By Diane Evans Updated Aug 30, 2022
Water contains two distinct types of bonds: covalent bonds that hold the oxygen and hydrogen atoms together within each molecule, and hydrogen bonds that link one water molecule to another. The covalent bonds give water its molecular structure, while the hydrogen bonds create the network that defines the bulk properties of the liquid.
In liquid water, hydrogen bonds are relatively weak, but their sheer number dominates the molecule’s behavior. They arise from electrostatic attraction between the partially positive hydrogen atoms and the partially negative oxygen atoms. Because the molecules are constantly in motion, these bonds form and break dynamically. Heating increases molecular kinetic energy, strengthening the tendency for bonds to break and allowing water to vaporize. In the gaseous phase, water molecules drift independently; once they cool, hydrogen bonds re‑establish and the liquid is re‑formed.
Ice adopts a crystalline lattice in which each water molecule is tetrahedrally coordinated by four neighbors through hydrogen bonds. This ordered arrangement restricts molecular motion, making ice less dense than liquid water. As a result, ice floats, providing a protective blanket over bodies of water that supports aquatic life during winter.
Water’s polarity—an uneven distribution of charge caused by the electronegative oxygen atom—allows it to surround and separate ions and polar molecules. The small size of water molecules lets many of them cluster around a solute, forming hydrogen bonds that draw the solute apart. This explains why water dissolves more substances than any other liquid, earning its title as the “universal solvent.”
Hydrogen bonding confers high cohesion and surface tension, evident when droplets bead on waxed surfaces. It also accounts for water’s high heat of vaporization, which makes sweating an effective cooling mechanism for mammals. The large energy required to break hydrogen bonds means that water remains liquid over a wide temperature range, supporting life’s processes.
Beyond self‑interaction, water hydrogen‑bonds with molecules that possess hydroxyl (OH) or amine (NH₂) groups, a feature critical to countless biochemical reactions. Water’s ability to stabilize structures and transport molecules, coupled with its thermal buffering capacity, has been indispensable for the evolution of life on Earth.
