Saltwater

Saltwater refers to water that contains dissolved salts, primarily sodium chloride. It is the type of water found in oceans and seas and makes up about 97% of Earth’s liquid freshwater. The global saltwater reservoir is not uniform; salinity typically averages around 35 practical salinity units (PSU), or 35 grams of dissolved salts per kilogram of seawater, but it can range from roughly 30 to 40 PSU in different regions and with depth.

The major dissolved ions in seawater are sodium, chloride, magnesium, sulfate, calcium, and potassium, with sodium and chloride together forming the dominant salt. The exact composition varies with location, depth, temperature, evaporation, and freshwater input from rivers and precipitation. In coastal areas, mixing with freshwater can create brackish water, while extreme evaporation in isolated basins can produce hypersaline conditions.

Physically, saltwater is denser than freshwater, with densities around 1.025 g/cm3 at 25°C, and it freezes at temperatures lower than pure water, typically around −2°C at standard salinity. Its electrical conductivity is high, and its pH generally lies near neutral to slightly alkaline, often around 8.

Ecologically, saltwater supports complex marine ecosystems, including coral reefs, kelp forests, and various fish and invertebrate communities. Organisms regulate internal salinity through osmoregulation to cope with external salinity. Humans interact with saltwater through shipping, climate regulation, fisheries, desalination for freshwater, and mineral extraction, as well as through challenges such as saltwater intrusion into coastal aquifers.