The water that fills the ocean is seawater—a complex solution of water, dissolved salts, gases, organic compounds, and suspended particles. The dominant ions are chloride, sodium, sulfate, magnesium, calcium, and potassium, which together account for most of the salinity.

Seawater’s physical properties—density, freezing point, heat capacity, and sound speed—differ from those of pure water because of the dissolved load. These properties govern circulation, climate interactions, and the propagation of acoustic signals. Chemical properties, including pH and the carbonate system, control the solubility of minerals and the availability of nutrients and toxins.

Although the major ions occur in nearly constant proportions, the concentrations of nutrients, trace metals, and gases vary widely with depth, location, and biological activity. Understanding seawater chemistry and physics is foundational to every branch of oceanography. Standard seawater is used worldwide to calibrate salinity measurements.

A single kilogram of typical seawater contains about 35 grams of dissolved salts.