What’s wrong with the following statement?

Nylon is a biodegradable condensation polymer.

C

O

(CH2)4 C

O

Cl

+ N

H

H

(CH2)6 N

H

H

condensation polymerization

C(CH2)4C N(CH2)6N

H

O

H

O

n n

n

+ (2n- 1)

Nylon

hexane- 1,6- diamine

Cl

hexane- 1,6- dioyl dichloride

HCl

66

Correct statement

Nylon is a non-biodegradable condensation polymer.

C

O

(CH2)4 C

O

Cl

+ N

H

H

(CH2)6 N

H

H

condensation polymerization

C(CH2)4C N(CH2)6N

H

O

H

O

n n

n

+ (2n- 1)

Nylon

hexane- 1,6- diamine

Cl

hexane- 1,6- dioyl dichloride

HCl

67

What’s wrong with the following statement?

Urea methanol is a thermosetting polymer.

68

Correct statement

Urea methanal is a thermosetting polymer.

O

CH H

CN N

O

H

H H

H CN N

O

H

H H

H

urea urea

methanal

CN N

O

H

H H

C CN N

OH H

H

H

H

69

What’s wrong with the following statement?

Isotopes have similar chemical properties.

70

Correct statement

Isotopes have same chemical properties.

71

What’s 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

What’s 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

What’s 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

What’s wrong with the following formula?

•NaSO4

•Zn(Cl2)

•KCr2O7

•(NH3)2SO4

78

Correct formula

•Na2SO4

•ZnCl2

•K2Cr2O7

•(NH4)2SO4

79

What’s wrong with the following structure?

repeating unit of poly(propene)

C C

H

H H

H

C

H

H

80

Correct structure

repeating unit of poly(propene)

C C

H

H H

CH3

81

What’s wrong with the following electronic structure?

nitrogen

N N

82

Correct electronic structure

Nitrogen – octet structure for each nitrogen atom

N N

83

What’s 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

What’s 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

What’s 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

What’s 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

What’s 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

What’s 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

What’s 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

What’s wrong with the following statement?

conc. H2SO4

C6H12O6(s) 6C(s) + 6H2O(l)

Table sugar (cube sugar) undergoes dehydration.

98

Correct statement

The main constituent of cube sugar/ table sugar is sucrose.

conc. H2SO4

• C12H22O11(s) 12C(s) + 11H2O(l)

99

What’s wrong with the following diagram?

A

Copper plate

Copper(II)

Solutionchloride

anodecathode

variable resistor

ammeterdry cell

100

Correct diagram

A

Copper plate

Copper(II)

Solutionchloride

anode cathode

variable resistor

ammeterdry cell

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 neededChlorine 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.

Cl2 + H2O HCl + HOCl

Chlorine water is a solution of hydrochloric acid and hypochlorous acid.

113

The following answer is incorrect, amend it.

Mean volume of titrant used = (15.0 + 15.7 + 15.8 + 15.9)/4 = 15.4 cm3

1st 2nd 3rd 4th

15.0 15.7 15.8 15.9

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 neededAqueous solution of ammonium chloride is acidic. H2O H+ + -OHNH4Cl 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 neededAqueous solution of sodium ethanoate is alkaline. H2O H+ + -OHCH3COONa Na+ + CH3COO-

CH3COOH is a weak acidic, CH3COO- combines with H+.

CH3COO- + H+ CH3COOHNaOH 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 neededOil undergoes hydrolysis (breaks down to form glycerol and sodium salt of alkanoic acid) in alkaline medium.

NaOH3+

RCOO-Na+

R'COO-Na+

R'COO-Na++

CH2OH

CHOH

CH2OH

CH2O C

O

CHO C

O

CH2O C

O

R

R'

R"

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.

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Why is an inverted funnel used here?

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

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Should you need further assistance, please send your e-mail to Mr. S.K. Leung (skleung2@emb.gov.hk.)

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Homework

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