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Substances that exhibit a change of colour or odour when added to acidic, basic and neutral solutions are called indicators.
Visual indicators show different colours in presence of an acidic and a basic solution. Similarly, olfactory indicators exhibit different odour when added to acids or bases. Thus, they help us in deciding whether a given solution is acidic or basic in nature.
Some of the naturally occurring indicators are china rose petals, litmus, turmeric, etc.