Water, a compound covering a significant portion of our planet and making up a large part of our bodies, participates in countless chemical processes. A common question is whether water functions as a reactant in chemical reactions. Water’s involvement can vary significantly depending on the specific chemical process.
What Defines a Reactant?
A reactant is a substance that undergoes a chemical change during a reaction and is consumed to form new products. Reactants are the starting materials, and their chemical bonds are broken and new ones are formed to create different substances. In a chemical equation, reactants are typically written on the left side of an arrow, pointing towards the products. For instance, in the formation of water, hydrogen gas (H₂) and oxygen gas (O₂) are the reactants that combine to produce liquid water (H₂O).
When Water Acts as a Reactant
Water acts as a reactant when its chemical structure is altered, meaning its hydrogen and oxygen atoms become incorporated into the newly formed molecules. One common example is hydrolysis, a term meaning “splitting with water.” In hydrolysis, water molecules break down larger compounds into smaller ones by cleaving chemical bonds. This process is fundamental in biology, such as during the digestion of food, where complex carbohydrates, proteins, and fats are broken into simpler units.
Photosynthesis, the process by which plants produce their own food, also uses water as a reactant. Plants take in carbon dioxide and water, and with light energy, convert them into glucose (a sugar) and oxygen. During this reaction, water molecules are broken down, and their hydrogen atoms are used to build glucose, while the oxygen atoms are released as oxygen gas.
Another instance where water is a reactant is in hydration reactions. In organic chemistry, hydration involves adding water across a double bond in unsaturated compounds like alkenes or alkynes. For example, ethanol, a common alcohol, can be produced industrially by the hydration of ethene, where a water molecule adds to the ethene molecule. In these reactions, the water molecule is chemically incorporated into the new product, demonstrating its role as a reactant.
Water’s Other Important Roles in Chemistry
While water can act as a reactant, it also plays other significant roles in chemical processes without being chemically consumed. Water is often called the “universal solvent” due to its ability to dissolve a wide variety of substances. Its polar nature, with slightly positive hydrogen atoms and a slightly negative oxygen atom, allows it to attract and surround other polar molecules and ions, enabling them to disperse evenly and react. Many chemical reactions, especially those in biological systems, occur in aqueous solutions where water serves as the medium for reactants to interact.
Water also provides the environment or medium for many reactions to occur. It can facilitate reactions by allowing reactants to come into contact and move freely. In some cases, water can even act as a catalyst, speeding up a reaction without being used up itself. However, water’s role as a solvent and reaction medium is far more common and broadly recognized in chemistry.