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Seawater is a complex solution with an average salinity of about 35 parts per thousand (ppt), dominated by sodium chloride but containing many dissolved ions. Its density depends on three variables: temperature (higher temperature lowers density), salinity (higher salinity raises density), and pressure (higher pressure raises density). These properties govern how water masses stratify and circulate globally. Seawater also has a higher heat capacity than freshwater and freezes at approximately −1.8°C.
Work through density calculations for water parcels with different T-S combinations using T-S diagrams. Observe how adding salt depresses freezing point and raises density, connecting to colligative properties from chemistry.
Seawater is not simply salty water — it is a complex solution of dissolved ions, gases, and organic matter with physical properties that differ meaningfully from pure freshwater. The average salinity of the open ocean is about 35 parts per thousand (ppt), meaning roughly 35 grams of dissolved salts per kilogram of seawater. The dominant ions are sodium and chloride (table salt), but seawater also contains magnesium, sulfate, calcium, potassium, and many trace elements. Importantly, the ratio of major ions remains nearly constant across the ocean even as total salinity varies — this is known as the rule of constant proportions.
The most consequential physical property of seawater is its density, and density is controlled by three variables: temperature, salinity, and pressure. Temperature and density are inversely related: warmer water is less dense because thermal energy causes molecules to spread apart. Salinity and density are directly related: dissolved ions add mass without proportionally increasing volume. Pressure effects are significant only at depth (thousands of meters) and can largely be ignored at the surface. This means that in the surface ocean, the density of a water parcel is almost entirely determined by its temperature and salinity — summarized on a T-S diagram that oceanographers use to identify and track distinct water masses.
Two properties set seawater apart from freshwater in climatically important ways. First, seawater has a high specific heat capacity — it can absorb and store large amounts of heat without a large temperature change. This is why the ocean acts as a thermal buffer for Earth's climate, absorbing excess heat during warming periods and releasing it slowly. Second, seawater freezes at about −1.8°C rather than 0°C because dissolved salts depress the freezing point (a colligative property you may recognize from chemistry). When seawater does freeze, the ice crystal lattice expels most dissolved ions, producing nearly fresh sea ice and leaving behind a saltier, denser brine that sinks — a process central to deep ocean circulation.
A key misconception to correct: surface salinity is not uniform. The tropics have higher salinity because intense evaporation concentrates dissolved ions, while polar regions have lower salinity due to ice melt and net precipitation exceeding evaporation. This spatial variation in salinity — alongside temperature differences — creates the density contrasts that drive the large-scale circulation patterns you will study next in thermohaline circulation.