Explain i) ozone layer acts as an umbrella for the earth against the harmful radiation from sun. ii) The wooden shelf under thé reagent bottle containing conc. `H_2 SO_4` blackens after sometime. iii Precipitation of secònd group sulphides in qualitative analysis is càrried out with `H_2 S` in presence of `HCl` and not nitric acid.

Explanation

i.Ozone possesses the tendency to absorb U. .V.. rays of sunlight and hence ozone layer acts as a sereen and prevent the harmful radiations from reaching the.surfaice of the earth:
ii. Conc. .H_2 SO_4. is a dehydrating agent and. when comes in contact with wood cellulose of the shelf. remoyes wąter from "it. leaving behind black carbon.
iii. HiNO is anoxidising agent and it will oxidise .H_2 ~S. to colloidal sulphiur. Hence precipitation of groupII sulphide. will not occur if nitric acid iś used.