Water molecules exhibit cohesion

The hydrogen bonds between water molecules are weak relative to covalent bonds. However, water molecules tend to be attracted to other water molecules to form hydrogen bonds. The tendency for molecules to be attracted to one another if they are the same kind of molecules is called cohesion.

Cohesion allows water molecules to “stick together” via hydrogen bonding. Imagine a water droplet on the surface of your desk or table. Water droplets form because water molecules tend to be attracted to one another through hydrogen bonds rather than to your desk or table or to the air surrounding the water. This creates surface tension where water meets another type of matter. Water droplets form because of the cohesive properties of water!

In the human body, cohesion of water molecules ensures that when water moves in and out of thin-walled blood vessels called capillaries, one water molecule will be followed by many water molecules. This bulk movement of water ensures fluid balance within the body.