Which of the following changes is evidence of a chemical reaction?

• A. Evaporation of alcohol
• B. Dissolving sugar in water
• C. Formation of a gas when two solutions are mixed
• D. Melting of ice into liquid water

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 gas formation, color change, temperature change, or precipitate formation.
2. Analyze each option to determine if it represents a physical change or a chemical change: Evaporation of alcohol is a physical change because it involves a phase change without altering the chemical identity.
3. Dissolving sugar in water is also a physical change since sugar molecules disperse in water but do not chemically transform into new substances.
4. Melting of ice into liquid water is a physical change involving a phase transition from solid to liquid without changing the chemical composition of H₂O.
5. Formation of a gas when two solutions are mixed indicates a chemical reaction because new substances are produced, often evidenced by bubbling or fizzing, which is a clear sign of chemical change.