Which of the following is an example of a chemical change, and what type of evidence supports this classification?

• A. Water freezing; change in state without new substance formation
• B. Salt dissolving in water; physical mixing without chemical reaction
• C. Breaking a glass; change in shape but no new substance formed
• D. Iron rusting; formation of a new substance (iron oxide)

Answer: (D)

Verified step by step guidance

1. Understand the difference between physical 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 one or more new substances with different properties.
2. Analyze each example given: Water freezing involves a change in state (liquid to solid) but the chemical identity of water (H2O) remains the same, so this is a physical change.
3. Salt dissolving in water is a physical process where salt dissociates into ions but no new substance is formed; the process is reversible, indicating a physical change.
4. Breaking a glass changes its shape and size but does not alter the chemical composition of the glass material, so this is also a physical change.
5. Iron rusting involves a chemical reaction between iron and oxygen in the presence of moisture, producing iron oxide, a new substance with different properties; this is evidence of a chemical change.