Water Structure, Properties, and Carbon

Water Structure

• A water molecule consists of 3 atoms: one oxygen and two hydrogen atoms.
• Oxygen is highly electronegative, attracting shared electrons more strongly than hydrogen.
• This uneven distribution of charge results in polar covalent bonds between oxygen and hydrogen.
• Electrons spend more time near the oxygen nucleus due to its electronegativity, giving oxygen a partial negative charge and each hydrogen a partial positive charge.
• Water molecules are linked by hydrogen bonds, which occur when a partially positive hydrogen atom is attracted to a partially negative atom (like oxygen or nitrogen).
• Hydrogen bonds are weaker than covalent bonds and are represented by dotted lines.
• Each water molecule can bond with up to 4 other water molecules at any moment.
• Hydrogen bonding is cooperative, meaning a hydrogen-bonded water molecule is a better acceptor/donor than an unbound one.

Water Properties

• Hydrogen bonding is crucial for water's unique properties.
• Property 1: Cohesion
  • Water molecules stay close together due to hydrogen bonding, giving water high cohesiveness.
  • Water's cohesiveness aids in transporting materials as it draws dissolved substances along with it.
  • Cohesion contributes to the transport of water and dissolved nutrients against gravity in plants, where evaporation from leaves pulls water upward from the roots.
  • Cohesion is related to surface tension; water has a high surface tension because molecules are hydrogen-bonded to each other and the water below, creating an "invisible film".
  • This allows some animals, like the basilisk lizard, to walk or run on water.
• Property 2: Temperature Moderation
  • Water has a high specific heat due to its ability to form hydrogen bonds.
  • Heat must be absorbed to break hydrogen bonds, and heat is released when hydrogen bonds form.
  • Specific heat is the amount of heat required to change the temperature of 1g of a substance by 1 degree Celsius; for water, it is 4.18 \frac{J}{g°C}.
  • Water is a great heat sink. Large bodies of water absorb and store heat from the sun during the day and warm the air at night, minimizing temperature fluctuations.
  • Water has a high boiling point (100°C), a high melting point (0°C), and a high heat of vaporization (40.65 \frac{kJ}{mol}).
  • High heat of vaporization contributes to water's evaporative cooling effect, allowing sweat evaporation from human skin to dissipate body heat.
• Property 3: Density
  • Ice is less dense than liquid water, a unique property due to hydrogen bonding.
  • In ice, hydrogen bonds are directional and linear, forming a rigid, open lattice structure.
  • Breaking some hydrogen bonds allows for closer packing of water molecules in the liquid state.
  • The lower density of ice prevents water from freezing from the bottom up, insulating the water below and allowing life to exist underneath.
  • Floating ice provides a habitat for animals like polar bears.
• Property 4: Solvent Properties
  • Water is an excellent solvent due to its polar nature.
  • Only hydrophilic (water-loving) materials dissolve in water.
  • Non-polar solutes are hydrophobic (water-fearing) and do not readily form hydrogen bonds.

Carbon

• Life on Earth is carbon-based; carbon is the base element for many macromolecules in the body.
• Carbon dating determines the age of biological artifacts by comparing the carbon-12 and carbon-14 ratios of a sample to a living organism.
• Carbon is a small element that can form 4 covalent bonds.
• Carbon can bond to other carbon atoms in straight chains, branched chains, and rings.
• All biomolecules have a carbon skeleton (or backbone) that may contain single, double, or triple bonds.
• If only hydrogen and carbon are present in the skeleton/backbone, the compound is called a hydrocarbon.
• Other elements may attach to the carbon skeleton/backbone to form functional groups, which are more reactive than the hydrocarbon portions of biomolecules.