Cohesion

Water molecules stick together due to hydrogen bonds formed between them. The oxygen atom in one water molecule has a partial negative charge that attracts the partial positive charge on a hydrogen atom in another water molecule. Each water molecule can form up to four hydrogen bonds with neighboring water molecules, creating strong cohesive forces that hold water together.

Surface tension is the phenomenon where water molecules at the surface are more attracted to each other than to air molecules, creating a pull inward. Water molecules at the surface cannot form hydrogen bonds on the air-facing side, so they form stronger interactions with neighboring water molecules below and beside them. This compressed surface layer can resist rupture and support small objects.

Water striders, also called pond skaters, can walk on water because of surface tension created by cohesion. The high surface tension of water and the buoyancy of the insect's legs allow it to remain on the water's surface without breaking through the cohesive forces. The water molecules' strong attraction to each other supports the insect's weight.

Water droplets form spheres because cohesive forces between water molecules create a compressed surface area. Water molecules are more attracted to each other than to surrounding air or surfaces, so they group together minimizing their exposed surface. The spherical shape represents the most efficient arrangement for water molecules held together by hydrogen bonds.

Both water and mercury are cohesive liquids, but they behave differently in glass containers. Mercury remains in spherical droplets because cohesive forces between mercury molecules are stronger than adhesive forces with glass. Water, though highly cohesive, also has affinity for silica in glass, so it disperses evenly at the bottom rather than forming droplets.

Water's polarity is essential to its cohesion. Water molecules are polar, meaning the oxygen atom carries a partial negative charge while hydrogen atoms carry partial positive charges. This polarity enables hydrogen bonds to form between water molecules, creating the intermolecular forces responsible for cohesion and surface tension.

Each water molecule can form up to four hydrogen bonds with other water molecules. This occurs because water has two hydrogen atoms with partial positive charges and two lone pairs of electrons on the oxygen atom with partial negative charges. The ability to form multiple hydrogen bonds creates the strong cohesive network that defines water's unique properties.