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chlorate, (c) manganese dioxide, (d) sulphuric acid, (e) mercuric oxide, (f) barium dioxide.

142. How much oxygen should be obtained from 100 lbs. of each of the above substances when used as sources of oxygen ?

143. How much mercuric oxide inust be heated to yield one litre of oxygen at N.T.P. ?

144. Equal weights of manganese dioxide are heated (a) alone, (6) with sulphuric acid. Which process yields the greater amount of oxygen?

145. Exactly 100 litres of oxygen at N.T.P. were obtained by heating potassium chlorate. How much of this salt was used ? 146. Some

pure manganese dioxide lost when strongly heated 137 grammes. Find the weight of the residue and the volume (at N. T.P.) of oxygen evolved.

147. A quantity of mercury was heated in air for several days, and gained in weight 1394 grammes.

Find the volume of oxygen at N.T.P. removed from the air.

148. Four hundred and seventy-nine grammes of potassium chloride remained after heating some potassium chlorate. Find the weight of the latter salt used.

149. A glass flask filled with air at N.T.P. weighs 52 grammes. What capacity must it have so that when filled with oxygen at 14° C. and 753 m.m. pressure, it shall weigh 53:47 grammes ?

150. What are the most important properties of oxygen? How is it distinguished from other gases?

151. How did Pictet obtain the gas in the liquid state and ascertain its density ?

152. The following bodies are separately burned in oxygen, (a) hydrogen, () sulphur, (c) charcoal, (d) iron, (e) phosphorus, (f) magnesium. Give the name of the oxide formed in each case, and express all the reactions by equations.

153. Explain the terms “combustion,” oxidation," “reduction,” and “ deoxidation.” 154. Sulphur is burnt in a jar of air, and also in a jar

Name the gases present in each jar after the combustion, and state the action of water on each.

155. Why was the name oxygen given to the gas, and why is it inappropriate ? To what element would the name more suitably apply?

156. What chemical changes occur when a candle burns in oxygen ? How may each of the products be recognised ?

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of oxygen.

157. At the bottom of a glass flask some sand is placed and on the top of it a fragment of phosphorus. The flask is then filled with dry oxygen, closed air-tight, and the whole weighed. The flask is then warmed until the phosphorus kindles, and when quite cold re-weighed. The stopper is removed for an instant and a third weighing made. How will the three weighings differ?

158. If air were used instead of oxygen in the previous question, what difference would be noticed in the weighings?

159. By burning a candle in oxygen 13'21 grammes carbon dioxide and 5'58 grammes water were produced. What loss in weight did the candle suffer, and what volume of oxygen at N.T.P. was required for the combustion ?

160. What weight of oxygen is contained in a room measuring 23 feet long, 17 broad, and 13 high? With how much phosphorus would this quantity of oxygen combine

OZONE (ALLOTROPIC OXYGEN).

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 ?

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