breaking an object into smaller pieces by using a hammer allowing an iron chain to remain in salt water until it rusts heating a brown substance until it changes into a gray powder reacting vinegar and baking soda to make white foam and a gas
I gave my students this multiple choice question: Which is an example of a physical change?

Only one answer is correct among these 4 choices:
- breaking an object into smaller pieces by using a hammer
- allowing an iron chain to remain in salt water until it rusts
- heating a brown substance until it changes into a gray powder
- reacting vinegar and baking soda to make white foam and a gas

Could you tell me which one is correct?

breaking an object into smaller pieces by using a hammer