Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every 1 kg of seawater has approximately 35 grams of dissolved salts (mostly, but not entirely, the ions of sodium chloride: Na+, Cl-). The average density of seawater at the surface of the ocean is 1.025 g/ml; seawater is denser than freshwater (which reaches a maximum density of 1.000 g/ml at a temperature of 4°C) because of the added mass of the salts. The freezing point of sea water decreases with increasing salinity and is about -2°C (28.4°F) at 35 gram per liter.

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