Question

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

Answer 1

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.