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 am hesitating between 4 options to answer the following question, which option should I choose?
Question: Which observation indicates that a chemical reaction has taken place?
Possibilities:
- 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.

A new chemical substance is formed.