By Catherine Carney | Updated Mar 24, 2022
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Water molecules are polar, with a slightly negative charge on the oxygen atom and a slightly positive charge on the hydrogen atoms. This polarity creates hydrogen bonds—attractive forces between the opposite charges of neighboring molecules. The result is cohesion (water sticks to itself) and adhesion (water sticks to other hydrophilic surfaces). These properties give water its high surface tension, enabling phenomena such as capillary action in plants and the “stickiness” of a puddle.
Water’s hydrogen bonding gives it a high specific heat, a high heat of vaporization, and a powerful evaporative cooling effect. Because it takes more energy to break these bonds, water resists rapid temperature changes, buffering ecosystems and organisms against extreme heat or cold. The evaporative cooling of sweating or transpiration is a prime example of this natural thermostat.
Polarity also makes water an excellent solvent for other polar, or hydrophilic, molecules. It readily dissolves salts, sugars, and many biological macromolecules, facilitating countless biochemical reactions. In contrast, non‑polar, hydrophobic substances such as oils remain largely undissolved, forming distinct phases.
Unlike most liquids, water expands when it freezes. The rigid lattice formed by hydrogen bonds occupies more volume than the fluid arrangement, so ice is less dense than liquid water. This causes ice to float, insulating bodies of water and preserving aquatic life during cold seasons.
Pure water dissociates into roughly equal concentrations of H⁺ and OH⁻ ions, yielding a neutral pH of 7. This neutrality allows it to serve as a buffer in biological systems, adjusting to slightly acidic or basic conditions required by diverse enzymes.
These five interconnected properties—cohesion/adhesion, temperature stability, solvent capability, expansion upon freezing, and neutral pH—make water the cornerstone of all known life.
