Composition of Sea Water & Dissolving Power of Water

Water as a “universal solvent”
- Owes its effectiveness mainly to its polarity.
- When an ionic or strongly polar compound is introduced, water molecules form a hydration shell around each ion/molecule (process = hydration), effectively pulling the substance apart and keeping particles in suspension.
Illustrative example: NaCl dissociation
- $\text{NaCl} \; \longrightarrow \; \text{Na}^+ + \text{Cl}^-$
- Sodium’s loosely-bound outer electron is “stolen” by chlorine → ions form.
- Water’s dipole aligns so that its partially negative oxygen surrounds $\text{Na}^+$ while its partially positive hydrogens surround $\text{Cl}^-$ .
Resulting properties
- Explains why seawater can hold vast quantities of dissolved ions.
- Drives nutrient and chemical cycling in oceans.

Geometry: Bent/angular; H–O–H bond angle ≈ $104.5^{\circ}$ .
Electronegativity contrast: O (3.5) » H (2.1) ⇒ electrons spend more time near O → partial charges: $\delta^-!(\text{O})$ , $\delta^+!(\text{H})$ .
Dipole moment leads to hydrogen bonding—the weak attraction between $\delta^+$ H of one molecule & $\delta^-$ O of another.
- Gives water high surface tension, high specific heat, and its exceptional dissolving capacity.
Comparison: Other small linear molecules (e.g., $\text{CO}_2$ ) may be non-polar overall despite polar bonds because their geometry cancels dipoles.

Often described as “a weak solution of almost everything.”
Approximate bulk composition:
- $96.5\%$ pure $\text{H}_2\text{O}$ .
- $3.5\%$ dissolved salts (mainly Na\^+, Cl\^-, Mg\^{2+}, $\text{SO}_4^{2-}$ , K\^+, Ca\^{2+}, etc.).
Total dissolved solids (TDS) in the global ocean ≈ $5.0 \times 10^{16}$ kg ("50 quadrillion tons").
Relative salinity: Average seawater is ≈ $220$ times saltier than typical freshwater.

Hypothetical: Remove all oceanic salt and spread it evenly over Earth’s land surface → a layer > $500$ ft (≈ $152$ m) thick, comparable to a 40-story building.
- Conveys scale of dissolved material involvement in marine chemistry.

Continental weathering
- Igneous & sedimentary rocks chemically weather → ions liberated (\textit{e.g.}, $\text{Na}^+, \text{K}^+, \text{Ca}^{2+}, \text{Cl}^-$ ).
- Transported via rivers/streams to the ocean.
Atmospheric & volcanic contributions
- Outgassing of $\text{CO}<em>2$ , $\text{SO}</em>2$ , HCl, and other volatiles from volcanoes—especially important in early Earth history.
- Early atmosphere likely richer in $\text{CO}_2$ , S, Cl ⇒ higher influx of corresponding ions.
Seafloor processes (implied for continuity)
- Hydrothermal vents remove/add Mg\^{2+}, Ca\^{2+}, K\^+, etc. (not explicitly in page but relevant to overall salt budget).

Salinity levels controlled over geologic time by balance between inputs (weathering/volcanism) and outputs (evaporite deposition, biological uptake, seafloor alteration).
Polarity-driven solvation chemistry underpins:
- Nutrient availability → marine productivity.
- Buffering of atmospheric gases (\textit{e.g.}, oceanic uptake of $\text{CO}_2$ ).

Water in seawater: $96.5\%$ .
Salt in seawater: $3.5\%$ .
Total oceanic dissolved solids: $5 \times 10^{16}$ kg (="50 million billion" tons).
Salinity factor vs. freshwater: $220$ .
Hypothetical salt layer thickness: $500$ ft (≈ $152$ m).
H–O–H bond angle: $104.5^{\circ}$ .

Polarity: Unequal distribution of charge within a molecule, leading to dipole moment.
Dipole: Molecule with partial positive & negative ends.
Hydration shell: Sphere of water molecules surrounding a dissolved ion/molecule.
Hydrogen bond: Weak intermolecular attraction between $\delta^+$ H and an electronegative atom (often O or N).
Salinity: Total concentration of dissolved salts, typically expressed in ‰ (parts per thousand).