Question

This lab question is: How can you distinguish a physical change from a chemical change?

Which statements below are an appropriate hypothesis in response to this question? Check all that apply.

If a substance changes state of matter (e.g., liquid to gas), then this is a physical change because the original properties of the substance have not changed.

If a substance undergoes a physical change, then it will retain most of its original properties because no new substance is formed.

If a substance undergoes a chemical change, then it will not retain its original properties because a new substance is formed.

If a precipitate is formed when two substances are mixed, then a chemical change has taken place because precipitates are new substances.

Answer 1

All four statements are appropriate hypotheses. Brief justification for each:

- If a substance changes state (liquid → gas), then this is a physical change because the chemical identity remains the same — a testable prediction about state changes.
- If a substance undergoes a physical change, then it will retain most of its original properties because no new substance is formed — matches the definition of a physical change and is testable.
- If a substance undergoes a chemical change, then it will not retain its original properties because a new substance is formed — describes the expected outcome of a chemical reaction.
- If a precipitate is formed when two substances are mixed, then a chemical change has taken place because precipitates are new (insoluble) products — precipitation is evidence of a chemical reaction.

Minor caveat: physical changes can alter some physical properties (e.g., density, shape, conductivity) even though composition stays the same — the hypotheses above still correctly distinguish chemical vs. physical changes.