It is chemically possible to create water (\(text{H}_2text{O}\)) by combining its constituent elements, hydrogen and oxygen. While manufacturing water is possible in a laboratory setting, the process is not a practical, large-scale method for solving water scarcity. The chemical synthesis of water is a high-energy reaction that illustrates the fundamental principles of chemical bonding and energy exchange.
The Chemical Recipe for Water
The creation of water relies on combining two volumes of pure hydrogen gas (\(text{H}_2\)) with one volume of pure oxygen gas (\(text{O}_2\)). Both hydrogen and oxygen naturally exist as diatomic molecules. The successful reaction requires these existing molecular bonds to be broken so the atoms can rearrange into water molecules.
The balanced chemical equation is \(2text{H}_2 + text{O}_2 rightarrow 2text{H}_2text{O}\). Simply mixing the two gases at room temperature will not result in a reaction because the mixture needs an energy input to begin the process. This necessary starting energy is known as the activation energy, which serves to destabilize the reactant bonds.
A small spark, high heat, or a catalyst is required to provide the initial activation energy to trigger the synthesis. Once initiated, the reaction proceeds rapidly in a combustion event. The product formed is pure water vapor, which must then be cooled and condensed to collect as liquid water.
The Energy Cost of Making Water
The synthesis of water is a highly exothermic reaction, meaning it releases a significant amount of energy, which is why the reaction often results in a loud explosion. The energy released when liquid water is formed is approximately \(-285.8 text{ kJ}\) for every mole of water produced. This negative value for the standard enthalpy of formation indicates that water has a much lower energy state than the reactants, making the reaction thermodynamically favorable once started.
The practical challenge lies in the energy required to obtain the pure hydrogen and oxygen reactants. The most common industrial method for producing these gases is through the electrolysis of water, which is the exact reverse of the synthesis reaction. This process requires a substantial input of electrical energy to break the strong bonds within the water molecule.
From a thermodynamic perspective, the energy used to create the reactants must always be greater than the energy released when the water is formed, resulting in a net energy loss. This makes synthesis an impractical and costly source of water compared to purification methods like desalination or filtration. The capital cost for the equipment required for water synthesis is estimated to be 50 to 420 times higher than for a reverse osmosis desalination plant.
Where Water Is Made Naturally
While chemical synthesis is an industrial process, the creation of water molecules is a constant process occurring throughout nature, from the smallest cells to the vastness of space. In biology, water is created continuously as a byproduct of aerobic respiration, the process cells use to convert food into usable energy. Cellular respiration uses glucose and oxygen to produce carbon dioxide, energy in the form of adenosine triphosphate, and six molecules of water (\(text{C}_6text{H}_{12}text{O}_6 + 6text{O}_2 rightarrow 6text{CO}_2 + 6text{H}_2text{O}\)).
Water is also formed in Earth’s atmosphere through chemical reactions that clean the air. The highly reactive hydroxyl radical (\(text{OH}\)), often called the atmosphere’s detergent, plays a role in breaking down pollutants. When the hydroxyl radical reacts with an organic compound, it strips a hydrogen atom, resulting in the formation of a water molecule.
The largest natural synthesis of water occurs on a cosmic scale in the interstellar medium, specifically within cold, dense molecular clouds. In these frigid environments, hydrogen atoms adhere to the surface of microscopic dust grains. These grains act as natural catalysts, allowing oxygen atoms to bond with hydrogen atoms, forming water molecules that freeze onto the grain surface. These icy mantles eventually become the building blocks for new stars and planets.