What are olfactory indicators ? Write an activity to prove them

Olfactory Indicators : There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators.
Activity :
Aim : To check the olfactory indicator.
Required Materials : 1. Onions 2. Knife 3. Plastic bag 4. Clean clothes. Procedure :

  1. Take some onions and finely chop them.
  2. Put the chopped onions in a plastic bag with some clean cloth.
  3. Tie up the bag tightly and keep it overnight in the fridge.
  • Observation : Check the odour of the cloth strips.
    Result: It is used as the acid indicator.