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Indicators are used in a reaction to identify the endpoint or determine whether it is an acid or a base based on the colour change.
Physical change: Any substance that changes its physical properties such as size, shape, colour, state of the substance etc., where no new substance is formed is known as physical change. Physical changes are reversible (i.e. they can be brought back to their original form) in nature.
Cutting vegetables
Chopping vegetables, cutting paper etc.
Chemical change: Any substance that changes its chemical properties is known as chemical change. In this change, a new substance is formed, and these changes are irreversible (i.e. they cannot be brought back to their original form) in nature.
Cooked food
Burnt paper turning into ash, baking of idly, rusting of iron, curd formation, etc.
The fermentation process takes place in idly batter with the help of bacteria, making the idly soft when cooked. Similarly, fermentation takes place during curd formation.

The rusting of in iron objects is an example of chemical change where iron reacts with atmospheric moisture and forms rust. The process involved here is the oxidation of iron. These are chemical changes that occur around us that are irreversible.
Natural indicators are obtained from naturally occurring substances, for example, turmeric, red cabbage juice and beetroot.
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Natural indicators
Natural indicators are mostly used for acid-base reactions. For example, curcumin is a substance present in turmeric, which helps to indicate whether it is an acid or a base based on colour change. These natural indicators are used for testing food substances, whether it's an acid or base.