An indicator is a chemical compound that changes its color in the presence of an acid or base. Indicators are used to determine whether a substance is acidic or basic by observing the change in color. There are two types of acid-base indicators: natural and synthetic. Natural indicators are derived from plants and include litmus, China rose, and turmeric. Synthetic indicators are man-made and include phenolphthalein. Indicators are used in various fields, including chemistry, biology, and medicine. In Class 7 Science, indicators are taught as part of the Acids, Bases, and Salts chapter.