Physicochemical Properties of Water and Air-Sea Interactions

Physicochemical Properties of Water

The Water Molecule (H₂O)

• Composition: Composed of $2$ hydrogen atoms and $1$ oxygen atom.

• Bonds: Atoms are held together by covalent bonds, meaning they share electrons.

  • These are strong bonds, requiring a significant amount of energy to break.

Structure and Polarity

• Lone Pair Electrons: The oxygen atom in water possesses $2$ lone pairs of electrons.

• Bent Geometry: The presence of these lone pairs pushes the hydrogen atoms, resulting in a "bent" molecular geometry rather than a linear one.

  • The angle between the hydrogen atoms is approximately $104.5^ ext{o}$.

  • This angle is less than a bonded tetrahedral (e.g., in methane, $ext{CH}_4$) due to the greater repulsion caused by the lone pairs.

• Polarity (Dipolar): The bent shape and the difference in electronegativity between hydrogen and oxygen create a polar molecule.

  • Dipolar means it has two distinct poles: a slightly negative end (around the oxygen) and a slightly positive end (around the hydrogens).

  • This polarity is crucial for water's unique properties and its lattice structure.

Electronegativity and Intermolecular Bonds

• Electronegativity ($\chi$): This is a measure of an atom's ability to attract electrons in a chemical bond.

  • Oxygen ($\chi_ ext{O} = 3.5$) is significantly more electronegative than hydrogen ($\chi_ ext{H} = 2.1$).

  • The large difference ($1.4$) in electronegativity between O and H creates the polarity of the water molecule.

• Intermolecular Hydrogen Bonds: The positive end of one water molecule is attracted to the negative end of another water molecule, forming a hydrogen bond.

  • These are intermolecular bonds (between molecules), which are much weaker than intramolecular covalent bonds (within a molecule).

  • However, H-bonds are the strongest type of intermolecular bond.

  • They form between hydrogen and highly electronegative elements like Fluorine ($\chi_ ext{F} = 4.0$), Nitrogen, and Oxygen (examples: H-F, H-N, H-O).

Properties Resulting from Polarity and H-Bonds

• Cohesion: Water molecules attract other water molecules.

  • This leads to surface tension, where the surface of water acts like a thin, elastic film.

• Adhesion: Water molecules can 'stick' to other surfaces (e.g., glass).

• Universal Solvent: Water's polarity makes it an excellent solvent, capable of dissolving more substances than most other liquids, especially polar and ionizable solutes.

  • When an ionic (charged) or polar substance dissolves, water molecules reorient themselves to surround and separate the solute particles based on their charges (positive ends of water to negative ions/poles, negative ends of water to positive ions/poles).

  • Example: Dissolving salt (NaCl) in water. Na⁺ and Cl⁻ ions are held by electrostatic attraction (ionic bonds) but are separated by water molecules.

  • Insoluble substances, like bears (as a humorous example), are non-polar.

• High Specific Heat Capacity: Discussed further below.

• Driving Earth’s Energy Balance: Discussed further below.

Thermal Properties of Water

States of Matter

• Water exists naturally in all three states: solid (ice), liquid, and gas (water vapor).

• Changes between these phases require the breaking or creation of bonds, which involves adding or removing energy.

Heat vs. Temperature

• Heat: The amount of energy transferred from one body to another due to a difference in temperature.

  • Proportional to the average kinetic energy of molecules.

  • SI unit for heat/energy: Joule (J).

• Temperature (T): A direct measure of the average kinetic energy of the molecules within a substance.

  • An object's response to an input or removal of heat.

  • SI unit for temperature: Kelvin (K).

    • Absolute zero: $0 ext{ K} = -273.15^ ext{o} ext{C}$.

    • A change of $1 ext{ K}$ is equivalent to a change of $1^ ext{o} ext{C}$ ($ext{Δ}1 ext{ K} = ext{Δ}1^ ext{o} ext{C}$).

    • Example: $20^ ext{o} ext{C} = 293.15 ext{ K}$.

  • Degrees Celsius ($^ ext{o} ext{C}$) is also very commonly used.

Specific Heat Capacity

• Definition: A measure of a substance's ability to absorb and hold heat.

  • SI unit: $ext{Joule} ext{ kJ} / ext{kg} / ext{K}$ (or $ext{kJ} / ext{kg} / ^ ext{o} ext{C}$).

• Water's Property: Pure water has a very high specific heat capacity ($4.184 ext{ kJ} / ext{kg} / ^ ext{o} ext{C}$).

  • For comparison, Iron (Fe) has a specific heat capacity of $451 ext{ J} ext{ kg}^{-1} ext{ K}^{-1}$.

• Implication: Water can absorb or release large amounts of heat with only a relatively small change in its own temperature. This means it takes a lot of time for water to heat up or cool down.

  • Example: Sand heats up quickly (low specific heat capacity), while water remains cool (high specific heat capacity) on a hot day. Conversely, water can retain heat longer than air on a cold day.

Latent Heat vs. Sensible Heat

• Latent Heat: The heat energy absorbed or released during a phase change (e.g., melting, freezing, evaporating, condensing) without a change in temperature.

  • This energy is used to break or form hydrogen bonds.

  • Example: It takes $334 ext{ kJ}$ of latent heat to melt $1 ext{ kg}$ of ice.

• Sensible Heat: The heat energy that causes a change in temperature of a substance.

• Calculation Example: If it takes $334 ext{ kJ}$ of latent heat to melt $1 ext{ kg}$ of water, how many $^ ext{o} ext{C}$ temperature increase does it take for the equivalent sensible heat?

  • Using water's specific heat capacity ($4.184 ext{ kJ/kg/}^ ext{o} ext{C}$):

  • $rac{334 ext{ kJ}}{4.184 ext{ kJ/kg}/^ ext{o} ext{C}} = 79.8^ ext{o} ext{C}$ (using more precise value for specific heat capacity, the slide used $4.218$ resulting in $rac{334}{4.218} = 79.2^ ext{o} ext{C}$)

Water as a Global Temperature Moderator

• Water's high specific heat and latent heat are critical for moderating Earth's climate.

• Heat is absorbed from tropical oceans (evaporation latitudes) through latent heat.

• This latent heat is carried towards the poles by atmospheric and oceanic circulation.

• The heat is then released at higher latitudes through precipitation (condensation), moderating global temperatures.

Physical Properties of Water - Density

• Pure Water Density: Approximately $1.0 ext{ g/cm}^3$.

• Factors Affecting Density: Water temperature, salinity, and pressure.

Why Ice Floats

• Density Maximum: Pure water reaches its maximum density at approximately $4^ ext{o} ext{C}$.

• Expansion upon Freezing: As liquid water cools from $4^ ext{o} ext{C}$ to $0^ ext{o} ext{C}$ and freezes into ice, it expands slightly.

  • This expansion occurs because the hydrogen bonds form a more rigid, open lattice structure, making the ice less dense than liquid water.

• Ice Density: The density of ice is about $0.917 ext{ g/cm}^3$, which is less than liquid water's maximum density.

• Consequence: Ice floats on water, which is crucial for aquatic life as it insulates the water below, preventing entire bodies of water from freezing solid.

  • The bond angle in ice changes from $105^ ext{o}$ (liquid) to $109^ ext{o}$ (solid lattice).

Effect of Salinity on Density and Thermal Properties

• Seawater Density: The average density of seawater is approximately $1.028 ext{ g/cm}^3$, higher than pure water due to dissolved salts.

• Freezing Point Depression: Dissolved solids in seawater reduce its freezing point.

  • Seawater freezes at about $-1.91^ ext{o} ext{C}$.

• Boiling Point Elevation: Salinity also slightly increases water's boiling point (by approximately $0.6^ ext{o} ext{C}$).

• Latent Heat Reduction: Salinity lowers the latent heat of water by about 4 ext{%}.

What is Salinity?

• Definition: The total amount of solid material dissolved in water, including dissolved gases.

• Average: Averages about 3.5 ext{%} or $35 ext{‰}$ (parts per thousand) for ocean water.

• Major Ions: $7$ ions account for 99 ext{%} of all dissolved solids, with Chloride (Cl⁻) and Sodium (Na⁺) being the most dominant.

  • Other major ions: Sulfate (SO₄²⁻), Magnesium (Mg²⁺), Calcium (Ca²⁺), Potassium (K⁺), Bicarbonate (HCO₃⁻ traces in seawater).

  • Many other elements (at least $80$) are present in trace amounts.

• Net Charge: Ocean water has an overall neutral charge, meaning 50 ext{%} positive ions balance 50 ext{%} negative ions by molar or charge equivalent ratios, not by mass.

  • This means the number of moles of positive charges equals the number of moles of negative charges.

Sources of Ions in the Ocean

Ions continuously enter the oceans through processes such as:

• River Discharge: Carries dissolved components from land.

• Volcanic Eruptions & Atmospheric Deposition: Release gases and particles that dissolve in water.

• Hydrothermal Activity: Chemical reactions at mid-ocean ridges add and remove dissolved components.

• Biological Processes: Organisms incorporate or release ions.

Removal of Ions from the Ocean

Ions are continuously removed from the ocean through processes like:

• Adsorption and Precipitation: Ions adhere to particles or form insoluble compounds.

• Sea Spray: Entrapment of ions in aerosols that return to land or are deposited elsewhere.

• Biological Processes: Marine organisms use ions to produce shells or skeletons (e.g., Ca²⁺, CO₃²⁻).

• Hydrothermal Activity: Removes certain dissolved components.

Ocean Equilibrium and Residence Time

• Equilibrium: The ocean is in a state of chemical equilibrium, where the rate of addition of dissolved solids equals their rate of removal.

  • This is why the oceans are not continuously getting saltier despite constant input from rivers.

• Residence Time: The average length of time an ion spends in the ocean.

  • Calculated as: $ext{Residence time (yrs)} = rac{ ext{Total amount ion in water (kg)}}{ ext{Input rate (kg/yr)}}$.

  • The input rate is often estimated as: $ext{Avg ion Conc in rivers (kg/km}^3 ext{)} imes ext{River discharge (km}^3 ext{/yr)}$.

  • Residence times vary greatly depending on how chemically active (reactive) an ion is; less reactive ions (e.g., Cl⁻, Na⁺) have very long residence times and are therefore more abundant in seawater.

Seawater vs. Freshwater Composition

• The dominant ions in seawater (Cl⁻, Na⁺) are very different from those in typical freshwater (HCO₃⁻, Ca²⁺).

• This discrepancy is due to the different reactivities and residence times of ions in the ocean versus in rivers.

  • Seawater salinity: $35 ext{‰}$.

  • Freshwater salinity: approximately $0.12 ext{‰}$.

Measuring Salinity

• Salinometer: Measures the electrical conductivity of seawater, which is directly related to salinity.

• Principle of Constant Proportions (William Dittmar):

  • States that while the absolute salinity of seawater varies, the ratios of major dissolved solids remain constant throughout the ocean.

  • This allows scientists to measure the concentration of just one major dissolved solid (typically chloride, Cl⁻, which accounts for 55.04 ext{%} of dissolved solids by weight) and use a formula to determine total salinity.

  • Formula: $ext{Salinity (‰)} = 1.80655 imes ext{Chlorinity (‰)}$.

• Remote Sensing (Satellites):

  • Instruments like the Aquarius on satellites (2011-2015) use microwave radiometers to measure sea surface salinity.

  • Salinity affects the electrical conductivity of seawater, which in turn alters the microwave radiation emitted from the ocean.

  • Radar scatterometers are used for data corrections to account for ocean roughness (waves).

Salinity Variability in the Ocean

• Global Patterns: Driven by the balance between water addition and removal:

  • Evaporation: Increases salinity by removing water, leaving dissolved solutes behind.

  • Precipitation (rain/snow): Decreases salinity by adding freshwater.

  • Freezing of seawater: Increases salinity of the remaining liquid water as salts are excluded from the ice lattice.

  • Melting of ice: Decreases salinity.

  • Groundwater flow and river runoff: Decrease salinity.

Salinity, Density, and Temperature Variations with Depth (Layered Ocean)

• Ocean waters are typically layered due to variations in density.

• Density generally increases with depth (from $1.022 ext{ g/cm}^3$ to $1.030 ext{ g/cm}^3$).

• Three Zones (Water Masses):

  • Surface Zone (Mixed Layer): Upper layer influenced by surface currents and waves, relatively uniform properties.

  • Pycnocline: A layer of rapidly changing density with depth.

    • This layer is absent at high latitudes where the water column is generally uniform (isopycnal).

  • Deep Zone: Cold, dense water below the pycnocline.

• Halocline: A layer of rapidly changing salinity with depth.

  • At high (and equatorial) latitudes: Precipitation >> Evaporation, leading to lower surface salinity and a halocline where salinity increases with depth.

  • At low to mid-latitudes: Evaporation > Precipitation, leading to higher surface salinity and a halocline where salinity decreases with depth.

• Thermocline: A layer of rapidly changing temperature with depth.

  • Temperature typically decreases with depth in lower latitudes.

  • Influenced by seasonality and location.

  • Absent at high latitudes, where the water column is isothermal (uniform temperature with depth).

Dissolved Gases in the Oceans

• Solubility Factors: The amount of gas that can dissolve in ocean water depends on:

  • Temperature (T): Gas concentration increases as temperature decreases.

  • Pressure (P): Gas concentration increases as pressure increases.

  • Solubility (s): Intrinsic solubility of the specific gas (e.g., $CO_2$ is highly soluble).

• Gas Distribution (Percentages):

  • Air: N₂ (78 ext{%}), O₂ (21 ext{%}), CO₂ (0.04 ext{%}).

  • Total Ocean: CO₂ (83 ext{%}), N₂ (11 ext{%}), O₂ (6 ext{%}).

  • Surface Ocean: N₂ (48 ext{%}), O₂ (36 ext{%}), CO₂ (15 ext{%}).

  • The high concentration of $CO_2$ in the total ocean is due to its high solubility and chemical reactions in water.

Air-Sea Interactions

Interconnected System

• Earth's ocean and atmosphere form an interdependent system.

• Winds are a major driver of ocean circulation.

• Massive exchanges of energy and matter occur between the oceans and atmosphere.

• Key features of global weather and climate are driven by these interactions.

• There are also significant biological connections (e.g., seabirds relying on marine productivity).

Latitude and Solar Radiation (Insolation)

• Unequal Intensity: The curvature of the Earth's surface causes unequal intensity of solar irradiation (insolation).

  • At low latitudes (equator), sunlight strikes the Earth at a high angle of incidence, concentrating energy over a smaller area. There's less atmospheric absorption and lower albedo (less reflection).

  • At high latitudes (poles), sunlight strikes at a low angle of incidence, spreading energy over a much larger area. There's more atmospheric absorption, and higher albedo (e.g., thick sea ice reflects up to 90 ext{%} of sunlight).

• Energy Balance: This results in:

  • Low-latitude oceans: Experience a net heat gain.

  • High-latitude oceans: Experience a net heat loss.

• Circulation: This surplus energy at low latitudes is transferred towards the poles through both atmospheric and oceanic circulation, moderating global temperature differences.

Hemispheric Temperature Differences

• The Southern Hemisphere has more ocean (81 ext{%}) than the Northern Hemisphere (61 ext{%}), suggesting it might be warmer due to greater solar absorption.

• However, the mean temperature of the Southern Hemisphere ($13.3^ ext{o} ext{C}$) is actually lower than the Northern Hemisphere ($15.2^ ext{o} ext{C}$).

• Reason: The strong Antarctic Circumpolar Vortex limits north-to-south energy transfer in the Southern Hemisphere, trapping cold air around Antarctica.

Characteristics of the Atmosphere

• Composition: Dry air is dominated by Nitrogen (N₂) and Oxygen (O₂).

• Temperature Profile: The troposphere (lowest atmospheric layer) is heated from below, so temperature generally decreases as you move higher.

  • This temperature profile is why the troposphere is well-mixed.

Interrelationship of Temperature, Density/Pressure, and Relative Humidity

• Convection Cells: Atmospheric density varies with temperature, creating convection cells.

  • Warm surface air: Is less dense, holds more water vapor, and creates low-pressure zones. This air rises, expands, and cools.

    • Rising air cools due to adiabatic expansion (occurs without heat exchange with surroundings).

  • Cool upper troposphere air: Becomes denser, holds less water vapor. This air sinks and compresses, creating surface high-pressure zones.

    • Sinking air warms due to adiabatic compression.

• Wind: The movement of air from areas of high pressure to areas of low pressure.

• Manifestation: This leads to phenomena like orographic precipitation (rain on the windward side of mountains from rising, cooling air) and rain shadows (dry conditions on the leeward side from sinking, warming air).

Takeaways from this Lecture

• Water's Polarity: Responsible for cohesion, adhesion, and its role as a universal solvent.

• Thermal Concepts: Understanding the distinction between heat, heat capacity, and temperature.

• Latent Heat: Its importance in phase changes and moderation of global temperatures.

• Ice Floats: Explanation based on water's unique density characteristics.

• Salinity Effects: How dissolved solids affect freezing point, boiling point, and latent heat.

• Salinity: Definition, drivers, variability, and measurement methods.

• Ocean Equilibrium: The balance between ion input and removal, maintaining constant proportions of major ions.

• Variations with Depth: How salinity, density, and temperature vary, leading to a layered ocean and the formation of pycnocline, halocline, and thermocline zones.

• Air-Sea Interactions: The fundamental relationship between solar radiation, latitude, atmospheric composition, and the complex interplay of air temperature, density, water vapor content, and pressure.