Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, or 599 mM). This means that every kilogram (roughly one litre by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na) and chloride (Cl) ions). Average density at the surface is 1.025 g/ml. Seawater is denser than both fresh water and pure water (density 1.0 g/ml @ 4 C (39 F)) because the dissolved salts add mass without contributing significantly to the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity it freezes at about 2 C (28 F). The coldest seawater ever recorded (in a liquid state) was in 2010, in a stream under an Antarctic glacier, and measured 2.6 C (27.3 F). The thermal conductivity of seawater is 0.6 W/mK at 25 degC and a salinity of 35 g/kg. The thermal conductivity decreases with increasing salinity and increases with increasing temperature. Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near
Contributions by Plumbago, Vsmith, and 18.104.22.168.
The type of matter that ocean water is would be a mixture.