(a) Indicators are substances that show a distinct change (colour or odour) simultaneous to a change in concentration of ions such as H+ and OH- ions.
Examples: Methyl orange, phenolphthalein and litmus solution.
(b) A purple dye is extracted from a plant called lichen. This dye is called a litmus solution and is commonly used as an indicator.
(c) When turmeric paper is put into an alkaline solution, it turns red.