Activity 2.9 NCERT Class 10 Science, Chapter 2 Acids, Bases, and Salts
Activity 2.9 asks us to react solid sodium chloride with concentrated sulphuric acid and check if the gas evolved turn the blue litmus paper into red or not.
Gas fumes produce but do not turn the blue litmus paper into the red.
Solid sodium chloride reacts with concentrated sulphuric acid and forms dry hydrochloric acid gas. Here HCl is produced in gaseous form as there is no water.
NaCl(S) + H2SO4(Conc.) ———> Na2SO4 (s)+ HCl(g)
A litmus paper works on H+ or OH- ion. Since there is no water in HCl gas, HCl does not dissociate into its ion and does not turn the blue litmus red.
HCl (aq) ——-> H+ + Cl–
Concentrated acids and bases need an aqueous medium to turn litmus paper.
If we use moist blue litmus paper (as depicted in the diagram) then HCl gas will turn blue litmus paper into the red.
Reference: NCERT textbook for class 10 Science.