Question

State reasons for the following observations: 

(i) The shining surface of some metals becomes dull when exposed to air for a long time. 

(ii) Zinc fails to evolve hydrogen gas on reacting with dilute nitric acid. 

(iii) Metal sulphides occur mainly in rocks but metal halides occur mostly in sea and lake waters.

Answer 1

(i) It is because metal reacts with substances present in atmosphere to form surface compounds which make it dull. 

(ii) It is because dil. HNO₃ is an oxidising agent therefore zinc gives NO and not H₂ with dil. HNO₃. 

(iii) It is because sea water contains sodium chloride due to which metal halides are formed, whereas sulphur is found below rocks. Therefore, metal – sulphides are formed in rocks.