Activity 2.2 NCERT Class 10 Science, Chapter 2 Acids, Bases, and Salts.
Activity 2.2 asks us to check the change in odour and colour of cloth soaked with onion and vanilla with acids and bases.
Bases deodorise onion and vanilla but do not change the colour.
The odour and colour of Onion:
Onion contains sulphurous allium. It gives the peculiar onion odour. It is acidic and reacts with bases like sodium hydroxide to give the odourless compound. So when we add bases to onion soaked clot, its smell disappears.
Allium does not react with acids, so when we add acids to onion soaked cloth smell remains in the cloth.
Colour of onion: There is no relation to the reaction and colour. So the colour remains the same.
The odour and colour of Vanilla:
Vanilla contains an aldehyde which gives it a pleasant odour. The aldehyde is also weak acids. They react with bases, and the smell disappears.
Similar to onion vanilla colour also has no relation with bases and acids. As a result, we do not see any changes in the colour of vanilla with acids or bases.
You must have seen onion dipped in a red colour solution in home or restaurant. This red colour solution is potassium permanganate. It is a slightly basic salt. It deodorises the onion smell and also helps in preserving onion for a long duration.
Reference: NCERT textbook for class 10 Science.