Water: Physical and Chemical Properties

Physical Properties of Water

  • Water molecules are polar and form hydrogen bonds with other molecules.

  • Ice: Water molecules are hydrogen bonded in a crystalline array, forming 4 H-bonds per molecule; it is less dense than liquid water.

  • Liquid Water: Hydrogen bonds rapidly break and re-form in irregular networks, with 2-3 H-bonds per molecule; it is denser than ice.

  • Attractive forces: ionic interactions, hydrogen bonds, and van der Waals interactions, all weak individually.

  • Polar and ionic substances dissolve in water; nonpolar substances do not.

  • Hydrophobic Effect: Exclusion of nonpolar groups to maximize the entropy of water molecules; nonpolar molecules aggregate to minimize water contact.

  • Amphiphilic Substances: Contain both polar and nonpolar groups, forming micelles or bilayers to hide hydrophobic groups and expose hydrophilic groups to water.

  • Osmosis: Solvent molecules diffuse across a permeable membrane from regions of higher solvent concentration to lower solvent concentration.

  • Dialysis: Solutes diffuse across a semipermeable membrane from regions of higher solute concentration to lower solute concentration.

  • Water's bent structure makes it polar, contributing to high boiling point, melting point, heat of vaporization, and surface tension.

  • Water has a high dielectric constant, solvating polar and ionic groups well through charge-dipole interactions.

Chemical Properties of Water

  • Water dissociates into H+H^+ and OH−OH^-.

  • The ion product of water: Kw=[H+][OH−]=10−14M2K_w = [H^+][OH^-] = 10^{-14} M^2 at 25°C.

  • pH: The acidity of a solution expressed as pH=−log10[H+]pH = -log_{10}[H^+].

  • pH < 7 is acidic ([H^+] > [OH^-]); pH > 7 is basic ([OH^-] > [H^+]). Neutral pH is 7, where [H+]=[OH−]=10−7M[H^+] = [OH^-] = 10^{-7} M.

  • An acid donates a proton; a base accepts a proton.

  • Weak Acids: Dissociate partially (HA⇌H++A−HA \rightleftharpoons H^+ + A^-).

  • Acid Dissociation Constant: K<em>a=[H+][A−][HA]K<em>a = \frac{[H^+][A^-]}{[HA]}. pK</em>a=−log<em>10K</em>apK</em>a = -log<em>{10}K</em>a.

  • Henderson-Hasselbalch Equation: Relates pH, pK<em>apK<em>a, and the concentrations of a weak acid and its conjugate base: pH=pK</em>a+log10[A−][HA]pH = pK</em>a + log_{10}\frac{[A^-]}{[HA]}.

    • When [HA]=[A−][HA] = [A^-], then pH=pKapH = pK_a.

  • Buffers: Solutions that resist changes in pH when acid or base is added.

    • Consist of a weak acid and its conjugate base.

    • Most effective at pH values near their pKapK_a (±1\pm 1 pH unit).

  • Enzyme activity and other biological molecules are highly sensitive to pH changes.

  • If solution pH < pKa, the protonated form (HAHA) predominates. If solution pH > pKa, the deprotonated form (A−A^-) predominates.