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

Changes in mass can occur without indicating that a chemical reaction has taken place. For example, during physical processes such as evaporation or dissolution, mass can change due to the phase of the substances involved but does not necessarily imply a chemical transformation has occurred.

In contrast, color change, gas production, and the formation of a precipitate are all strong indicators of a chemical reaction. A color change often signifies that new substances have formed which absorb or emit light differently than the reactants. Gas production is another clear sign, suggesting that gas is being released or created, indicative of a reaction taking place. The formation of a precipitate, which is an insoluble solid that emerges from a solution, is also a hallmark of a chemical reaction where new compounds have formed.