Question

what are indicators and olfactory indicators ?

definition will be ncert based

Answer 1

Those indicators whose odour(smell) changes in acidic or basic medium are called olfactory indicators. Onion(juice or extract) and Vanilla are the two examples of olfactory indicators. 

Those chemical substances (dyes) which behave differently in acidic and basic medium and indicate by physical changes.For ex-Litmus

Answer 2

Heya !!

★ Indicators :-

(1) An indicator is a dye that changes colour when it is put into an acid or base.

(2) An indicator tells whether the substance we are testing is an acid or a Base by changes in it's colour.

(3) The three common indicators to test for acids and bases are :
(a) litmus (b) Methyl orange (c) phenolphthalein

(4) An acids turns blue litmus to red .

(5) A base ( or alkali ) turns red litmus to blue.

(6) Litmus is a natural indicator whose natural colour is purple.

(7) Methyl orange and phenolphthalein are synthetic indicators.

(8) The natural colour of Methyl orange is orange.

★ Olfactory indicator :- Those substance whose smell ( or odors ) change in acidic or basic solution are called olfactory indicator.

Onion and vanilla extract are olfactory indicators.

Hope it will help you :-)