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In litmus paper test HCl gas turns moist litmus paper from blue to red as it is acidic in nature.


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Hint: litmus is defined as the water soluble mixture of different dyes that are extracted from lichens. Litmus is used to test a solution whether it is acidic or it is basic. It is absorbed into filter paper to form a pH indicator.

Complete step by step answer:
Litmus is used as a pH indicator and it is used to test the acidity and the basicity of material. The use of litmus is to test whether the solution is acidic or basic.
HCl is a hydrogen halide which is colourless at room temperature and forms white fumes when comes in contact with water vapour.
If you see a blue litmus paper changes its color to red litmus paper, then the compound is acidic in nature having pH value less than seven. And if you see a red litmus paper changes its colour to blue litmus paper, then the compound is basic in nature having the pH more than seven.
HCl gas is acidic only when it is moist or in an aqueous solution. Dry HCl does not dissociate to give H+ but in aqueous solution it dissociates to give H+ ions which gives a change the colour of litmus paper red ,showing acidic nature. In litmus paper test HCl gas turns moist litmus paper from blue to red as it is acidic in nature.

Note: pH is defined as a scale which is used to measure the acidity and basicity of an aqueous solution or liquid. If a pH is less than seven, it is acidic. If the pH is more than seven, it is basic. If the pH value is exactly seven, then it is neutral. It is calculated by the negative logarithm of the concentration of H+ ions.
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