# Why does gaseous HCl not change dry blue litmus paper to red?

I am doing a practical in lab, in which HCl is to be evolved. I found that when blue litmus is near the evolved gas, it doesn't turn red. But, when litmus is dipped in water and then brought near the HCl gas it does change to red. Why is this happening?

• Dry $\ce{HCl}$ gas doesnt furnish $\ce{H+}$ ions. Moist $\ce{HCl}$ however, can. – Pritt says Reinstate Monica Apr 27 '17 at 14:47
• – Berry Holmes Apr 27 '17 at 18:40

$$\ce{H2O(l) + HCl(aq) -> H3O+ (aq) + Cl−(aq)}$$
Thus it acts as a typical Brønsted acid (proton donor) in an aqueous environment. In this case, water is gaining a proton $\ce{(H+)}$, so it is a base, while HCl is giving one away, so it is an acid.