Final answer:
The breakdown of water molecules into hydrogen and oxygen gas is a chemical change involving the rearrangement of atoms and the breaking and forming of chemical bonds.
Step-by-step explanation:
The breaking down of water molecules into hydrogen and oxygen gas is classified as a chemical change. This process, also known as electrolysis, involves the rearrangement of the atoms within water (H₂O) molecules to produce hydrogen gas (H₂) and oxygen gas (O₂).
The reaction can be represented by the chemical equation 2H₂O(l) → 2H₂(g) + O₂(g). During this reaction, the chemical bonds between the atoms in the water molecules are broken, and new bonds are formed between the hydrogen atoms and between the oxygen atoms, resulting in the formation of hydrogen and oxygen gases.