Question

which evidence is an indication that a chemical reaction has occurred? melting of a substance formation of a gas boiling of a substance freezing of a substance

Explanation:

Melting, boiling, and freezing are physical changes (changes in state) where the chemical composition of the substance remains the same. Formation of a gas (in a non - phase change context, i.e., when it is a result of a chemical reaction like in a reaction between an acid and a carbonate producing carbon dioxide) is an indication of a chemical reaction. In a chemical reaction, new substances are formed.

Answer:

formation of a gas