2.15
Cohesion
Cohesion is the attraction between molecules of the same type, such as water molecules. Water molecules have an overall neutral charge but are polar m…
Cohesion is the tendency for the same type of molecules to stick together due to intermolecular forces.
Water molecules are cohesive because of their polarity and ability to form hydrogen bonds. An oxygen atom with a partial negative charge in one water molecule can bind to a hydrogen atom with a partial positive charge in another water molecule, forming a hydrogen bond.
Each water molecule can form up to four hydrogen bonds with other water molecules.
Molecules of water will group together to form droplets. The molecules at the surface are exposed to air but are more attracted to themselves than to the molecules in the air.
The stronger attraction to other water molecules because of the hydrogen bonds creates a pull inwards, which generates surface tension.
Due to this phenomenon, the surface of the water can resist rupture. Because of this, small objects appear to float, and small insects can walk on water as long as they don't break the cohesive forces on the surface.
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Q1: Why do water molecules stick together?
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.
Q2: What is surface tension and how does it form?
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.
Q3: How can water striders walk on water?
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.
Q4: Why do water droplets form spheres?
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.
Q5: How does cohesion differ between water and mercury?
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.
Q6: What role does polarity play in water's cohesion?
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.
Q7: How many hydrogen bonds can each water molecule form?
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.