Final answer:
A change in phase (solid to liquid) is not a sign of a chemical reaction. Changes in temperature, formation of a precipitate, and changes in color are all signs of a chemical reaction.
Step-by-step explanation:
A change in phase (solid to liquid) is not a sign that a chemical reaction has occurred. A change in phase is considered a physical change, not a chemical change. In a chemical reaction, the substances involved undergo a change in their chemical composition, resulting in the formation of new substances with different properties.
On the other hand, changes in temperature, formation of a precipitate, and changes in color are all signs that a chemical reaction has occurred. For example, if a reaction gives off heat or absorbs heat, it indicates a change in energy and suggests a chemical reaction. Formation of a solid (precipitate) from two aqueous solutions is another indication of a chemical reaction, as is a noticeable change in color.
Learn more about Signs of a chemical reaction