Which of the following best describes a situation when reactants are not used to make new substances?

• A. A chemical reaction, such as rusting iron
• B. A physical change, such as melting ice
• C. Combustion of methane
• D. Neutralization of an acid with a base

Answer: (B)

Verified step by step guidance

1. Understand the difference between a chemical reaction and a physical change: A chemical reaction involves the rearrangement of atoms to form new substances, while a physical change affects the state or appearance without creating new substances.
2. Analyze the examples given: Rusting iron, combustion of methane, and neutralization of an acid with a base all involve chemical reactions where new substances are formed.
3. Consider the example of melting ice: This is a physical change where water changes from solid to liquid, but the chemical composition (H2O) remains the same.
4. Identify that the situation where reactants are not used to make new substances corresponds to a physical change, such as melting ice.
5. Conclude that the best description for when reactants are not used to make new substances is a physical change, exemplified by melting ice.