Cohesion is the tendency of water molecules to stick to each other. This “stickiness” comes from hydrogen bonds, temporary electrical attractions that form between neighboring water molecules because of the way charge is distributed across each molecule. Water is significantly more cohesive than other simple liquids, and that extra grip between molecules explains many of water’s most familiar and important properties, from its ability to form droplets to its role in keeping trees alive.
Why Water Molecules Attract Each Other
A water molecule has one oxygen atom bonded to two hydrogen atoms, arranged at an angle of about 104.5 degrees. Oxygen pulls on shared electrons much more strongly than hydrogen does, so the oxygen side of the molecule carries a slight negative charge while each hydrogen side carries a slight positive charge. This uneven charge distribution makes water a polar molecule.
When two water molecules come near each other, the positively charged hydrogen on one molecule is attracted to the negatively charged oxygen on another. That attraction is a hydrogen bond. Each water molecule can form up to four of these bonds at once, two through its hydrogens and two through its oxygen’s available electron pairs, creating a dynamic, interconnected network throughout any body of liquid water. A hydrogen bond involves both classical electrical attraction and a smaller quantum-level sharing of electrons between molecules, making it stronger than the weaker attractions found in most simple liquids.
A useful comparison is hydrogen sulfide, a gas that has a nearly identical molecular shape to water. Sulfur sits in the same column of the periodic table as oxygen but is larger and holds its electrons less tightly, so the bonds in hydrogen sulfide are far less polar. Even though hydrogen sulfide is almost twice as heavy as water (34 vs. 18 grams per mole), it boils at a frigid negative 60°C and exists as a gas at room temperature. Water stays liquid up to 100°C. The difference is cohesion: hydrogen bonding gives water roughly twice the vaporization energy (about 41 kilojoules per mole compared to 19 for hydrogen sulfide).
Cohesion vs. Adhesion
Cohesion and adhesion often work together, but they describe different forces. Cohesion is water attracting water. Adhesion is water attracting other substances, like glass, soil, or the walls of a plant’s internal tubes. A raindrop holds its rounded shape because cohesion pulls the water molecules inward toward each other. That same drop clinging to the tip of a pine needle is adhesion at work, water molecules bonding to the needle’s surface.
You can see both forces in a glass of water. The water curves slightly upward where it meets the glass because adhesion between water and glass is strong enough to pull the edges up. Meanwhile, cohesion keeps the bulk of the water together, preventing it from simply spreading across every available surface like a thin film.
Surface Tension: Cohesion at the Boundary
At the surface of any body of water, molecules have neighbors only below and beside them, not above. The cohesive pull from surrounding molecules draws those surface molecules inward and to the sides, creating a thin, tight “skin” known as surface tension. At 20°C, water’s surface tension measures about 72.75 millinewtons per meter, strong enough to support small objects that would otherwise sink.
Water striders are a vivid example. These insects stand and jump on water without breaking through it. Their legs press the surface into tiny dimples, and the cohesive force between water molecules pushes back, providing enough support for locomotion. Research published in the Proceedings of the National Academy of Sciences found that smaller water strider species keep the surface intact even while jumping, deriving their thrust almost entirely from surface tension rather than pushing water downward. Their leg speed is precisely tuned to avoid puncturing the surface.
Surface tension weakens as temperature rises because heat makes molecules move faster, which disrupts hydrogen bonds. At 0°C, water’s surface tension is about 75.6 millinewtons per meter. By 100°C it drops to 58.9, and by 200°C it falls to 37.7. At water’s critical temperature of 374°C, surface tension reaches zero and the distinction between liquid and gas disappears entirely.
How Cohesion Moves Water Through Trees
One of cohesion’s most dramatic roles is pulling water from roots to leaves in tall trees. The cohesion-tension theory, first proposed in the 1890s, explains how this works. When water evaporates from leaf surfaces (a process called transpiration), it tugs on the water molecules just behind it in the narrow tubes of the plant’s vascular system. Because those molecules are hydrogen-bonded to the ones below them, the tug travels downward through an unbroken chain of water stretching all the way to the roots.
This creates a tension gradient, essentially negative pressure, that can reach several megapascals, enough to overcome both gravity and friction along the tube walls. Water under this tension is in what physicists call a metastable state: it’s being pulled rather than pushed, which means the column would snap if air were introduced. The continuous, cohesive chain of water molecules is what makes the entire system possible. Without hydrogen bonding strong enough to hold the column together under tension, tall trees simply could not exist.
Cohesion and Heat Storage
Breaking hydrogen bonds requires energy, and forming them releases energy. This gives water an unusually high heat capacity, meaning it absorbs a lot of heat before its temperature rises significantly. Water’s specific heat capacity is about 75.3 joules per mole per degree, more than double that of hydrogen sulfide (34.6 J/mol·K). In practical terms, a pot of water on a stove heats slowly compared to the same mass of cooking oil, and it cools slowly too.
This thermal buffering shapes climate on a global scale. Coastal regions experience milder temperature swings than inland areas because nearby ocean water absorbs heat during the day and releases it at night. Inside your body, water’s high heat capacity helps maintain a stable internal temperature. Both effects trace directly back to the energy stored in and released from hydrogen bonds between cohesive water molecules.
Cold Water Behaves Differently
The hydrogen bond network in water changes with temperature in a way that sets water apart from almost every other liquid. Cold water retains more of an open, cage-like structure that resembles the ordered arrangement found in ice. As water warms, this structure progressively breaks down and the molecules pack together in a less organized way. This is why water reaches its maximum density at about 4°C rather than at its freezing point: the partial cage structure of very cold water actually takes up slightly more space than the denser, more disordered arrangement at 4°C.
That density quirk has major consequences for aquatic life. In a freezing lake, the densest water (at 4°C) sinks to the bottom while colder, lighter water rises and eventually freezes at the surface. The ice layer insulates the liquid below, allowing fish and other organisms to survive winter. This behavior is a direct product of how cohesive hydrogen bond networks reorganize at low temperatures.