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Water: Polarity, Hydrogen Bonding, and pH (Lecture Notes)

Water as a Polar Molecule and Hydrogen Bonding

  • Water is a polar molecule with a slight negative charge on the oxygen end and a slight positive charge on the hydrogen ends.
  • Polar molecule definition: has opposite charges on opposite ends of the molecule.
  • Electrons aren’t shared equally in water: negatively charged electrons spend more time around the oxygen atom, giving that end a negative charge and the hydrogens a relative positive charge.
  • Chemical representation: \mathrm{H_2O}
  • Hydrogen bonding: a very weak molecular bond where water molecules bond to each other via hydrogen bonds.
  • Hydrogen bonds are weak electrical attractions between polar molecules and are important to many of water’s properties.
  • Polar molecules (like sugar) readily dissolve in water.
  • Hydrophilic substances dissolve readily in water (e.g., salt).
  • Hydrogen bonds form with water causing atoms to pull apart when dissolved or interacting with water.
  • Polar Molecules & Dissolution:
    • Polar molecules have an asymmetrical distribution of electrical charge.
    • The polarity drives dissolution in water.
    • Example categories: polar molecules (sugars) dissolve in water; salts are hydrophilic.
  • Non-polar molecules: do not have a distribution of charge; do not dissolve in water.
  • Non-polar characteristics:
    • Non-polar: symmetrical distribution of electrical charge.
    • Lipids are typically non-polar and hydrophobic (do not dissolve in water).
  • Hydrophobic vs Hydrophilic:
    • Hydrophobic: compounds that do not dissolve readily in water (e.g., oil).
    • Hydrophilic: compounds that dissolve readily in water (e.g., salts).
  • Amphipathic molecules: have both hydrophobic and hydrophilic regions.
  • Emulsifiers: amphipathic molecules used to keep hydrophilic and hydrophobic mixtures from separating.

Physical Properties and Roles of Water

  • Water can dissolve many other substances due to polarity; it is often referred to as the "universal solvent."
  • Water is a liquid at moderate temperatures; many other substances of similar molecular size are gases at those temperatures.
  • Water can hold a lot of heat relative to its volume (high heat capacity per unit volume).
  • The polarity of water molecules and hydrogen bonding explain water’s life-supporting properties:
    • Water molecules stick to each other (cohesion).
    • Water molecules cohere (stick to each other) and cohesion contributes to surface tension.
    • Water molecules adhere (cling to other substances) and wet surfaces (adhesion).
  • Cohesion and adhesion contribute to surface phenomena and movement of water in structures such as plant cells (e.g., water conduction in xylem).
  • Water cohesion leads to surface tension (water’s surface acts like a membrane).
  • Water adhesion describes how water clings to other substances, aiding wetting.
  • Water’s properties enable it to act as a common solvent for life-sustaining reactions.
  • Water can dissolve many other substances due to polarity; thus it is known as the "universal solvent."
  • Water’s physical properties support life: cohesion, adhesion, surface tension, high heat capacity, and high solvent capability.
  • Water expands when it freezes; ice is less dense than liquid water, so frozen water floats.
  • Water is a common solvent for life-sustaining reactions.

Water as a Solvent for Ions and Salts

  • In aqueous solutions, salts dissolve into ions:
    • Sodium ion in solution: \mathrm{Na^+}
    • Chloride ion in solution: \mathrm{Cl^-}
  • Salt crystals (e.g., NaCl) dissolve into their constituent ions when placed in water.
  • Illustration reference (Figure 2.14): the separation of ions in solution demonstrates dissolution and ionic charges.

The pH Scale and Neutralization

  • The pH scale measures the amount of hydrogen ions (H⁺) in solution.
  • Acidic solution: more H⁺ than OH⁻.
  • Basic (alkaline) solution: more OH⁻ than H⁺.
  • Neutral solution: equal amounts of H⁺ and OH⁻.
  • Acidic solution + basic solution = neutral (neutralization relationship).
  • Practical note: The slide emphasizes the qualitative rule that mixtures of acids and bases move toward neutrality when combined.

Connections to Real-World Relevance

  • Water’s polarity and hydrogen bonding underpin biological processes such as digestion, circulation, skin health, and overall homeostasis.
  • The universal solvent property enables biochemical reactions, nutrient transport, and waste removal.
  • The distinction between polar (water-loving) and non-polar (water-avoiding) substances explains how organisms manage dissolved nutrients and lipids.
  • Amphipathic molecules and emulsifiers are crucial in biological membranes and food science to stabilize mixtures of hydrophilic and hydrophobic substances.
  • Understanding pH and neutralization is essential for metabolic processes, enzyme activity, and environmental health.