There are several indications that a chemical reaction has occurred. Heat and/or light are produced in the case of combustion and some other exothermic reactions. The generation of gas, often seen as bubbles in a liquid, is another indication. An example is the reaction of vinegar with baking soda, which produces carbon dioxide gas. A color change is another indication that a chemical reaction has taken place, as it provides evidence that a new substance has formed.
Chemical indicators are substances that exist is different forms, having different colors, at different pHs. A common example is litmus, which is red in its acid form and blue in its base form. Because a color change takes place at a specific pH an indicator can be used to show pH changes in a chemical reaction. They are commonly used to signal the endpoint in an acid-base titration. for example, as soon as all of the hydroxide ion in a sample being titrated has reacted with hydrogen ion that is being added, additional hydrogen ion added will react with the indicator molecules in the sample being titrated, changing them to the base form of the molecule and changing the color.