Which pair of olfactory indicators loses its smell in a basic solution?

The correct option is Vanilla essence and clove oil.

What are Olfactory Indicators?
Olfactory indicators are substances whose smell (odour) changes depending on whether they are mixed with an acidic or a basic solution. These indicators are particularly useful for visually impaired students in laboratory experiments to identify acids and bases.

Behavior of Olfactory Indicators in Acidic and Basic Solutions:
1. Vanilla Essence: It has a characteristic sweet smell. When treated with an acidic solution, its smell remains unchanged. However, when it is treated with a basic solution (like sodium hydroxide, NaOH), it loses its characteristic smell.
2. Clove Oil: It has a distinct, strong odor. Similar to vanilla, the smell of clove oil is retained in acidic medium, but completely disappears or becomes undetectable in basic medium.
3. Onion: Freshly chopped onion or cloth strips treated with onion juice retain their characteristic smell in acids, but lose it in basic solutions.

Evaluating the Given Options:
- Vanilla essence and clove oil: Both are olfactory indicators, and both lose their smell in a basic solution. Hence, this is the correct pair.
- Onion and blue litmus: While onion is an olfactory indicator, blue litmus is a chemical indicator that changes color (turns red in acid and stays blue in base), not smell.
- Turmeric and onion: Onion is an olfactory indicator, but turmeric is a natural indicator that changes its color to reddish-brown in basic solutions instead of changing its smell.
- Phenolphthalein and onion: Onion is an olfactory indicator, but phenolphthalein is a synthetic color indicator that turns deep pink in basic solutions.