Water and Life

Water and Life

Overview of Water's Importance

  • Water makes life possible on Earth.

  • It is the only common substance to exist in the natural environment in all three physical states of matter: solid, liquid, and gas.

  • Unique emergent properties of water contribute to making Earth suitable for life.

  • The molecular structure of water allows interaction with other molecules.

Concept 3.1: Polar Covalent Bonds and Hydrogen Bonding

  • In the water molecule, the electrons of the polar covalent bonds spend more time near the oxygen atom than the hydrogen atoms.

  • The result is that water is a polar molecule with an uneven distribution of charge.

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

Figure 3.2: Charge Distribution in Water Molecule

  • Key Components:

    • δ+ (partial positive charge) regions around hydrogen atoms.

    • δ– (partial negative charge) region around the oxygen atom.

    • Visualization of hydrogen bonds and their formation.

Concept 3.2: Emergent Properties of Water

Properties Contributing to Life

  • The four emergent properties of water that facilitate an environment supportive of life include:

    1. Cohesive behavior.

    2. Ability to moderate temperature.

    3. Expansion upon freezing.

    4. Versatility as a solvent.

Cohesion of Water Molecules

  • Hydrogen bonds collectively hold water molecules together (cohesion).

  • This cohesion aids in the transport of water against gravity in plants.

  • Adhesion refers to the attraction between different substances, like water and plant cell walls.

Surface Tension

  • Surface tension is a measure of how difficult it is to break the surface of a liquid. Water has a high surface tension due to hydrogen bonding at the air-water interface.

Temperature Moderation

  • Water can absorb heat from warmer air and releases stored heat to cooler air efficiently.

  • It can absorb large amounts of heat with minimal temperature change.

Temperature and Heat

  • Kinetic Energy: The energy of motion.

  • Thermal Energy: Kinetic energy associated with random motion of atoms/molecules.

  • Temperature: Represents the average kinetic energy of molecules in a body of matter.

  • Heat: Thermal energy in transfer from one body of matter to another.

Caloric Measurements

  • A calorie (cal) is the amount of heat needed to raise the temperature of 1 g of water by 1ºC.

  • "Calories" on food packages are kilocalories (kcal); where 1 kcal = 1,000 cal.

  • The joule (J) is another energy unit; with conversions:

    • 1 J = 0.239 cal,

    • 1 cal = 4.184 J.

Water’s High Specific Heat

  • Specific heat of a substance = heat required for 1 g to change temperature by 1ºC.

  • Water’s specific heat = 1 cal/(g • ºC).

  • Water resists temperature changes due to its high specific heat, attributed to hydrogen bonding.

  • Heat absorption occurs when hydrogen bonds break; heat is released when they form.

  • This property minimizes temperature fluctuations conducive to life.

Evaporative Cooling

  • Evaporation transforms liquid to gas; the heat of vaporization is the heat absorbed for 1 g of liquid to become gas.

  • Evaporative cooling of water stabilizes temperatures in organisms and bodies of water.

Floating Ice on Liquid Water

  • Ice floats because hydrogen bonds in ice are more ordered, making ice less dense than liquid water.

  • Water reaches maximum density at 4ºC.

  • If ice sank, aquatic life would suffer due to solid freezing of bodies of water.

Global Warming Concerns

  • Global warming negatively impacts ice environments, challenging species dependent on ice.

  • Increased glacier and Arctic sea ice melting observed, posing a threat to these ecosystems.

Water: The Solvent of Life

Solution Components

  • A solution is a homogeneous mixture of substances where the solvent is the dissolving agent, and the solute is what is dissolved.

  • An aqueous solution is a solution where water is the solvent.

Water as a Solvent

  • Water’s polarity enhances its solvent capabilities.

  • Ions in ionic compounds are surrounded by hydration shells when dissolved in water.

  • Nonionic polar molecules and even large proteins can dissolve in water if they possess polar/ionic regions.

Hydrophilic and Hydrophobic Substances

  • Hydrophilic substances: Have an affinity for water.

  • Hydrophobic substances: Lack affinity for water (e.g., oil molecules).

  • Major components of cell membranes are hydrophobic due to their nonpolar bonds.

Concept 3.3: Acidic and Basic Conditions

  • Hydrogen atoms can transfer between water molecules, leaving an electron behind as a proton (H+).

  • The molecule that loses a proton becomes a hydroxide ion (OH–), and the one gaining it becomes a hydronium ion (H3O+), often represented as H+.

  • This dissociation occurs at a dynamic equilibrium rate in pure water, affecting cell chemistry significantly.

Concentration Effects

  • In pure water, concentrations of H+ and OH– are equal.

  • Certain solutes (acids/bases) alter these concentrations, measured using the pH scale.

Acids and Bases

  • An acid increases H+ concentration in a solution.

  • A base reduces H+ concentration.

  • Strong acids/bases dissociate completely, while weak acids/bases release H+ reversibly, shifting balance from neutrality.