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Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%, or 35 parts per thousand (also expressed 35‰ or 35 ppt). 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 fresh water (which reaches a maximum density of 1.000 g/ml at a temperature of 4°C) because of the added weight of the salts and electrostriction.^[1] The freezing point of sea water decreases with increasing salinity and is about -2°C (28.4°F) at 35 parts per thousand. ^[2]

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 melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in the formation of unusually salty seawater. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater.

The density of surface seawater ranges from about 1020 to 1029&_160;kg·m^-3, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1050&_160;kg·m^-3 or higher. Seawater pH is limited to the range 7.5 to 8.4. The speed of sound in seawater is about 1500 m·s^-1, and varies with water temperature and pressure.

Seawater is more enriched in dissolved ions of all types than fresh water.^[3] However, the ratios of various solutes differ dramatically. For instance, although seawater is about 2.8 times more enriched with bicarbonate than river water based on molarity, the percentage of bicarbonate in seawater as a ratio of all dissolved ions is far lower than in river water; bicarbonate ions constitute 48% of river water solutes, but only 0.41% of all seawater ions.^[3][4] Differences like these are due to the varying residence times of seawater solutes; sodium and chlorine have very long residence times, while calcium (vital for carbonate formation) tends to precipitate out much more quickly.^[4]

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