Which of the following would not necessarily indicate that a chemical reaction has occurred?

• A. Production of a precipitate in a solution
• B. Formation of a gas when two solutions are mixed
• C. A change in state, such as melting of ice
• D. A color change after mixing two substances

Answer: (C)

Verified step by step guidance

1. Understand that a chemical reaction involves the formation of new substances with different chemical properties, often accompanied by observable changes such as color change, gas production, or precipitate formation.
2. Recognize that the production of a precipitate (a solid formed in a solution) indicates a chemical reaction because it shows new substance formation.
3. Note that the formation of a gas when two solutions are mixed also indicates a chemical reaction, as gas evolution is a sign of new products being formed.
4. Consider that a color change after mixing two substances usually signals a chemical reaction, as it often results from the creation of new compounds with different light absorption properties.
5. Realize that a change in state, such as melting of ice, is a physical change, not a chemical reaction, because the substance (water) remains chemically the same despite the phase change.