Question

If litmus paper turns blue on reacting with gas dissolved in water, it is:
(A) Acidic in nature
(B) Neutral in nature
(C) Basic in nature
(D) Both A and C

Answer 1

Hint: The colour that a litmus paper shows when dipped in a basic medium has shorter wavelength in comparison to its colour in acidic medium. In fact, it shows a colour which has the largest wavelength amongst all the colours in acidic medium.

Complete step by step solution:
We know that litmus paper is used to check the pH of the solution. Here, it is given that the litmus paper reacts with gas dissolved in water.
- Litmus paper has certain dyes absorbed on a paper. These dyes are sensitive to the presence of protons and hydroxyl ions. They both react with the dyes in a way that the structure of dye changes and so that the colour change in dye is observed.
- The dyes that are used in litmus paper are extracted from lichens.
- There are two types of litmus paper available; red and blue litmus paper. However, both litmus papers will show the same colour in the solutions with the same pH.
- If we dip the litmus paper in an acidic solution, then the litmus paper attains red colour. Here, the pH of the solution is less than 7.
- If we dip the litmus paper in a basic solution, then the litmus paper attains blue colour. Here the pH of the solution is more than 7.
So, here, it is given that the litmus paper turns blue. So, the solution should be basic.

Therefore, the correct answer is (C).

Note: Remember that litmus paper does not retain its colour in neutral solutions. In neutral solution, litmus paper shows violet colour. Do not forget that it gives blue colour in basic medium and red colour in acidic medium.