Indicators change color when placed different substances because they undergo a chemical reaction with them. This chemical reaction causes a change in the structure of the indicator molecule, which in turn changes the way it absorbs light. As a result, the indicator appears to change color.

The specific color change that an indicator undergoes depends on the type of indicator and the substance it is reacting with. For example, phenolphthalein is an indicator that turns pink in the presence of a base. This is because the hydroxide ions in the base react with the phenolphthalein molecule to form a new compound that is pink in color.

Other indicators, such as litmus, change color in the presence of both acids and bases. Litmus turns red in the presence of an acid and blue in the presence of a base. This is because the hydrogen ions in an acid react with the litmus molecule to form a new compound that is red in color, while the hydroxide ions in a base react with the litmus molecule to form a new compound that is blue in color.

The ability of indicators to change color makes them a valuable tool in chemistry. They can be used to identify the presence of acids and bases, and to determine the pH of a solution.