Question

Sulphurous acid turns ⋯⋯
  (A) blue litmus into red
  (B) red litmus into blue
  (C) no change to any of the litmus paper
  (D) can't say

Answer 1

Hint: The litmus paper is mainly of two types; red and blue and the litmus paper can be used to determine the basic or acidic nature of a substance based on color change of litmus paper. The reason behind the colour change in litmus paper is due to the presence of hydrogen ions.

Complete step by step solution:
- The solutions can be classified as basic or acidic by their hydrogen ion concentration relative to that of pure water. If the pH is greater than seven the solution is said to be basic and if it is less than seven, then the solution is considered as an acidic solution.
- If we use a litmus paper, we can easily find whether a solution is acidic, basic or neutral. Hence, we can say that litmus paper is a type of acid base indicator.
- The litmus paper is available in neutral, blue and red variations. Depending on the nature of the solution the litmus paper will change its colour on interaction with the solution to be tested.
- The blue litmus paper is used to test identify the acidic nature and will turn the blue shade into a red shade if it comes in contact with a solution of acidic nature. The red litmus paper is used to detect the alkaline pH and this basic solution will turn the red shade into a shade of blue. Similarly, the neutral litmus paper is purple in colour and this colour will also change accordingly depending on the nature of the solution which needs to be tested.
- Sulphurous acid (${{H}_{2}}S{{O}_{3}}$) is a dibasic acid which releases two hydrogens when dissolved in water. Since it's an acid it will change the blue litmus to red.

Hence the correct answer is option(A).

Note: Bear in mind that there is no evidence for the existence of Sulphurous acid in solution, the molecule has been detected in the gas phase. It is also an intermediate species in the formation of acid rain from the oxides of Sulphur. Also, in equilibrium, the solutions of sulfurous acids and protonated sulfites contain a mixture of isomers.