Water

Availability of Water

• Distribution of Water on Earth

  • Oceans: Approximately $97\%$ of Earth's water is salt water found in the oceans.

  • Glaciers and Ice Caps: About $2\%$ of the total water is frozen in glaciers and polar ice caps, representing the largest reservoir of freshwater.

  • Freshwater Accessibility: Only about $1\%$ of all water on Earth is liquid freshwater available for human use, found in rivers, lakes, and underground aquifers.

  • Biological Importance: While exact percentages fluctuate, understanding the hierarchy (Oceans > Glaciers > Groundwater > Surface Water) is crucial for resource management.

Physical Properties of Water

• The Universal Solvent

  • Water is known as the "universal solvent" because it dissolves more substances than any other liquid. This is due to its polar nature, which allows it to disrupt the attractive forces of other polar or ionic compounds.

• General Characteristics

  • Appearance: Colorless, odorless, and tasteless in its pure form.

  • Boiling and Freezing Points: Pure water freezes at $0^{\circ}\text{C}$ ($32^{\circ}\text{F}$) and boils at $100^{\circ}\text{C}$ ($212^{\circ}\text{F}$) at standard atmospheric pressure.

  • Palatability: The specific taste of water often comes from dissolved gasses, minerals (like calcium or magnesium), or treatment chemicals (like chlorine).

• Density and Phase Changes

  • Density Standard: The density of pure water is approximately $1\text{ g/mL}$ at $4^{\circ}\text{C}$.

  • Seawater Density: Seawater is more dense than freshwater (approximately $1.025\text{ g/mL}$) because of its salinity ($3.5\%$ average salt content).

  • Solid vs. Liquid: Unlike most substances, water expands when it freezes.

    • Ice Floats: The density of ice is about $0.92\text{ g/mL}$. This occurs because hydrogen bonds form a rigid crystalline lattice that keeps molecules further apart than in the liquid state.

    • Ecological Impact: Floating ice provides an insulating barrier for lakes and oceans, preventing them from freezing solid and allowing aquatic organisms to survive at the bottom in warmer, liquid water.

Chemical Properties of Water

• Molecular Polarity

  • Water ($H_2O$) is a polar covalent molecule. Oxygen is highly electronegative, pulling shared electrons closer to itself.

    • This creates a dipole: a partial negative charge ($\delta^-$) near the oxygen atom and partial positive charges ($\delta^+$) near the hydrogen atoms.

• Hydrogen Bonding

  • The attraction between the $\delta^+$ hydrogen of one molecule and the $\delta^-$ oxygen of another creates a hydrogen bond.

  • Cohesion and Adhesion:

    • Cohesion: Water molecules sticking to each other (leads to high surface tension).

    • Adhesion: Water molecules sticking to other surfaces (leads to capillary action in plants).

• Thermodynamic Properties

  • High Specific Heat Capacity: Water can absorb a lot of heat energy before its temperature changes ($4.18\text{ J/g}\cdot^{\circ}\text{C}$), which helps regulate Earth's climate.

  • High Heat of Vaporization: A large amount of energy is required to convert liquid water into gas because hydrogen bonds must be broken. This is the basis for evaporative cooling (sweating).

• Solubility Dynamics

  • "Like Dissolves Like": Polar substances (solutes) dissolve in water (the solvent). Non-polar substances, like oils and fats, do not mix with water.

  • Dissociation of Ionic Compounds (e.g., NaCl):

    • Water molecules surround ions in a process called hydration shells.

    • The negative oxygens orient toward positive Sodium ions ($Na^+$).

    • The positive hydrogens orient toward negative Chloride ions ($Cl^-$).

  • This physical separation allows the salt to dissolve completely without changing the chemical identity of the sodium or chlorine.

Summary and Extensions

• Recap: The unique combination of polarity and hydrogen bonding dictates water's physical state, density, and role as a solvent.

• Implications: These properties are fundamental to biochemistry, meteorology, and environmental science.