Seawater

Seawater is the saline water found in the world’s oceans and seas. It is a complex solution containing dissolved salts, inorganic compounds, organic matter, and gases. The typical salinity is about 35 practical salinity units (PSU), roughly 3.5% by mass, though salinity varies with latitude, depth, and proximity to freshwater sources.

The composition is dominated by sodium and chloride ions, which together constitute most of the dissolved salts. Other major ions include sulfate, magnesium, calcium, and potassium, with bicarbonate and other minor constituents present at lower concentrations. Seawater also contains dissolved gases such as oxygen and carbon dioxide, and trace nutrients essential for marine life. The pH of surface seawater is generally slightly alkaline, around 7.8 to 8.3, with regional variations influenced by carbon dioxide levels and biological activity.

Physically, seawater is denser than freshwater, and density increases with salinity while decreasing with temperature. Its density at typical surface conditions is about 1025 kg/m3. The freezing point of seawater is well below 0°C, while its boiling point is slightly higher than that of pure water. Salinity influences vertical and horizontal stratification, affecting ocean circulation and mixing.

Seawater plays a central role in Earth's climate and biogeochemical cycles. It covers about 71% of the planet’s surface and contains the majority of Earth’s dissolved inorganic carbon, supporting marine life and driving thermohaline circulation that distributes heat and nutrients. Humans interact with seawater through desalination, shipping, and resource use, while facing challenges from pollution, ocean acidification, and sea-level rise.