Question

Why does dry $HCl$ gas not change the color of the dry litmus paper.

Answer 1

Hint : Acids are those which give hydrogen ion $({{H}^{+}})$ in the aqueous solution. The dipping of red litmus paper in acidic solution leads to the change in its color which is blue. This change in color occurs due to the reaction between litmus and free hydrogen ions $({{H}^{+}})$ .

Complete Step By Step Answer:
Acids are those which give hydrogen ion $({{H}^{+}})$ in the aqueous solution. The dipping of red litmus paper in acidic solution leads to the change in its color which is blue. This change in color occurs due to the reaction between litmus and free hydrogen ions $({{H}^{+}})$ . In dry $HCl$ gas, water is not present at all and hence it doesn’t dissociate into hydrogen ion $({{H}^{+}})$ and chloride ion $(C{{l}^{-}})$ . In the case of gaseous (dry) substance, wet litmus paper somewhat works because some amount of substance gets dissolved in wet litmus and gives free hydrogen ion $({{H}^{+}})$ . In contrast, dry litmus paper doesn’t work at all in case of gaseous substances because of the unavailability of free hydrogen ions $({{H}^{+}})$ .
Therefore, dry $HCl$ gas does not change the color of the dry litmus paper because of the unavailability of free hydrogen ion $({{H}^{+}})$ .

Note :
It is important to note that dry HCl gas does not change the color of the dry litmus paper because of the unavailability of free hydrogen ions $({{H}^{+}})$ . In the case of gaseous substances, wet litmus paper works because some amount of substance gets dissolved in wet litmus and gives free hydrogen ion $({{H}^{+}})$ . However, dry litmus paper doesn’t work at all in case of gaseous substances.