Question

why is HCL gas unable to change the colour of dry blue Litmus Paper what happen if the Litmus Paper is moisture

Answer 1

Hi
Dry HCl gas does not change the colour of dry litmus paper because in the absence of water, it cannot ionise to give H+ ions and therefore, the colour of the dry litmus paper does not change.
And if the litmus paper is moisturised the HCl gas can turn it Red in colour as it is acidic in Nature.
Hope this helps you.......☺