Acids, bases & salts
Indicator — নির্দেশক
Quick answer
A substance that changes colour in acid versus base — litmus, phenolphthalein and methyl orange are the classics.
Definition
An indicator is a substance that changes colour depending on whether it meets an acid or a base, so chemists use it to test solutions quickly. Litmus turns red in acid and blue in base; phenolphthalein stays colourless in acid but turns pink in base; methyl orange gives red in acid and yellow in base. Universal indicator or a pH meter goes further and reveals the actual pH value. Many indicators come from plants.
For example
Red cabbage water is a home-made indicator: lemon juice turns it pink-red, and soap water turns it green.