What is a key indicator that a chemical reaction has taken place?

The formation of gas is a key indicator that a chemical reaction has taken place because it often signifies that new substances are being created as a result of the reaction. When reactants combine to form products, one of the common outcomes can be the release of a gas. This might happen in various types of chemical reactions, such as combustion or acid-base reactions, where bubbles or fumes can be observed.

In addition to this, gas formation can often be accompanied by other indicators of a chemical change, such as a change in color or the release of energy in the form of heat or light. These observations collectively signal that a chemical transformation has occurred, as opposed to a mere physical change, where substances maintain their chemical identity. In contrast, other options do not necessarily indicate a chemical change has occurred; for example, physical mixing of substances does not alter their chemical properties and typically maintains the individual characteristics of each substance involved.