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2.19: Cohesion

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2.19: Cohesion

Cohesion is the attraction between molecules of the same type - such as water molecules. The partially charged negative oxygen of one water molecule binds to the partially charged positive hydrogen of a second water molecule via hydrogen bonding. Each water molecule can form up to four hydrogen bonds with other water molecules.

On a surface, when multiple water molecules come together through cohesion, a droplet is formed. Water does not typically spread out across the surface. This is because the water molecules are more attracted to each other than to molecules that make up the surface or the surrounding air.

Surface tension results from cohesion. Pond skaters, also known as water striders, are insects that use this phenomenon to walk on water. Surface tension occurs at the interface between water and air (i.e., the water’s surface). Again, water molecules are more attracted to each other than they are to molecules in the air. Thus, the water molecules at the surface form bonds with neighboring water molecules beside and below them. Since the surface water molecules cannot form bonds with other water molecules on one side (the side next to the air), they form stronger bonds with their neighboring water molecules. The strongly-bonded molecules have a compressed surface area, creating spherical droplets of water molecules. The high surface tension of water and the buoyancy of the pond skater’s legs enable the insect to remain on the water’s surface.

Mercury is another cohesive molecule that can be readily seen. In a glass container, mercury does not spread out or wet the glass, because the cohesive forces between mercury molecules are stronger than the adhesive forces between the mercury and the glass. Although water is highly cohesive, it also has an affinity for silica. Thus, water disperses evenly in the bottom of a glass container.

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