The correct option is (a).
Explanation
Litmus is a commonly used natural acid-base indicator. It is a water-soluble mixture of different dyes used to test the acidity or alkalinity of a solution based on colour changes.
Statement-wise Analysis
- Statement 1 is Correct: Litmus solution is a purple dye extracted from lichens, a plant belonging to the division Thallophyta. It is a symbiotic association between algae and fungi.
- Statement 2 is Incorrect: When litmus solution is in a neutral medium, such as distilled water, its natural colour is purple, not green.
- Statement 3 is Incorrect: The colour change of litmus depends on the pH of the solution:
- An acidic solution turns blue litmus to red.
- A basic (alkaline) solution turns red litmus to blue.
Key Takeaway
Litmus is a natural indicator obtained from lichens. Its neutral colour is purple; it turns red in the presence of acids and blue in the presence of bases.