question_answer

  State   reasons   for   the   following observations (a) The shining surface of some metals becomes dull when exposed to air for a long time. (b) Zinc fails to evolve hydrogen gas on reacting with dilute nitric acid. (c) Metal sulphides occur mainly in rocks but metal halides occur mostly in sea and lake waters.

Answer:

                  (a) Metals react with oxygen, sulphur compounds, carbon dioxide and moisture to form layers of their oxides, hydroxides, carbonates or sulphides. This process is called corrosion. Due to this process, their surface becomes dull.          (1) (b) Dilute nitric acid is an oxidising acid and hence oxidises  formed into , so zinc does not evolve with dilute .         (1) (c) Sulphides are insoluble in water but metal halides are water soluble. Therefore, metal sulphides occur in rocks but halides in sea and lake water.                                                          (1)