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).[1] 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 °C and a salinity of 35 g/kg.[3] The thermal conductivity decreases with increasing salinity and increases with increasing temperature.[4] [edit]Salinity Main article: Salinity Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas[5] Salinity 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 unusually salty water. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater still. The density of surface seawater ranges from about 1,020 to 1,029 kg·m?3, depending on the temperature and sa inity. Deep in the ocean, under high pressure, seawater can reach a density of 1,050 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 1,500 m/s, and varies with water temperature, salinity, and pressure. Climate change, rising atmospheric carbon dioxide, excess nutrients, and pollution in many forms are altering global oceanic geochemistry. Rates of change for some aspects greatly exceed those in the historical and recent geological record. Major trends include an increasing acidity, reduced subsurface oxygen in both near-shore and pelagic waters, rising coastal nitrogen levels, and widespread increases in mercury and persistent organic pollutants. Most of these perturbations are tied either directly or indirectly to human fossil fuel combustion, fertilizer use, and industrial activity. Concentrations are projected to grow in coming decades, with negative impacts on ocean biota and other marine resources.[6] [edit]Compositional differences from freshwater Seawater contains more dissolved ions than all types of freshwater.[7] However, the ratios of solutes differ dramatically. For instance, although seawater contains about 2.8 times more 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 also constitute 48% of river water solutes but only 0.14% of all seawater ions.[7][8] 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 much more quickly.[8] The most abundant dissolved ions in seawater are sodium, chloride, magnesium, sulfate and calcium.