Question

Why HCl gas is unable to change the colour of a dry blue litmus paper ? What happens if the paper is moistened ?

Answer 1

$HCl$ is unable to change any colour of dry blue litmus paper because it shows its acidic behavior by dissociating into $H^{+}$ molecules in the presence of water. So when it comes in contact with moistened blue litmus paper it dissociates into $H^{+}$ ions and ${Cl}^{-}$ ions thus by showing its acidic behavior now it changes blue litmus red.