Water and Life (Chapter 3)

Concept 3.1: Polar covalent bonds in water molecules result in hydrogen bonding

  • Water is a polar molecule due to oxygen's higher electronegativity, creating partial negative and positive charges.

  • This polarity enables water molecules to form hydrogen bonds with each other.

  • Hydrogen bonding is fundamental to water's emergent properties that support life.

Concept 3.2: Four emergent properties of water contribute to Earth’s suitability for life

  • Cohesive behavior: Water sticks to itself (cohesion) and other molecules (adhesion) due to hydrogen bonding, essential for water transport in plants.

  • Ability to moderate temperature: Water has a high specific heat (c=1 cal g−1 °C−1c = 1\ \text{cal}\,\text{g}^{-1}\,\text{°C}^{-1}) and high heat of vaporization. Hydrogen bonds absorb heat when they break and release heat when they form, allowing water to resist temperature changes and stabilize environments and organisms.

  • Expansion upon freezing: Ice is less dense than liquid water because hydrogen bonds form a stable, open hexagonal lattice, causing ice to float. This insulates aquatic environments, allowing life to survive below.

  • Versatility as a solvent: Water's polarity allows it to dissolve many ionic and polar substances (hydrophilic materials) through hydrogen bonding, making it the solvent of life. Non-polar substances (hydrophobic materials) do not dissolve.

  • Solution concentration is often measured in molarity (Molarity=nV\text{Molarity} = \frac{n}{V}).

Concept 3.3: Acidic and basic conditions affect living organisms

  • Water molecules can dissociate into hydrogen ions (H+ or hydronium, H3O+) and hydroxide ions (OH−).

  • In pure water, [H+]=[OH−]=10−7 M[H^+] = [OH^-] = 10^{-7}\,\text{M} at dynamic equilibrium.

  • Acids increase H+ concentration, while bases decrease it (often by increasing OH−).

  • The pH scale measures H+ concentration: pH=−log⁡[H+]\mathrm{pH} = -\log [\mathrm{H^+}].

  • For any aqueous solution, [H+] [OH−]=10−14[H^+]\,[OH^-] = 10^{-14}. pH values less than 7 are acidic, greater than 7 are basic, and 7 is neutral. Each pH unit represents a tenfold change in H+ concentration.

  • Buffers are substances that resist pH shifts, maintaining stability crucial for biological systems.

  • Ocean acidification is caused by human-generated CO2 dissolving in seawater to form carbonic acid (CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−), increasing H+ and reducing carbonate ions (CO3^2−) needed by marine calcifiers like corals.