Here's why:
* Chemical reactions involve the rearrangement of atoms: In a chemical reaction, bonds between atoms in the reactants break, and new bonds form to create the products. This changes the arrangement of atoms and therefore the chemical composition of the substances.
* No change in composition = no reaction: If the products have the same chemical composition as the reactants, it means the atoms have not rearranged, and no chemical reaction has taken place.
Example:
* Reactants: Hydrogen gas (H₂) and oxygen gas (O₂)
* No reaction: If you mix hydrogen and oxygen, they remain as separate gases. The composition of the mixture is still hydrogen and oxygen.
* Reaction: If you ignite the mixture, a reaction occurs, forming water (H₂O). The composition has changed from hydrogen and oxygen to water.
