Water is a fundamental substance on Earth, making up a significant portion of living organisms and covering most of the planet’s surface. Its unique characteristics arise directly from how its molecules are structured and how these molecules interact.
The Strong Bonds Inside a Water Molecule
A single water molecule, H2O, consists of one oxygen atom covalently bonded to two hydrogen atoms, sharing electrons.
The sharing of electrons within a water molecule is unequal; oxygen has a stronger pull (electronegativity) than hydrogen. This unequal sharing means the oxygen atom develops a slight negative charge, while each hydrogen atom acquires a slight positive charge. This makes the water molecule polar, with distinct positive and negative ends. The molecule also has a bent shape due to unshared electron pairs on the oxygen atom.
The Weaker Bonds Between Water Molecules
Beyond the strong internal covalent bonds, water molecules also form weaker attractions with neighboring water molecules. These intermolecular attractions are called hydrogen bonds. A hydrogen bond forms when the partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of an adjacent water molecule.
While a single hydrogen bond is considerably weaker than a covalent bond, the sheer number of hydrogen bonds formed in a sample of water provides significant collective strength. Each water molecule can potentially form up to four hydrogen bonds. In liquid water, these hydrogen bonds are constantly breaking and reforming, allowing molecules to move past one another. In ice, these bonds become more stable, creating a fixed, organized network.
How These Bonds Make Water Special
The extensive network of hydrogen bonds among water molecules gives water several unusual properties important for life and natural processes.
One such property is its high specific heat capacity. This means water can absorb a large amount of heat energy with only a small increase in temperature because much of the absorbed energy is used to break the numerous hydrogen bonds rather than increasing molecular motion. This property helps moderate Earth’s climate and allows organisms to maintain stable internal temperatures.
Water also exhibits a high heat of vaporization, which is the energy required to change it from a liquid to a gas. To evaporate, water molecules must gain enough energy to completely overcome their hydrogen bonds and escape into the air as vapor. This characteristic is responsible for the cooling effect of sweating, as evaporating water draws heat away from the body.
The cohesive and adhesive properties of water also stem from hydrogen bonding. Cohesion refers to water molecules sticking to each other, forming droplets and allowing for surface tension. Adhesion describes water’s ability to stick to other polar surfaces, which is important for processes like capillary action in plants, where water travels upward through narrow tubes against gravity.
Water displays a unique density anomaly: its solid form, ice, is less dense than its liquid form. As water freezes, its hydrogen bonds arrange molecules into a more open, crystalline lattice structure. This arrangement causes ice to expand and float on liquid water, preventing entire bodies of water from freezing solid from the bottom up and allowing aquatic life to survive in colder climates.