A student prepared hydrogen chloride gas by treating sodium chloride with concentrated sulphuric acid in a test-tube. He held a strip of dry blue litmus paper in HCl gas coming out of the test-tube. The student observed that on coming in contact with HCl gas, the colour of blue litmus paper :
(1) turned yellow
(2) turned violet
(3) remained blue
(4) turned red
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Solution
In acidic solution, colour blue litmus paper is changed to red. So, hydrochloric acid gas turns blue litmus to red.