Which of the following most likely represents a chemical change?

• A. Salt dissolving in water
• B. Water boiling to form steam
• C. Ice melting into liquid water
• D. Iron rusting when exposed to air

Answer: (D)

Verified step by step guidance

1. Step 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. Step 2: Analyze each option to determine if it involves a change in chemical composition or just a change in physical state or form.
3. Step 3: Salt dissolving in water is a physical change because the salt dissociates into ions but does not chemically react or form a new substance.
4. Step 4: Water boiling to form steam and ice melting into liquid water are both physical changes because they involve changes in the state of matter (liquid to gas, solid to liquid) without altering the chemical identity of H2O.
5. Step 5: Iron rusting when exposed to air is a chemical change because iron reacts with oxygen and moisture to form a new compound, iron oxide, which has different properties from elemental iron.