Question

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 1

a) oxidation

b) redox reaction

c) metal sulphides are insoluble while metal halides are soluble

Explanation:

a) When some metals get exposed to air, oxidation takes place on the outer layer of the metal. A new layer of the oxide formed will cover the metal inside, thus giving it a dull appearance instead of the original shiny look.

b) When zinc granules are added to nitric acid, a redox reaction takes place. So hydrogen is not liberated. Instead, the redox reaction of HNO3 results in the formation of oxides of nitrogen (N2O, NO, NO2 can all be formed), or dinitrogen (N2).

c) Metal halides are soluble in the water. So they get dissolved in rainwater and are washed away to the lakes and seas. But metal sulphides are insoluble and hence left behind in the rock as residue.