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161. What is meant by the term allotropism ? Give examples of elements which occur in allotropic forms.

162. Give a short account of the history of ozone, and state how it was first prepared.

163. How does ozone occur in nature ? How is its presence usually detected ?

164. Describe six distinct methods by which ozone may be prepared.

165. Name all the gases which liberate iodine from potassium iodide.

166. How may oxygen be converted into ozone, and vice versâ ?

167. Can a given volume of oxygen be entirely converted into ozone, and if so, by what method ?

168. What change in bulk occurs when oxygen is converted into ozone ? Describe the apparatus of Andrews and Tait for showing this.

169. The formula of ozone is 03. Describe fully the experiments of Andrews and of Soret in support of this formula.

170. What occurs when ozone is heated to 300° C. and when sand is shaken up in contact with the gas ?

171. Sketch the apparatus used by Andrews to demonstrate the presence of ozone in air.

172. Express by equations the action of ozone, and of chlorine, on potassium iodide.

173. What occurs when a hot glass rod is placed in a mixture of ether vapour and air, and when turpentine is shaken up with air or oxygen ?

174. To what causes has the presence of ozone in air been attributed ?

175. Compare the properties of ozone with those of oxygen, and show how they differ from each other?

176. A flask contains a colourless gas, which is found to liberate iodine from potassium iodide. How could you ascertain with certainty whether or not this reaction is due to the presence of ozone?

177. What is the action of ozone on each of the following bodies ?-(a) mercury, (6) turpentine, (c) caoutchouc, (d) manganese dioxide, (e) lead sulphide, (f) barium dioxide, (g) finely-divided silver, and (h) silver oxide.

178. What is the density of ozone, and how has it been ascertained ?



179. Name the most important compounds of boron which are found native.

180. How was boron first isolated by Gay-Lussac and Thenard, and by Davy?

181. How may crystallized boron be obtained ? By whom was it first prepared ?

182. Compare the properties of the allotropic modifications of boron.

183. Express by equations the reactions which occur when potassium is heated (a) with boron trioxide, (6) with potassium þorofluoride.

184. Amorphous boron is heated in (a) air, (b) oxygen, (c) nitrogen, (d) chlorine. What occurs in each case ? Give the names and formulæ of the products.

185. What occurs when adamantine boron is strongly heated in air or oxygen ? What occurs when it is fused with soda or potash ?

186. One hundred and thirty-seven grammes of boric acid are heated until all the water is expelled. What weight of trioxide remains, and with how much potassium must it be heated in order to prepare boron?

187. Under what conditions is boron soluble in water?



188. How does carbon occur in nature, (a) free, (6) combined ?

189. How are the three allotropic forms of carbon proved to consist of the same substance ?

190. The Koh-i-noor diamond weighs 106 carats (1 carat = 3-17 grains). What volume of oxygen at N.T.P. would be necessary to burn it, and what volume of carbon dioxide would result ?

191. Give the physical properties of the three varieties of carbon, and mention the chief uses of each.

192. How may the presence of carbon in organic matter be shown ?

193. By what other names is graphite known? Give the names of the different kinds of amorphous carbon, and state how each is prepared.

194. 2*62 grammes of a sample of coke yielded when

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