The separation of water into oxygen and hydrogen is a chemical change because the original substance (water) undergoes a chemical reaction and is transformed into new substances (oxygen and hydrogen). This process, known as electrolysis, involves the breakdown of water molecules (H2O) into their constituent elements, oxygen (O2) and hydrogen (H2), through the application of an electric current.

During electrolysis, water molecules are subjected to an electric current, which causes the H2O molecules to split apart. This splitting occurs because electricity provides the energy (activation energy) needed to break the chemical bonds holding the water molecules together. At the cathode (negative electrode), hydrogen ions (H+) gain electrons from the electric current and are reduced to hydrogen gas (H2).

2H+ (aqueous) + 2e- → H2 (gas)

Simultaneously, at the anode (positive electrode), oxygen gas (O2) is produced when oxygen ions (O2-) react with water molecules and release electrons back into the electric circuit.

4OH- (aqueous) → 2H2O(l) + O2 (g) + 4e-

Overall, the equation for water electrolysis can be represented as:

2H2O (l) → 2H2 (g) + O2 (g)

Since new substances are formed during the process, namely hydrogen and oxygen gas, through a chemical reaction driven by an electric current, the electrolysis of water is considered a chemical change.