Which of the following events indicates that a chemical reaction might have occurred?

• A. Evaporation of water
• B. Change in physical state from solid to liquid due to melting
• C. Dissolving sugar in water
• D. Formation of a precipitate

Answer: (D)

Verified step by step guidance

1. Understand the difference between physical changes and chemical changes. A physical change affects the form of a substance but not its chemical composition, while a chemical change results in the formation of new substances with different properties.
2. Analyze each event given: Evaporation of water is a physical change because water changes from liquid to gas without altering its chemical identity.
3. Melting of a solid to a liquid is also a physical change since the substance's chemical structure remains the same despite the change in state.
4. Dissolving sugar in water is a physical change because sugar molecules disperse in water but do not chemically react to form new substances.
5. Formation of a precipitate indicates a chemical reaction because it involves the creation of a new solid substance from a solution, signaling that new chemical species have formed.