Which of the following is NOT a sign of a chemical reaction?

A change in pressure is not typically considered a sign of a chemical reaction. While changes in pressure can occur during physical changes or due to environmental factors, they do not directly indicate that a chemical transformation has taken place. Chemical reactions are more definitively indicated by observable signs such as a color change, the formation of a gas, or the production of a precipitate.

Color change often occurs when new substances are formed during a chemical reaction due to the rearrangement of atoms and the formation of bonds. The formation of a gas can be a result of the reaction of solids or liquids, creating gaseous products that signify chemical change. Similarly, the formation of a precipitate indicates that a new solid substance has formed in a solution, marking the occurrence of a chemical reaction. These signs are associated with the alteration of chemical structures, while a change in pressure alone does not necessarily imply that a chemical change has occurred.