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Structure of Water and Hydrogen Bonding

Structure of Water and Hydrogen Bonding

Introduction

  • Water is essential for life.
  • Most organisms and their environments are primarily water.
  • The human body is approximately two-thirds water.
  • Water has unique properties compared to other molecules.
  • Water can exist as a liquid, solid, or gas at reasonable temperatures.

Water's Polarity

  • Water's shape makes it a polar molecule.
  • Polarity means having two different sides or charges.
  • A single drop of water contains billions of water molecules (H_2O).
  • Each water molecule comprises two hydrogen atoms and one oxygen atom.
  • The arrangement of hydrogen and oxygen gives water its polar properties.
  • Oxygen has a high electronegativity, while hydrogen has a low electronegativity.
  • One side of the water molecule has a slight positive charge, while the other side has a slight negative charge.
  • Nonpolar molecules have an even distribution of charge.

Hydrogen Bonds

  • Water's polarity enables it to form hydrogen bonds.
  • Hydrogen bonds are attractions between different water molecules, not bonds holding atoms together within a molecule.
  • Positive sides of one water molecule attract negative sides of another.
  • These bonds are constantly forming and breaking in liquid or gaseous water.
  • Frequent hydrogen bonds can become strong.

Unique Properties of Water

  • Hydrogen bonds give water unique properties.
  • Three key properties: cohesion, adhesion, and surface tension.

Cohesion

  • Cohesion: water molecules stick to each other due to hydrogen bonding.
  • Water molecules attract each other because of positive and negative ends.
  • Water tends to form droplets due to its polarity and hydrogen bonds.
  • Water is "sticky" because it clings to itself.

Adhesion

  • Adhesion: water molecules stick to polar surfaces.
  • Water is adhesive, like tape, meaning it sticks to things.
  • Water easily sticks to polar surfaces due to hydrogen bonds and polarity.
  • Cohesion refers to water sticking to itself, while adhesion refers to water sticking to other surfaces.

Practical Application of Adhesion

  • Water's stickiness allows plants to transport it upward from the ground against gravity.
  • Water moves up through the xylem (water-conducting cells) in trees.
  • Cohesion helps water molecules pull each other up as water evaporates from leaves due to transpiration.
  • Adhesion allows water to stick to the walls of water-conducting cells, preventing it from falling back down.

Surface Tension

  • Surface tension allows some animals to glide on water.
  • Surface tension: measure of how difficult it is to stretch or break the surface of a liquid.
  • Water striders can sit on the surface because their force isn't strong enough to break the hydrogen bonds.
  • Water has high surface tension compared to other liquids.

Conclusion

  • Cohesion, adhesion, and surface tension are due to hydrogen bonds.
  • Hydrogen bonds result from the polarity of water molecules.
  • The properties of large bodies of water are related to the atomic level.