Question 3 Page 25 - Chapter 2 Class 10 - Acids, Bases and Salts

Why does dry HCl gas not change the colour of the dry litmus paper?

Answer: 

• Litmus paper shows change in colour only in the presence of H ⁺ ions.
• Acids dissociate to give H ⁺ ions only in the aqueous solution.
• Dry acid does not dissociate to give H ⁺ ions.
• In this case HCL is not in aqueous form nor is the  litmus paper wet and thus the lack of presence of H ⁺ ions leads to the litmus paper not changing its colour.