Question

dry hydrogen chloride gas does not affect a dry strip
of blue litmus paper but it turns red in the presence
of a drop of water.
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Answer 1

Answer:

Thus a dry litmus paper will not show any effect in the presence of dry HCL due to the absence of moisture(H+ ions) in the the litmus paper , whereas on putting a drop of water,the blue litmus paper immediately turns to red due to the presence of water in it hence presence of hydrogen ions(H+ ions)