Biochemistry Quiz on Water Properties

Water

• Water is the most abundant substance in living organisms, comprising 70% or more of their weight
• Early life likely originated in an aqueous environment
• Water's properties (e.g., attractive forces, ionization tendencies) deeply affect biomolecules' structure and function
• Water molecules are polar, enabling them to form hydrogen bonds with each other and solutes
• Hydrogen bonds are relatively weak (23 kJ/mol) compared to covalent bonds (470 kJ/mol) but still very influential
• Hydrogen bonding leads to water's high melting point, boiling point and heat of vaporization compared to other solvents
• Water molecules cluster together in ice forming a crystal lattice, causing ice to be less dense than liquid water
• Water dissolves salts (e.g., NaCl) by hydrating ions, weakening electrostatic attraction and making them soluble
• The dielectric constant of water (78.5 at 25°C) screens electrostatic interactions between dissolved ions
• Nonpolar molecules cluster together in water
• Water's ability to form hydrogen bonds with itself and solutes influences solute solubility
• The ordering (reduced entropy) of water surrounding nonpolar solutes is energetically unfavorable

Weak Interactions in Aqueous Systems

• Hydrogen bonds between water molecules provide cohesive forces, making water a liquid at room temperature
• Water's unique properties are crucial to life
• Weak interactions (hydrogen bonds, ionic, hydrophobic, and van der Waals) determine the three-dimensional structures of biological molecules (e.g., proteins, nucleic acids)
• These weak interactions collectively significantly influence biological molecules' structures contributing to their strength, flexibility and precise shapes
• Water's ability to form hydrogen bonds with polar solutes facilitates the solubility of polar biomolecules
• The strength of hydrogen bonds is determined by the orientation of the molecules, maximizing electrostatic attraction when the molecules are in line

Ionization of Water, Weak Acids, and Weak Bases

• Water ionizes slightly into H⁺ and OH⁻ ions.
• The ionization of water is an equilibrium reaction expressed by a constant (K)
• Water's ionization is significant because it contributes to the pH of solutions.
• Weak acids and bases also ionize in water, influencing the pH.
• Equilibrium constants describe the proportion of reactants compared to products in an equilibrium reaction
• Equilibrium constants can be calculated from the concentrations of reactants and products at equilibrium,
• The ionization of water is expressed by an equilibrium constant (K)
• The product of [H⁺] and [OH⁻] in water is called K_w, which has a constant value at a given temperature
• The pH scale measures hydrogen ion concentration
• Changes in pH can affect the structure and function of biomolecules.