Here's why:

* Chemical reactions involve the rearrangement of atoms and molecules. This means the original reactants are transformed into entirely new products with unique properties.

* Physical changes like melting, freezing, boiling, or dissolving, don't involve the creation of new substances. The chemical composition remains the same.

Signs of a chemical reaction:

While the formation of new substances is the definitive proof, here are some common signs that often indicate a chemical reaction:

* Change in color: A clear solution turning cloudy or a solid changing color.

* Production of gas: Bubbles forming, fizzing, or an odor being released.

* Production of heat or light: A reaction that gets hot or emits light (think of a fire).

* Formation of a precipitate: A solid forming out of a solution.

* Change in pH: A solution becoming more acidic or alkaline.

Remember: These signs are indicators, but they can also occur in some physical changes. It's essential to look for the fundamental change in chemical composition for conclusive evidence.