Question

Dry HCL gas do not change the colour of the dry litmus paper.why

Answer 1

Because the ions of Hydrogen don't ionises so it can't change the colour of litmus paper

Answer 2

Only aqueous material react with litmus paper, think if air react with litmus paper then all litmus change their colour according to air.. Litmus react with hydrogen ion of aqueous solution and turn their colour but gas can not react with litmus paper. Litmus can be turn red by H+ ion.