Water is a fundamental substance on Earth, present in vast oceans and within the microscopic interiors of cells. Its remarkable properties, unlike many other liquids, arise from a specific attraction between its molecules. These characteristics enable water to perform diverse biological and environmental roles.
Hydrogen Bonds: The Name and Nature
The attraction between water molecules is called a hydrogen bond. These are not true chemical bonds, like the strong covalent bonds within a water molecule, but weaker intermolecular forces. A hydrogen bond forms as an electrostatic attraction between a partially positive hydrogen atom in one water molecule and a partially negative, highly electronegative atom, such as oxygen, in another. This force is weaker than covalent or ionic bonds but stronger than other intermolecular forces. The concept was introduced by American chemist Linus Pauling in 1931.
The Polarity That Drives Bonding
Water’s ability to form hydrogen bonds stems from its molecular structure and polarity. A water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms, forming a bent shape. Oxygen is highly electronegative, strongly attracting shared electrons.
Because oxygen pulls electrons more strongly than hydrogen, shared electrons spend more time closer to the oxygen atom. This unequal sharing creates a slight negative charge on the oxygen atom and slight positive charges on each hydrogen atom. This uneven charge distribution makes water a polar molecule, allowing hydrogen bonds to form between adjacent water molecules.
How These Bonds Shape Water’s Unique Qualities
Hydrogen bonds are responsible for several properties of water significant for life. One property is cohesion, the tendency of water molecules to stick together. This mutual attraction contributes to phenomena like surface tension, allowing some insects to walk on water and supporting droplet formation. Water also exhibits adhesion, its attraction to other polar surfaces. This adhesive quality, combined with cohesion, enables capillary action, where water moves upwards against gravity in narrow tubes, such as within plant vascular tissues.
Water’s high specific heat capacity is another consequence of hydrogen bonding. This property means water can absorb or release substantial heat energy with only a small temperature change. When heat is added, much energy is used to break existing hydrogen bonds before molecular kinetic energy increases, which would raise the temperature. This thermal buffering helps organisms maintain stable internal temperatures and moderates aquatic environment fluctuations.
The unusual density of ice, which allows it to float, also results from hydrogen bonding. As water cools and freezes, its molecules arrange into a more ordered, crystalline lattice. Hydrogen bonds hold water molecules further apart than in liquid water, creating more empty space within the solid structure. This makes ice less dense than liquid water, explaining why it floats and provides an insulating layer that protects aquatic life in colder climates.