Unit 1: Chemistry of Life — Water and Hydrogen Bonding (Vocabulary Flashcards)

Water Structure and Hydrogen Bonding – Comprehensive Notes

Water facts
- Three-quarters of the Earth’s surface is submerged in water.
- The abundance of water is a primary reason the Earth is habitable.
Water is a polar molecule, meaning the distribution of electrons between the covalently bonded atoms is unequal (unequal sharing of electrons).

Water’s Polarity
- Polar molecule with partial positive charges on hydrogen atoms and a partial negative charge on the oxygen atom.
- Polar covalent bonds enable molecules to form hydrogen bonds with each other.
- Polarity contributes to the emergence of water’s unique properties.

Water has six (listed as seven in slides) emergent properties that contribute to Earth’s suitability for life:

1) Cohesion
- Water molecules bond to other water molecules via hydrogen bonds.
- Cohesion helps pull water up through the microscopic vessels of plants (xylem).
2) Adhesion
- Water molecules stick to other molecules (e.g., plant cell walls).
3) Surface Tension
- Surface tension is a measure of how hard it is to break the surface of a liquid; related to cohesion.
4) Temperature Moderation
- Water resists temperature changes due to hydrogen bonding, helping to stabilize climates.
5) High Specific Heat
- Water has a high specific heat, which minimizes temperature fluctuations within life-sustaining ranges.
- Heat is absorbed when hydrogen bonds break and released when hydrogen bonds form.
6) Evaporative Cooling
- Water’s high heat of vaporization allows surfaces to cool as water evaporates.
- Example: sweating cools the body as body heat is used to vaporize sweat.
7) Universal Solvent
- Water can dissolve more substances than any other liquid due to its polarity.
- Process: DISSOLVE THE SOLUTE IN THE SOLVENT.

Pathway illustrating water movement:
- Roots absorb water via osmosis into root hairs.
- Water moves through the xylem via cohesion (between water molecules) and adhesion (to xylem walls).
- Evaporation from the stomata in leaves drives the upward pull (transpiration stream).
- This cohesion-tension mechanism enables water transport from roots to leaves.

Adhesion
- Water molecules sticking to other molecules (e.g., cell walls, other surfaces).
Cohesion
- Water molecules bonding to other water molecules via hydrogen bonds.
Interactions with polar/charged objects
- Water’s polar regions interact with ionic compounds and other polar substances.

Surface Tension
- A measure of how difficult it is to break the surface; results from cohesive forces.
Specific Heat
- The amount of heat needed to change the temperature of 1 gram of a substance by 1°C.
- Water’s high specific heat buffers environmental and physiological temperature changes.
Moderation of Temperature (Illustrative Data)
- Water’s heat capacity contributes to milder climates in coastal regions compared with inland areas.
- Example temperatures (illustrative snapshot): Santa Barbara 73°, Los Angeles 90°, Burbank 96°, Riverside 96°, Palm Springs 70s–100s (°F range shown), Santa Ana 75°, Pacific Ocean 90s–100s, San Diego 72°.
- Overall takeaway: Ocean and water bodies moderate temperatures in nearby areas.

Definition
- Transformation of a substance from a liquid to a gas requires energy.
Mechanism and Benefit
- Evaporation consumes heat, cooling the remaining liquid or surface.
- Example: Sweating cools the body as heat energy from the body is used to vaporize sweat.
Key concept
- Evaporative cooling relies on water’s high heat of vaporization.

Universal Solvent
- Water dissolves more substances than any other liquid because of its polarity.
Dissolution process
- Dissolve the solute in the solvent (solute + solvent interaction).
Polar water interactions with solutes
- The polar regions of water interact with ionic compounds and polar solutes to dissolve them.

Dissociation of water and pH relevance
- Dissociation of water leads to acidic and basic conditions affecting living organisms.
- Organisms must maintain homeostasis in their internal and external pH.
Water autoionization (acid-base chemistry)
- Water can dissociate to hydronium ions (H⁺) and hydroxide ions (OH⁻).
- Equation (typical representation):
  \mathrm{H2O + H2O \rightleftharpoons H_3O^+ + OH^-}
- Changes in the concentrations of these ions alter pH and biological processes.
Definitions of acids and bases
- An acid: any substance that increases hydrogen ion concentration in a solution (more H⁺ or H₃O⁺).
- A base: any substance that reduces hydrogen ion concentration in a solution (more OH⁻).

The pH scale (acute focus)
- The pH scale ranges from 1 to 14.
- Acids have pH values below 7; bases have pH values above 7; neutral is around pH 7.
Common substances and their approximate pH trends (illustrative list)
- Acidic examples: gastric acid, lemon juice, tomato juice, apple juice, black coffee, milk (varies but can be mildly acidic).
- Basic/alkaline examples: baking soda solution, ammonia solution, hand soap, bleach (basic range).
- Neutral: pure water.

Hydrophilic substances
- Have an affinity for water; are polar or ionic (e.g., carbohydrates, salts).
Hydrophobic substances
- Do not have an affinity for water; are nonpolar (e.g., lipids).
Rule of thumb
- “Like dissolves like”: polar substances dissolve in polar solvents like water; nonpolar substances are more soluble in nonpolar solvents.

Ice behavior
- Solid water (ice) has a more ordered hydrogen-bond structure than liquid water, making ice less dense.
- Ice floats on liquid water.
Ecological significance
- Floating ice insulates the liquid water and the organisms beneath the ice layer, helping to stabilize aquatic ecosystems during cold periods.

Water’s polarity and hydrogen bonding underpin its unique properties that support life.
Cohesion and adhesion enable transport in plants and interactions with surfaces.
Water’s high heat capacity and high heat of vaporization contribute to temperature regulation and evaporative cooling.
As a universal solvent, water facilitates chemical reactions and nutrient transport.
Amphipathic interactions with acids, bases, and pH balance are central to biochemistry and homeostasis.
Hydrophilic/hydrophobic distinctions explain solubility patterns and molecular interactions in biological systems.
Ice insulation preserves aquatic life by maintaining liquid water below the ice layer.