* Formation of a new substance: Bromine (Br₂) is a reddish-brown liquid, and sodium (Na) is a silvery-white solid. When they react, they form sodium bromide (NaBr), a white solid. This new substance has different properties than the original reactants.

* Breaking and forming of bonds: During the reaction, the bonds within the bromine molecule (Br-Br) and the metallic bonds in sodium are broken. New bonds are formed between sodium and bromine ions (Na⁺Br^-) in the sodium bromide crystal structure.

* Energy change: Chemical reactions typically involve an energy change, either releasing or absorbing heat. The reaction between bromine and sodium is exothermic, meaning it releases heat.

These factors all indicate that a chemical change has occurred, not just a physical change like a change in state or appearance.