1. 1 Common Mistakes in Chemistry

2. 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. 3 What’s wrong with the following statement? Oxygen relights a growing splint.

4. 4 Correct statement Oxygen relights a glowing splint.

5. 5 What’s wrong with the following statement? Nitrogen dioxide is an acidic oxide. It dissolves in water to form nitric acid.

6. 6 Correct statement Nitrogen dioxide is an acidic oxide. It dissolves in water to form nitric acid and nitrous acid. 2NO 2 + H 2 O  HNO 3 + HNO 2

7. 7 What’s wrong with the following statement? Ethanol turns green acidified potassium dichromate solution to orange.

8. 8 Correct statement Ethanol turns orange acidified potassium dichromate solution to green.

9. 9 What’s wrong with the following statement? Ammonia gas turns red litmus paper blue.

10. 10 Correct statement Ammonia gas turns moist red litmus paper blue.

11. 11 What’s wrong with the following statement? Water vapour turns cobalt chloride to pink.

12. 12 Correct statement Water vapour turns dry/ anhydrous/ blue cobalt chloride to pink.

13. 13 What’s wrong with the following statement? Nitrogen is an inert gas.

14. 14 Correct statement Nitrogen is an inactive gas. N 2 (g) + 3H 2 (g) 2NH 3 (g) Nitrogen acts as oxidising agent. N 2 (g) + O 2 (g)  2NO(g) Nitrogen acts as reducing agent.

15. 15 What’s wrong with the following statement? Chlorine gas turns moist blue litmus paper red.

16. 16 Correct statement Chlorine gas turns moist blue litmus paper red and then white.

17. 17 What’s wrong with the following statement? Sulphur dioxide turns potassium dichromate from orange to green.

18. 18 Correct statement Sulphur dioxide turns acidified potassium dichromate from orange to green.

19. 19 What’s wrong with the following statement? Ethene turns acidified potassium dichromate from orange to green.

20. 20 Correct statement Ethene turns acidified potassium permanganate from purple to colourless.

21. 21 What’s wrong with the following statement? Carbon dioxide turns limewater milky. The white precipitate is not soluble in excess carbon dioxide.

22. 22 Correct statement Carbon dioxide turns limewater milky. Ca(OH) 2 (aq) + CO 2 (aq)  CaCO 3 (s) + H 2 O(l) Excess carbon dioxide turns the milky solution colourless. CaCO 3 (s) + CO 2 (aq) + H 2 O(l)  Ca(HCO 3 ) 2 (aq)

23. 23 What’s wrong with the following statement? Hydrogen burns with a ‘pop’ sound.

24. 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. 25 What’s wrong with the following statement? Seperating funnel is used to seperate two immiscible liquids.

26. 26 Correct statement Separating funnel is used to separate two immiscible liquids.

27. 27 What’s wrong with the following statement? We use mortar to grind the calcium carbonate into powder.

28. 28 What’s wrong with the following statement? We use mortar and pestle to grind the calcium carbonate into powder.

29. 29 What’s wrong with the following statement? We use spoon to transfer chemicals from reagent bottle to the test-tube.

30. 30 What’s wrong with the following statement? We use spatula to transfer chemicals from reagent bottle to the test-tube.

31. 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. 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. 33 What’s wrong with the following statement? pH of 1M hydrochloric acid is higher than that of 1M ethanoic acid.

34. 34 Correct statement pH of 1M hydrochloric acid is lower than that of 1M ethanoic acid.

35. 35 What’s wrong with the following statement? The oxidation number of silver in Ag 2 O is 1.

36. 36 Correct statement The oxidation number of silver in Ag 2 O is +1.

37. 37 What’s wrong with the following statement? Acid is a hydrogen-containing compound which dissolves in water to give H + (aq).

38. 38 Correct statement Acid is a hydrogen-containing compound which dissolves in water to give H + (aq) as the only cation.

39. 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. 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. 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. 42 Precise answer Salt is a compound derived from acid. The hydrogen ion in acid is replaced by metal ion or ammonium ion.

43. 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. 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. 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. 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. 47 What’s wrong with the following structure?

48. 48 Correct structure

49. 49 What’s wrong with the following statement? The functional group of ethanol is –HO.

50. 50 Correct statement The functional group of ethanol is –OH.

51. 51 What’s wrong with the following structure? III

52. 52 Correct structure

53. 53 What’s wrong with the following statement? Helium has an octet structure.

54. 54 Correct statement Helium has a duplet structure.

55. 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. 56 Correct statement Na is a stronger reducing agent than Mg because Na loses electron more readily than Mg.

57. 57 What’s wrong with the following statement? During the electrolysis of dilute sulphuric acid, hydrogen discharges at the cathode.

58. 58 Correct statement During the electrolysis of dilute sulphuric acid, hydrogen ion discharges at the cathode.

59. 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. 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. 61 What’s wrong with the following statement? All sulphates are soluble in water except calcium sulphate.

62. 62 Correct statement All sulphates are soluble in water except calcium sulphate, barium sulphate, strontium sulphate and lead sulphate.

63. 63 What’s wrong with the following statement? All carbonates are insoluble in water except sodium and potassium carbonate.

64. 64 Correct statement All carbonates are insoluble in water except sodium, potassium and ammonium carbonate.

65. 65 What’s wrong with the following statement? Nylon is a biodegradable condensation polymer.

66. 66 Correct statement Nylon is a non-biodegradable condensation polymer.

67. 67 What’s wrong with the following statement? Urea methanol is a thermosetting polymer.

68. 68 Correct statement Urea methanal is a thermosetting polymer.

69. 69 What’s wrong with the following statement? Isotopes have similar chemical properties.

70. 70 Correct statement Isotopes have same chemical properties.

71. 71 What’s wrong with the following statement? Electrolyte is a compound which in aqueous solution or when molten, conducts electricity.

72. 72 Correct statement Electrolyte is a compound which in aqueous solution or when molten, conducts electricity and is decomposed in the process.

73. 73 What’s wrong with the following statement? Members of a homologous series have the same chemical properties but different physical properties.

74. 74 Correct statement Members of a homologous series have similar chemical properties but graduation in physical properties.

75. 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. 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. 77 What’s wrong with the following formula? NaSO 4 Zn(Cl 2 ) KCr 2 O 7 (NH 3 ) 2 SO 4

78. 78 Correct formula Na 2 SO 4 ZnCl 2 K 2 Cr 2 O 7 (NH 4 ) 2 SO 4

79. 79 What’s wrong with the following structure? repeating unit of poly(propene)

80. 80 Correct structure repeating unit of poly(propene)

81. 81 What’s wrong with the following electronic structure? nitrogen

82. 82 Correct electronic structure Nitrogen – octet structure for each nitrogen atom

83. 83 What’s wrong with the following ionic equation? Ca 2+ (aq) + Na 2 CO 3 (aq)  CaCO 3 (s) + 2Na + (aq)

84. 84 Correct ionic equation Ca 2+ (aq) + CO 3 2- (aq)  CaCO 3 (s) Na 2 CO 3 (aq) is an electrolyte.

85. 85 What’s wrong with the following ionic equation? CaCO 3 (aq) + 2H + (aq)  Ca 2+ (aq) + H 2 O(l) + CO 2 (g)

86. 86 Correct equation CaCO 3 (s) + 2H + (aq)  Ca 2+ (aq) + H 2 O(l) + CO 2 (g) Calcium carbonate is insoluble in water and hence no mobile carbonate ions are available for the reaction.

87. 87 What’s wrong with the following equation? CH 3 COOH(aq)  CH 3 COO - (aq) + H + (aq)

88. 88 Correct equation CH 3 COOH(aq) CH 3 COO - (aq) + H + (aq) Ethanoic acid is a weak acid. Ionization is incomplete.

89. 89 What’s wrong with the following equation? NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + - OH(aq)

90. 90 Correct equation NH 3 (aq) + H 2 O(l) NH 4 + (aq) + - OH(aq) Aqueous ammonia is a weak base. Ionization is incomplete.

91. 91 What’s wrong with the following statement? Solid iodine is purple.

92. 92 Correct statement I 2 (s) is black I 2 in KI is brown (iodine is slightly soluble in water, but highly soluble in KI solution.) I 2 vapour is purple

93. 93 What’s wrong with the following equation? NH 3 (g) + HCl(g)  NH 4 Cl(g)

94. 94 Correct equation NH 3 (g) + HCl(g)  NH 4 Cl(s) Ammonium chloride is an ionic compound. It is a solid at room temperature and pressure.

95. 95 What’s wrong with the following equation? Cu(s) + 2HCl(aq)  CuCl 2 (aq) + H 2 (g)

96. 96 Correct equation Copper is below hydrogen in the electrochemical series, it does not react with dilute hydrochloric acid.

97. 97 What’s wrong with the following statement? conc. H 2 SO 4 C 6 H 12 O 6 (s)  6C(s) + 6H 2 O(l) Table sugar (cube sugar) undergoes dehydration.

98. 98 Correct statement The main constituent of cube sugar/ table sugar is sucrose. conc. H 2 SO 4 C 12 H 22 O 11 (s)  12C(s) + 11H 2 O(l)

99. 99 What’s wrong with the following diagram?

100. 100 Correct diagram

101. 101 The following answer is imprecise, amend it. Ester can be formed by heating alkanoic acid with an alkanol.

102. 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. H 2 SO 4 e.g. CH 3 COOH + C 2 H 5 OH CH 3 COOC 2 H 5 + H 2 O

103. 103 The following answer is imprecise, amend it. Copper(II) ion is blue in colour.

104. 104 Precise answer is needed Aqueous copper(II) ion is blue in colour.

105. 105 The following answer is imprecise, amend it. Aluminium foil is corrosion resistant because it has an oxide layer on its surface.

106. 106 Precise answer is needed Oxide layer on the surface of aluminium foil is impervious/ impermeable to oxygen or water

107. 107 The following answer is imprecise, amend it. Electrical conductivity of copper(II) sulphate solution is due to ‘free ions’

108. 108 Precise answer is needed Electrical conductivity of copper(II) sulphate solution is due to ‘mobile ions’ not ‘free ions’

109. 109 The following answer is imprecise, amend it. Mixing of toilet cleaner and bleaching solution is dangerous.

110. 110 Precise answer is needed Toxic chlorine gas is produced on mixing toilet cleaner and bleaching solution. Cl - + ClO - + 2H +  Cl 2 + H 2 O

111. 111 The following answer is imprecise, amend it. Both chlorine bleach and chlorine water are alkaline.

112. 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. Cl 2 + H 2 O HCl + HOCl Chlorine water is a solution of hydrochloric acid and hypochlorous acid.

113. 113 The following answer is incorrect, amend it. Mean volume of titrant used = (15.0 + 15.7 + 15.8 + 15.9)/4 = 15.4 cm 3 1st2nd3rd4th 15.015.715.815.9

114. 114 Correct answer 1st2nd3rd4th 15.015.715.815.9 Mean volume of titrant used = (15.7 + 15.8 + 15.9)/3 = 15.8 cm 3 the datum 15.0 should be discarded

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

116. 116 Correct answer Formula mass of methanol is 30

117. 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. 118 Precise answer is needed In sacrificial protection, a more reactive metal (e.g. Zn) is connected to a less reactive metal (Fe)

119. 119 The following answer is imprecise, amend it. PVC is formed by additional polymerisation of chloroethene

120. 120 Correct answer PVC is formed by addition polymerisation of chloroethene

121. 121 Common Mistakes in Chemistry (Part 2)

122. 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. 123 The following answer is imprecise, amend it. Metal reacts with dilute hydrochloric acid to give hydrogen.

124. 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)  Mg 2+ (aq) + H 2 (g)

125. 125 Precise answer is needed

126. 126 The following answer is incorrect, amend it. Copper does not react with acid.

127. 127 Precise answer is needed Copper does not react with dilute acid. Cu reacts with concentrated sulphuric acid and concentrated nitric acid. Cu(s)  Cu 2+ (aq) + 2e - SO 4 2- (aq) + 2e - + 4H + (aq)  SO 2 (g) + 2H 2 O(l)

128. 128 The following answer is incorrect, amend it. Rusting of iron is a redox reaction. Iron is oxidised in the presence of water.

129. 129 Precise answer is needed Rusting of iron is a redox reaction. Iron is oxidised in the presence of water and oxygen. Fe(s)  Fe 2+ (aq) + 2e - O 2 (g) + 2H 2 O(l) + 4e -  4 - OH(aq)

130. 130 The following answer is incorrect, amend it. After scratching, galvanized iron rusts faster than pure iron.

131. 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. 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. 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. 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. 135 Precise answer is needed Calcium granules react with dilute sulphuric acid readily and the reaction stops after a short while. This is because CaSO 4 is insoluble in water. It forms a layer on the surface of the calcium granules and stops further reaction.

136. 136 The following answer is incorrect, amend it. Excess calcium is added to dilute hydrochloric acid. The resulting solution is calcium chloride.

137. 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. 138 The following answer is incorrect, amend it. The following reactions are reversible: esterification ionization of ethanoic acid SO 2 combines with O 2 to form SO 3 neutralization

139. 139 Precise answer is needed The following reactions are reversible: esterification ionization of ethanoic acid SO 2 combines with O 2 to form SO 3 except neutralization

140. 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. 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. 142 The following answer is incorrect, amend it. Aqueous solution of ammonium chloride is alkaline.

143. 143 Precise answer is needed Aqueous solution of ammonium chloride is acidic. H 2 O H + + - OH NH 4 Cl  NH 4 + + Cl - NH 3 is a weak alkali, NH 4 + combines with – OH. NH 4 + + - OH NH 3 + H 2 O HCl is a strong acid and Cl - does not combine with H +. - OH is removed, but not H +, so the solution is acidic.

144. 144 The following answer is incorrect, amend it. Aqueous solution of sodium ethanoate is acidic.

145. 145 Precise answer is needed Aqueous solution of sodium ethanoate is alkaline. H 2 O H + + - OH CH 3 COONa  Na + + CH 3 COO - CH 3 COOH is a weak acidic, CH 3 COO - combines with H +. CH 3 COO - + H + CH 3 COOH NaOH is a strong alkali & Na + does not combine with - OH. H + is removed, but not - OH, so the solution is alkaline.

146. 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. 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)  Cu 2+ (aq) + 2Ag(s)

148. 148 The following answer is incorrect, amend it. Flame colour of sodium compounds is intense yellow while that of calcium is bluish-green.

149. 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. 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. 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 Cu 2+ (aq), Fe 2+ (aq), Fe 3+ (aq), Zn 2+ (aq).

152. 152 The following answer is incorrect, amend it. Carbon monoxide can be used to reduce metal oxide to metal.

153. 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. 154 The following answer is incorrect, amend it. Alkanols are miscible with water.

155. 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. 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. 157 Precise answer is needed Soapy detergent acts as a wetting agent because it can decrease the surface tension of water.

158. 158 The following answer is incorrect, amend it. Detergent particles have hydrophilic heads which are soluble in oil or grease.

159. 159 Precise answer is needed Detergent particles have hydrophobic tails which are soluble in oil or grease.

160. 160 The following answer is incorrect, amend it. Detergent forms scum in hard water.

161. 161 Precise answer is needed Soapy detergent forms scum in hard water.

162. 162 The following answer is incorrect, amend it. The melting point range of petrol is narrow because petrol is a mixture.

163. 163 Precise answer is needed The melting point range of petrol is wide because petrol is a mixture.

164. 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. 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. 166 The following answer is incorrect, amend it. Incinerator is commonly used to dispose of plastic wastes in Hong Kong.

167. 167 Precise answer is needed Landfilling is commonly used to dispose of plastic wastes in Hong Kong.

168. 168 The following answer is incorrect, amend it. Injection moulding is used to manufacture thermosetting polymer.

169. 169 Precise answer is needed Injection moulding is used to manufacture thermoplastic polymer. OR Compression moulding is used to manufacture thermosetting polymer.

170. 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. 171 Precise answer is needed Catalytic converter in car is used to reduce the emission of carbon monoxide, nitrogen oxides and unburnt hydrocarbons.

172. 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. 173 Precise answer is needed Hydrogen gas is formed at the cathode because hydrogen ion accepts electrons more readily than sodium ion.

174. 174 The following answer is imprecise, amend it. Sodium hydroxide, because it is alkaline, can emulsify oil.

175. 175 Precise answer is needed Oil undergoes hydrolysis (breaks down to form glycerol and sodium salt of alkanoic acid) in alkaline medium.

176. 176 The following answer is imprecise, amend it. Quartz has giant covalent bond.

177. 177 Precise answer is needed Quartz has giant covalent network/ structure.

178. 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. 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 CaCO 3 volume of HCl(aq)

180. 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. 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. 182 The following answer is incorrect, amend it. Reactivity increases down group 7. Reactivity decreases down group 1.

183. 183 Correct answer is needed Reactivity decreases down group 7. Reactivity increases down group 1.

184. 184 The following answer is imprecise, amend it. The attraction between atoms in gold is ionic bond.

185. 185 Precise answer is needed The attraction between atoms in gold is metallic bond.

186. 186 The following answer is imprecise, amend it. Relative atomic mass is the weighted average of isotopic masses.

187. 187 Precise answer is needed Relative atomic mass is the weighted average of isotopic masses on the carbon-12 scale.

188. 188 The following answer is imprecise, amend it. Ethyne (C 2 H 2 ) burns with a more sooty flame than ethane (C 2 H 6 ) because ethyne has a smaller molecular size.

189. 189 Precise answer is needed Ethyne (C 2 H 2 ) burns with a more sooty flame than ethane (C 2 H 6 ) because ethyne has a higher carbon content than ethane.

190. 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. 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. 192 The following answer is imprecise, amend it. Sulphur dioxide is toxic.

193. 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. 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. 195 Precise answer is needed Methyl orange changes from yellow to orange. Initial and final colour should be specified.

196. 196 Name a gas which could be collected by method (a):

197. 197 Examples Gaseous substances should be heavier than air and do not react with air, e.g. HCl, Cl 2, SO 2. Average molar mass of air is 28.8g mol -1.

198. 198 Name a gas which could be collected by method (b):

199. 199 Examples Gaseous substances should be lighter than air and do not react with air, e.g. H 2, NH 3.

200. 200 Name a gas which could be collected by method (c):

201. 201 Examples Gaseous substances should be insoluble in water e.g. H 2, N 2.

202. 202 How can the Bunsen value prevent sucking back?

203. 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. 204 Why is an inverted funnel used here?

205. 205 It provides a larger contact area for the dissolution of ammonia, sucking back is prevented.

206. 206 Should you need further assistance, please send your e-mail to Mr. S.K. Leung (skleung2@emb.gov.hk.)skleung2@emb.gov.hk

207. 207 Homework Based on the mistakes you have made in your assignment, prepare ten similar questions and answers.