Molecular Structure
Water is a simple molecule composed of two hydrogen atoms covalently bonded to one oxygen atom. The oxygen atom has a higher electronegativity compared to hydrogen, creating a polar covalent bond. This polarity arises because the shared electrons are more strongly attracted to the oxygen atom, giving it a partial negative charge (δ−) and leaving the hydrogen atoms with partial positive charges (δ+). The V-shaped, or bent, structure of the water molecule, with a bond angle of approximately 104.5°, enhances its polarity.
Hydrogen Bonding
One of the most significant aspects of water’s chemistry is hydrogen bonding. Due to the polar nature of water molecules, the positive hydrogen end of one molecule is attracted to the negative oxygen end of a neighboring molecule. These hydrogen bonds are relatively weak individually but collectively result in water’s high cohesion, adhesion, surface tension, and its ability to dissolve many substances.
Physical Properties
Water exhibits several anomalous physical properties due to hydrogen bonding. One notable feature is its high specific heat capacity, which allows water to absorb and retain substantial amounts of heat with only a slight change in temperature. This characteristic is critical in stabilizing aquatic environments and regulating climate. Additionally, water has a high heat of vaporization, meaning it requires significant energy to transition from liquid to gas, contributing to cooling mechanisms like sweating and transpiration.
Another unusual property is water’s density behavior. Most substances become denser as they cool; however, water reaches its maximum density at 4°C and becomes less dense as it freezes. This occurs because the hydrogen bonds arrange water molecules into a hexagonal lattice structure, which occupies more space than in the liquid state. As a result, ice floats on liquid water, insulating aquatic life in cold environments.
Solvent Properties
Water is often called the “universal solvent” due to its ability to dissolve a wide variety of substances, particularly ionic and polar compounds. When ionic compounds, like sodium chloride (NaCl), dissolve in water, the positive and negative ions are surrounded by water molecules, which disrupts the ionic bonds. This process, known as hydration, results in the ions becoming solvated and dispersed throughout the solution.
Acid-Base Chemistry
Water can act as both an acid and a base, a property known as amphoterism. It undergoes self-ionization, forming hydronium (H₃O⁺) and hydroxide (OH⁻) ions:
2 H₂O ⇌ H₃O⁺ + OH⁻
The concentration of these ions in pure water at 25°C is 1 × 10⁻⁷ M, making water neutral with a pH of 7. The ability of water to donate or accept protons makes it integral to acid-base reactions and buffering systems in biological and environmental contexts.
Role in Biological Systems
In biological systems, water is a medium for chemical reactions, a reactant in processes like photosynthesis, and a product in cellular respiration. Its role as a solvent facilitates the transport of nutrients, gases, and waste products. Additionally, the cohesive and adhesive properties of water are essential for processes such as capillary action, which enables water movement through plant xylem.
Environmental Significance
Water’s high heat capacity helps stabilize Earth’s climate, while its solvent properties contribute to the geochemical cycling of nutrients. Its phase changes are crucial for weather patterns and the distribution of heat across the globe.
