Water's Polarity, Cohesion, Adhesion, and Surface Tension

Water's Polarity and Properties

Polar Covalent Bonds in Water

• Water (H₂O) consists of two hydrogen atoms and one oxygen atom.
• These atoms are joined via polar covalent bonds.
• Oxygen is more electronegative than hydrogen, meaning it attracts the shared electrons more strongly.
• This unequal sharing leads to:
  • A partial negative charge (δ-) on the oxygen atom.
  • Partial positive charges (δ+) on each hydrogen atom.
• This charge distribution makes water a polar molecule.

Consequences of Polarity

• Opposite charges attract, and like charges repel.
• Due to its polarity, water attracts and bonds with other polar or charged molecules.
• Water is often called the "universal solvent" because of its ability to dissolve many substances.
• Water's unique properties stem from its polarity and its capacity to form hydrogen bonds.

Hydrogen Bonding

• A hydrogen bond forms between the partial positive hydrogen of one water molecule and the partial negative oxygen of another.
• In liquid water, hydrogen bonds are constantly breaking and reforming.
• Boiling water requires breaking these hydrogen bonds, which is why water has a high specific heat.
• When water freezes, hydrogen bonds become stable, pushing water molecules farther apart, thus reducing density. This is why ice floats.

Key Properties of Water

• The three major properties of water are cohesion, adhesion, and surface tension.

Cohesion

• Cohesion: Water is attracted to other water molecules.

Adhesion

• Adhesion: Water is attracted to other polar or charged substances.

Cohesion and Adhesion Working Together

• Cohesion and adhesion allow water to defy gravity (e.g., in plants).
• Water moves from roots to leaves via:
  • Adhesion to the walls of plant vasculature.
  • Cohesion to other water molecules.
• Transpiration in leaves pulls water molecules upward in a "barrel of monkeys" action.

Meniscus Example

• A meniscus (curved surface) forms in a graduated cylinder because:
  • Adhesion forces to the glass are stronger than cohesion between water molecules.
  • Water literally climbs up the sides of the cylinder.

Surface Tension

• Surface tension arises from cohesive and adhesive forces.
• Examples include water droplets or morning dew on grass.
• Surface tension allows certain insects to walk on water because the interaction of their bodies with the water is weaker than the hydrogen bonds between water molecules.