Water (H₂O) is a common and stable compound, consisting of two hydrogen atoms covalently bonded to a single oxygen atom. The strength of these bonds contributes to the molecule’s stability. Despite this stability, water can be intentionally broken down at a molecular level. The process required to split the water molecule is a pure chemical change, fundamentally altering its identity.
Distinguishing Chemical Change from Physical Change
Any change to matter is categorized as either physical or chemical. A physical change alters the form or state of a substance, but the molecular structure remains the same. For example, when water freezes or boils, it is still H₂O. This transformation is typically reversible and does not involve breaking chemical bonds.
A chemical change, conversely, involves the breaking and forming of chemical bonds, creating entirely new substances with different properties. To break down water, the strong covalent bonds between hydrogen and oxygen atoms must be disrupted. This bond disruption requires a significant energy input, confirming the process as a non-spontaneous chemical reaction.
Electrolysis The Method for Breaking Down Water
The method used to force the chemical breakdown of water is called electrolysis, which means “breaking down using electricity.” This process uses electrical energy to drive a non-spontaneous chemical reaction, providing the necessary energy to overcome the strong H-O bonds. A basic setup involves two inert electrodes, typically platinum, submerged in water that has been made conductive by adding an electrolyte like an acid or salt.
An external power source is connected to these electrodes, creating a cathode (negative terminal) and an anode (positive terminal). At the cathode, a reduction reaction occurs where water molecules gain electrons, forming hydrogen gas (\(\text{H}_2\)). Simultaneously, an oxidation reaction takes place at the anode, where water molecules lose electrons to form oxygen gas (\(\text{O}_2\)).
The overall chemical equation for the decomposition is \(2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2\). This stoichiometry means that the volume of hydrogen gas produced is always twice the volume of oxygen gas produced. Electrolysis is an endothermic process, meaning it absorbs energy from the surroundings to facilitate the decomposition. The electric current acts as the driving force, pulling the water molecule apart into its constituent elements.
The Resulting Elements and Reforming Water
The result of this decomposition is the formation of two distinct elemental gases: hydrogen (\(\text{H}_2\)) and oxygen (\(\text{O}_2\)). These products are fundamentally different from liquid water, possessing unique properties such as being highly flammable (hydrogen) and a strong oxidizer (oxygen). The transformation confirms a true chemical change has occurred, as the original substance is replaced by new substances with altered chemical identities.
The process of breaking down water is chemically reversible, meaning the products can be recombined to reform the original compound. When hydrogen and oxygen gases are mixed and supplied with activation energy, such as a spark, they undergo a rapid and highly exothermic combustion reaction. This synthesis reaction, \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\), releases the energy that was originally put in to split the water molecules. The heat and light released during the recombination confirms the conservation of mass and energy.