Water and Aquasolution Lecture Notes

Jul 24, 2024

Lecture on Water and Aquasolution

Introduction

  • Lecturer: خالد معروف العرف
  • Topic: Water and Aquasolution
  • Program: إرشِدني educational program

Overview

  • Water (H2O): The medium for life, also known as the universal solvent.
  • Importance: Facilitates enzyme activity and biochemical reactions; constitutes about 70% of the human body weight.
  • Protection: Water offers UV protection.
  • Transportation: Eases movement of nutrients and chemicals within organisms.
  • Crucial for Proteins and Nucleic Acids: Essential for the structure and function of proteins, nucleic acids, and cell membranes.

Structure of Water Molecule

  • Composition: One oxygen atom and two hydrogen atoms (H2O).
  • Electron Pairs: Four electron pairs around the oxygen atom.
  • Bonding: Two pairs are covalently bonded to hydrogen atoms; other two pairs are lone pairs.
  • Polarity: Water is a polar molecule, having partial positive and negative charges.
  • Hydrogen Bonds: Formed due to polarity; allows water molecules to bond with each other.

Dipole Moment and Electronegativity

  • Dipole Moment: Arises from the electronegativity difference between oxygen and hydrogen.
  • Electronegativity: Oxygen's high electronegativity induces a dipole moment, allowing water to engage in hydrogen bonding.
  • Hydrogen Bonding: Strongest when three atoms are aligned. Water can act as both an acid and a base due to hydrogen bonding.

Properties of Water

  • Boiling and Melting Points: High due to strong hydrogen bonds.
  • Surface Tension: Allows small insects to walk on water due to strong cohesion among water molecules.
  • Solvent Properties: Dissolves polar substances well; like dissolves like principle (polar substances dissolve in polar solvents).
  • Weak Hydrogen Bonding: Individually weak but collectively strong due to cooperation among many bonds.

Interaction with Nonpolar Substances

  • Poor Solvent for Nonpolar Substances: Nonpolar substances do not form hydrogen bonds with water, leading to phenomena like oil not mixing with water.
  • Examples: Nonpolar gases, aromatic rings, and aliphatic chains.

Amphipathic Molecules

  • Amphipathic: Contains both hydrophobic (nonpolar) and hydrophilic (polar) parts.
  • Example: Phenylalanine - has both nonpolar hydrocarbon chains and polar groups.

Solubility of Polar and Nonpolar Solutes

  • Polar Solutes: Dissolve well in water; e.g., amino acids, peptides, carbohydrates.
  • Nonpolar Solutes: Poor solubility in water; e.g., hydrocarbons, nonpolar gases.
  • Ion Dissolution: Water can dissolve salts, reducing system energy and stabilizing ions.

Ionization and pH of Water

  • Self-Ionization: Water dissociates into hydrogen (H+) and hydroxide ions (OH-).
  • Equilibrium Constant (Keq): Dependents on temperature; typically low, indicating minimal dissociation.
  • pH Scale: Measures acidity/alkalinity; 7 is neutral, <7 is acidic, >7 is basic.

pH Calculation

  • Definition: Negative logarithm of hydrogen ion concentration (-log[H+]).
  • Relationships: pH + pOH = 14.
  • Neutral Solution: [H+] = [OH-] = 10^-7 M, leading to pH of 7.

Buffers and Buffering Capacity

  • Buffer Systems: Resist changes in pH upon addition of acids or bases.
  • Buffers in the Body: Maintain pH for enzyme activity and metabolic processes; examples include bicarbonate and phosphate buffers.
  • Buffering Region: Optimal buffering occurs when pH ≈ pKa.

Biological and Laboratory Buffers

  • In Vivo Buffers: Such as bicarbonate in blood plasma, histidine in intracellular fluids.
  • In Vitro Buffers: Examples include phosphate buffers used in lab settings.

Summary

  • Water’s unique properties are due to its molecular structure and ability to form hydrogen bonds. Its role as a solvent facilitates numerous biological processes. Understanding water's behavior, pH balance, and buffer systems is crucial for comprehending biochemical reactions.