Which of the following is necessary to be certain that a chemical change has occurred?

• A. Observe a change in the physical state, such as melting or boiling.
• B. Detect a change in the size or shape of the material.
• C. Measure a change in temperature without any other evidence.
• D. Demonstrate that a new substance with different properties has formed.

Answer: (D)

Verified step by step guidance

1. Understand the difference between physical changes and chemical changes. Physical changes affect the form of a substance but not its chemical composition, such as changes in state (melting, boiling) or shape.
2. Recognize that observing a change in physical state or size/shape alone does not guarantee a chemical change, because these can be physical changes.
3. Note that a temperature change by itself is not definitive evidence of a chemical change, as physical processes can also absorb or release heat.
4. Identify that a chemical change involves the formation of one or more new substances with different chemical properties from the original substances.
5. Therefore, to be certain a chemical change has occurred, you must demonstrate that a new substance with different properties has formed, which can be confirmed through chemical tests or observing new chemical behavior.