Question

Why does litmus paper have to be damp?​

Answer 1

because the substance you are testing needs to be wet for testing.

Answer 2

Because if u test HCL gas(hydro-chloride gas) with a litmus paper it won't change the colour.Because in gaseous state HCL doesn't show acidic property .But if you damp the litmus paper the gas in it's contact will change into some droplets of the acid HCL(hydrochloric acid) and will surely change the colour of the litmus paper .So for the best utilization of a litmus paper it is advisable to damp it first .Hope this will be helpful...