Why Water Sticks Together

By the end of this lesson, you will be able to describe cohesion, adhesion, and surface tension in water and explain everyday things they cause.

Think about washing a car on a hot Texas afternoon. When you spray the hood, the water does not spread out flat like a sheet of paper. Instead it pulls itself into round beads that sit on the paint, and if you tilt the hood, those beads run together into bigger drops. That behavior is not an accident. It happens because water molecules are attracted to each other and to other materials.

Cohesion is the attraction between molecules of the same substance, so water molecules pulling on other water molecules is cohesion. This is why water forms droplets instead of falling apart into a thin film. Adhesion is the attraction between molecules of different substances, such as water clinging to glass, soil, or the waxy surface of a leaf. Both attractions are happening at the same time, and whichever is stronger decides whether the water beads up or spreads out.

At the surface of water, the molecules have no water above them, so they pull sideways and downward on each other more strongly. This creates surface tension, a skin like layer at the top of water that resists being broken. Surface tension is strong enough to support a water strider walking across a stock tank, because the insect's light weight and spread out legs press on the surface without pushing through it.

These same properties move water through plants. Water adheres to the walls of narrow tubes inside a stem, and cohesion keeps the column of water connected, so as water evaporates from the leaves, more water is pulled up from the roots. A tall pecan tree lifts water many feet this way without any pump.

Try it: if you drip water onto a freshly waxed truck hood and it beads up tightly, is cohesion or adhesion stronger there?