GENERAL QUESTIONS 177 



150. On heating water in an open vessel it gradually disappears. Mercuric 

 oxide and carbon, when strongly heated in open crucibles, also disappear. 

 Has matter been destroyed in any of these cases ? Briefly outline the 

 experiments you would make to ascertain the nature of the change in each 

 case. 



151. When a candle is burned in air enclosed over mercury, it soon goes 

 out, the vessel being bedewed with moisture and the mercury rising slightly ; 

 if lime-water be substituted for mercury it rises to a greater extent and 

 turns milky. Give reasons for all the phenomena here stated. 



152. How do you account for the fact that sea- water has such a pro- 

 nounced salty taste, whereas the water from the rivers which flow into 

 seas has a scarcely perceptible taste ? How would you estimate the amount 

 of soluble matter present in one litre of sea- water ? 



153. Give a detailed account of any investigations you have made as to 

 the effect of heat on green vitriol, sal ammoniac and red oxide of mercury 

 respectively. 



154. You have placed before you four jars containing respectively 

 hydrogen, carbon dioxide, oxygen and nitrogen. What experiments 

 would enable you to say which was which ? 



I .")">. Explain how it is that phosphorus readily oxidises in air which 

 consists of about 23 per cent, oxygen, while it remains unchanged in water 

 which consists of about 89 per cent, oxygen. Explain also why it is im- 

 possible to keep metallic sodium in water. 



156. Carefully weighed quantities of the following substances are left 

 exposed to the air and weighed from time to time : iron filings, caustic 

 potash, oil of vitriol. State whether you would expect any change in weight, 

 and, if so, account fully for such change. 



157. Describe exactly what happens when each of the following bodies 

 is heated and then allowed to cool : sulphur, ammonia solution, chalk, 

 nitre. 



Compare the physical and chemical properties of the residues with those 

 of the original substances. 



158. Describe fully the effect of strongly heating (a) red oxide of mercury, 

 (b) green vitriol. Show from your knowledge of the history of chemistry 

 that special interest attaches to these experiments. 



159. Show that the introduction of the balance had an important influence 

 on the progress of chemistry as a science. 



160. State clearly your views on combustion. How would you investigate 

 the conditions which limit the combustion of a candle in an enclosed volume 

 of air ? 



J.O.8, 



