Chapter 5: Water and Seawater

Atom

An atom is the basic building block of all matter, consisting of a nucleus of protons and neutrons surrounded by electrons. Atoms combine to form molecules, which retain the properties of the substance.

Molecule

A molecule is a group of two or more atoms held together by shared electrons (chemical bonds). Water (H₂O) is a molecule made of two hydrogen atoms and one oxygen atom.

Polarity

Polarity refers to the uneven distribution of electrical charge within a molecule, resulting in positive and negative ends. Water’s bent geometry makes it a polar molecule, enabling unique interactions.

Hydrogen Bond

A hydrogen bond is a weak attraction between the positive end of one water molecule and the negative end of another. These bonds give water high cohesion and surface tension.

Universal Solvent

Water is called the universal solvent because its polarity allows it to dissolve nearly all ionic and polar substances. This property enables water to carry nutrients and waste in living organisms.

Salinity

Salinity is the total amount of dissolved solids in water, usually measured in parts per thousand (‰). Ocean salinity averages about 35‰, mostly from six major ions.

Halocline

A halocline is a layer in the ocean where salinity changes rapidly with depth, typically found between 300 and 1000 meters. It separates water masses of different salinity.

Thermocline

A thermocline is a layer of rapidly changing temperature with depth, usually present in low-latitude oceans. It acts as a barrier to vertical mixing.

Pycnocline

A pycnocline is a layer of rapidly changing density with depth, resulting from combined effects of temperature and salinity. It isolates surface water from deep water.

Buffering

Buffering is the process by which chemical reactions in seawater minimize changes in pH, keeping the ocean environment stable. The carbonate buffering system is key to this process.

Water’s Structure

Water molecules have a bent geometry, with hydrogen atoms on the same side of the oxygen atom, making water polar and able to form hydrogen bonds.

Thermal Properties

Water has high freezing and boiling points, high heat capacity, and high latent heats due to hydrogen bonding. These properties help regulate Earth’s climate.

Density

Water reaches maximum density at 4°C; below this, density decreases as bulky ice crystals form, causing ice to float.

Salinity Variation

Surface salinity varies with latitude—highest near the tropics due to evaporation, lowest at high latitudes due to precipitation and runoff. Salinity also varies with depth, but deep ocean salinity is consistent.

Seawater vs. Pure Water

Seawater has higher density, boiling point, and pH, but a lower freezing point than pure water.

Acid/Base Properties

Ocean surface water is slightly alkaline (average pH 8.1), but pH decreases with depth due to respiration and lack of photosynthesis. Buffering reactions keep pH stable.

Carbon and Oxygen Distribution

Carbon enters the ocean as CO₂, which reacts to form carbonate and bicarbonate ions. Oxygen is produced by marine photosynthetic organisms and consumed by respiration; both gases’ concentrations are affected by temperature, pressure, and biological activity.

Ice Floats

Unlike most substances, water expands as it freezes, so ice floats on liquid water. This insulates aquatic life in cold climates.

Egg in Saltwater

An egg floats in saltwater but sinks in freshwater due to the higher density of saltwater.

Dead Sea

The Dead Sea’s hypersaline water (330‰) is so dense that swimmers float easily, even with limbs above water.

Desalination

Distillation and reverse osmosis are methods used to remove salt from seawater, providing freshwater in arid regions.

Carbonate Buffering

When ocean pH rises, carbonic acid releases H⁺ ions to lower pH; when pH falls, bicarbonate ions remove H⁺ to raise pH, maintaining stability.