Answer :
Litmus is a soluble substance. Litmus is extracted from naturally occuring plants, the lichens. Litmus changes its colour to RED in an acidic solution, and it changes its colour to BLUE in basic or alkaline solution.
acid shows acidic property only in the presence of water. dry HCl not contain water it do not show acidic property, thus blue litmus doesn't turn red. wet the litmus paper and pass HCL gas so it turn the blue to red. HCL gas become acidic when it comes contect in water.