Question

What do you observe when blue litmus paper is dipped in dil. Hydrochloric acid?

Answer 1

We can define litmus paper as an indicator that is used to determine the acidity or basicity of a solution. Litmus paper provides less precise indication as it can’t give the exact $pH$ value of a solution. In simple words we can say that Litmus paper tells whether the given solution is acid or base.

Complete answer:
We know that litmus paper is a $pH$ indicator that changes its colour depending upon the $pH$ of the solution. Litmus paper is available as red litmus paper and blue litmus paper. Also, neutral litmus paper is purple in colour
When blue litmus paper is dipped in dil. Hydrochloric acid, it will change its colour from blue to red. This happens because the blue litmus paper already contains the blue conjugate base. It reacts with acid and changes its colour to red.
Thus, we will observe colour change from blue to red when blue litmus paper is dipped in dil. Hydrochloric acid.

Additional Information:
We have seen that blue litmus paper turns red under acidic conditions, the red litmus paper on the other hand turns blue under basic conditions. It should be noted that blue litmus paper doesn’t show any colour change under basic conditions, similarly red litmus paper doesn’t show any colour change under acidic condition.

Note:
Litmus paper can be used as a $pH$ indicator but it can’t give the exact $pH$ value of the solution. It is observed that sometimes the colour of the litmus paper changes when it is kept in open, which is because of the acidic or basic species present in the atmosphere.