BackKey Concepts: Water Chemistry and Its Biological Importance
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Water: Chemistry and Biological Significance
Polar Covalent Bonds and Hydrogen Bonding
Water's unique properties stem from its molecular structure and the interactions between its molecules.
Polar Covalent Bonds: Water molecules have polar covalent bonds, meaning electrons are shared unequally between hydrogen and oxygen atoms. This creates a partial positive charge on hydrogen and a partial negative charge on oxygen.
Hydrogen Bonding: The polarity of water molecules allows them to form hydrogen bonds with each other. These are weak attractions between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another.
Example: Hydrogen bonds are responsible for many of water's unique properties, such as its high boiling point and surface tension.
Four Emergent Properties of Water
Water exhibits several properties that are essential for life, largely due to hydrogen bonding.
Cohesion and Adhesion:
Cohesion: Water molecules stick to each other, contributing to surface tension.
Adhesion: Water molecules stick to other substances, aiding processes like capillary action in plants.
Example: Water moving up plant vessels against gravity (capillary action).
Moderation of Temperature:
Water has a high specific heat, meaning it can absorb or release large amounts of heat with little temperature change.
This property helps stabilize temperatures in organisms and environments.
Example: Oceans and lakes moderate coastal climates.
Equation: (where is heat absorbed or released, is mass, is specific heat, is temperature change)
Expansion Upon Freezing:
Water is less dense as a solid than as a liquid due to the arrangement of hydrogen bonds in ice.
This allows ice to float, insulating aquatic life in winter.
Example: Ice forming on the surface of lakes protects organisms below.
Versatility as a Solvent:
Water's polarity allows it to dissolve many substances, making it a universal solvent.
Hydrophilic substances dissolve easily in water, while hydrophobic substances do not.
Example: Salts, sugars, and gases dissolve in water, facilitating biochemical reactions.
Acids and Bases
Acids and bases are substances that alter the hydrogen ion concentration in a solution.
Acids: Substances that increase the concentration of H+ ions in solution.
Bases: Substances that decrease the concentration of H+ ions, often by increasing OH- ions.
pH Scale: Measures the concentration of H+ ions; lower pH is more acidic, higher pH is more basic.
Equation:
Buffers: Substances that minimize changes in pH by accepting or donating H+ ions.
Example: Blood contains buffers to maintain a stable pH.
Ocean Acidification
Increased atmospheric CO2 dissolves in oceans, forming carbonic acid and lowering ocean pH. This process, known as ocean acidification, threatens marine life, especially organisms that build calcium carbonate shells.
Equation:
Example: Coral reefs are at risk due to decreased carbonate ion availability.
