* New substances are formed: The reaction between sodium (Na) and water (H₂O) produces sodium hydroxide (NaOH) and hydrogen gas (H₂). These are completely different substances from the original reactants.

* Chemical bonds are broken and formed: The sodium metal bonds with water molecules, breaking the existing bonds and forming new ones to create hydroxide and hydrogen gas.

* Energy is released: The reaction is exothermic, meaning it releases heat, which is often enough to ignite the hydrogen gas, leading to the observed fire.

Key indicators of a chemical change:

* Formation of new substances

* Change in color

* Release of heat or light

* Production of gas bubbles

In this case, all of these indicators are present, confirming the reaction as a chemical change.