Water Structure, Hydrogen Bonding, and Properties

Water Molecules

  • Water is a polar molecule, meaning it has ends with opposite partial charges due to unequal charge distribution.
  • Chemical formula: H_2O (two hydrogen atoms and one oxygen atom).
  • Hydrogens carry partial positive charges (\delta+), and oxygen has a partial negative charge (\delta−).
  • "Like attracts like":
    • Polar substances attract polar substances.
    • Nonpolar substances attract nonpolar substances.
  • Oil and water do not mix because oil molecules are nonpolar with balanced charges unlike the positive and negative poles of water. Oil and water are immiscible.
  • Terminology:
    • Hydrophilic: Substance with an affinity for water (attracts water).
    • Hydrophobic: Substance that avoids water (does not have an affinity for water).
    • Lipids are hydrophobic due to relatively nonpolar bonds, while water is polar.

Hydrogen Bonding

  • Hydrogen bonding occurs when a hydrogen atom bonds to a highly electronegative atom (oxygen, nitrogen, or fluorine).
  • Creates a dipole moment where hydrogen has a partial positive charge and the electronegative atom has a partial negative charge.
  • Water molecules bond together through hydrogen bonding, giving water its polarity.
  • Hydrogen bonds are weaker than covalent bonds, but still relatively strong.
  • Biological importance:
    • Occurs in water, DNA, and proteins.
    • Responsible for the shape and function of proteins, stability of chemical compounds, and formation of intermolecular interactions in crystalline solids.
  • Hydrogen bonds between water molecules give water properties: cohesion, adhesion, surface tension, specific heat, and evaporative cooling.

Properties of Water

  1. Cohesion:
    • Attraction between water molecules.
    • Strong cohesive forces due to hydrogen bonds.
    • Example: Transport of water and nutrients against gravity in plants through transpiration.
    • Transpiration: Loss of water from a plant in the form of water vapor.
  2. Adhesion:
    • Attraction of water to other molecules and surfaces.
    • Example: Water on a glass surface.
  3. Surface Tension:
    • Difficulty in breaking the surface of water due to cohesive forces.
    • Example: Water strider insects walking on the surface of freshwater.
  4. Specific Heat:
    • The amount of heat energy required to raise or lower the temperature of one gram of a substance by one degree Celsius.
    • Water has a high specific heat, allowing it to absorb or release a large amount of heat with only a slight temperature change.
    • Large bodies of water take a while to evaporate due to this.
  5. Evaporative Cooling:
    • Water has a high heat of vaporization, so it can absorb a lot of heat and leave the surface cooler.
    • Example: Excess body heat is used to convert sweat into vapor, cooling the body.
  6. Dissociation of Water:
    • Hydrogen shifts from one water molecule to another.
    • Acid: Increases hydronium concentration by releasing hydrogen ions into solutions. Example: HCl \rightarrow H^+ + Cl^-.
    • Base: Increases hydroxide concentration by absorbing or accepting hydrogen ions. Example: NaOH \rightarrow Na^+ + OH^-.
    • Water has a neutral pH of 7.0 due to the counterbalance of hydronium and hydroxide.
    • pH scale range: 0-14 (acidic < 7, neutral = 7, alkaline/basic > 7).
    • Each level on the pH scale is a ten-fold change (logarithmic scale).
    • Most biological fluids are in the pH range of 6-8.
    • Formula: pH = -log[H^+]