A substance becomes warmer. A material changes from a solid to liquid. Bubbles form when a liquid is heated. A new chemical substance is formed.
I gave my students this multiple choice question: Which observation indicates that a chemical reaction has taken place?

Only one answer is correct among these 4 choices:
- A substance becomes warmer.
- A material changes from a solid to liquid.
- Bubbles form when a liquid is heated.
- A new chemical substance is formed.

Could you tell me which one is correct?

A new chemical substance is formed.