Water (H₂O) exhibits diverse chemical behaviors due to its unique structure. Its bent shape and the significant electronegativity difference between oxygen and hydrogen atoms result in a polar molecule, with a slight positive charge on the hydrogen side and a slight negative charge on the oxygen side. This polarity enables water molecules to form hydrogen bonds with each other and with other polar substances, contributing to its designation as the “universal solvent.” Beyond dissolving compounds, water’s interactive nature extends to direct chemical reactions with various elements and compounds. This reactivity is fundamental across chemical and biological systems.
Reacting with Metals
Water’s interaction with metals varies significantly based on the metal’s reactivity. Highly reactive metals, such as alkali metals like sodium and potassium, and some alkaline earth metals like calcium, react vigorously with water. These reactions are often exothermic, releasing substantial heat, and produce hydrogen gas along with a metal hydroxide. For instance, when sodium reacts with water, it displaces hydrogen to form sodium hydroxide and hydrogen gas, often igniting the hydrogen due to the heat generated: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g).
Less reactive metals, like iron, react much slower with water, typically in the presence of oxygen, leading to corrosion or rusting. Rusting is a complex electrochemical process where iron reacts with oxygen and water to form hydrated iron(III) oxide, commonly known as rust. A simplified representation is 4Fe(s) + 3O₂(g) + 6H₂O(l) → 4Fe(OH)₃(s), which then dehydrates to form rust. Noble metals such as gold and platinum are generally unreactive with water under normal conditions, exhibiting high resistance to corrosion.
Reacting with Non-Metals and Gases
Water reacts with various non-metals and atmospheric gases. Carbon dioxide, a common atmospheric gas, dissolves in water to form carbonic acid. This reversible reaction, H₂O(l) + CO₂(g) ⇌ H₂CO₃(aq), plays a significant role in natural phenomena, contributing to ocean acidification as increased atmospheric CO₂ lowers the water’s pH.
Halogens, like chlorine, react with water in processes used for disinfection. When chlorine gas is added to water, it forms hypochlorous acid (HOCl) and hydrochloric acid (HCl). This reaction, Cl₂(g) + H₂O(l) ⇌ HOCl(aq) + HCl(aq), is important for water treatment because hypochlorous acid is a potent disinfecting agent. Similarly, sulfur dioxide and sulfur trioxide, common pollutants, react with water in the atmosphere to form sulfurous and sulfuric acids. These reactions contribute significantly to acid rain, impacting ecosystems and structures. Conversely, some non-metals, such as nitrogen gas, are largely unreactive with water under normal conditions.
Reacting with Acids, Bases, and Salts
Water possesses an amphoteric nature, meaning it can act as both a weak acid and a weak base. This is evident in its autoionization, where two water molecules react to produce a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻): 2H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq).
When strong acids are added to water, water acts as a base, accepting a proton to form hydronium ions. For example, hydrochloric acid reacts with water as HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq). Conversely, with strong bases, water facilitates their dissociation to produce hydroxide ions. For weak bases, water acts as an acid, donating a proton; for instance, ammonia reacts with water to form ammonium ions and hydroxide ions: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).
Water is a product in neutralization reactions between acids and bases. In these reactions, hydrogen ions from the acid combine with hydroxide ions from the base to form water molecules. Water can also participate in the hydrolysis of certain salts, where salt ions react with water to produce acidic or basic solutions. For example, salts from a strong acid and a weak base, like ammonium chloride (NH₄Cl), yield an acidic solution when dissolved in water. Salts from a strong base and a weak acid, like sodium carbonate (Na₂CO₃), produce a basic solution.