Which sign does not typically indicate a chemical reaction has occurred?

The observation of no observable change in color is typically not an indicator of a chemical reaction. In a chemical reaction, various signs can indicate that reactants have transformed into products. These signs include the formation of a precipitate, which shows that a solid has formed from the solution; gas production, which may be evidenced by bubbles or a change in pressure; and temperature changes, which can indicate either an exothermic or endothermic reaction.

In contrast, the absence of observable changes, such as in color, does not signify that a reaction has occurred. It's possible for a reaction to take place without any noticeable changes to the substances involved, particularly in cases where reactants and products are similar in appearance or when the reaction occurs below the threshold of human perception. Thus, the lack of observable change, particularly in color, does not provide evidence for a chemical reaction and could suggest that no reaction has occurred at all.