Ocean Salinity and Water Properties

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Seawater is about 3.5% dissolved salt (35 g per kg), mostly sodium chloride, from rock weathering and volcanic vents. Salinity varies: higher in hot, dry regions with strong evaporation (Red Sea) and lower where rivers or ice melt dilute it.

Salinity and temperature determine density, driving deep ocean circulation. Salt water freezes at around −2 °C and is denser than fresh water. Water has a high specific heat, which moderates climate. Desalination turns seawater into drinking water.

A quick calculation: 35 g of salt per kg gives about 3.5 percent. If you could evaporate all the sea salt and spread it on land, it would cover the continents to a depth of around 150 m. The Dead Sea is about 34 percent salt, nearly ten times the ocean, so swimmers float easily, whereas the Baltic Sea is brackish at only 0.5 to 1 percent because many rivers dilute it. Salinity is measured by conductivity, since salty water conducts electricity better.

Cold, salty water is densest and sinks, driving thermohaline circulation; freezing sea ice expels salt, which adds to the effect near the poles. Because of water's high specific heat (about 4.2 joules per gram per degree), the ocean warms and cools slowly, moderating coastal climates, with milder winters and cooler summers than inland areas. Desalination by reverse osmosis supplies much of the drinking water in Saudi Arabia and Israel, but it uses energy and creates brine. A common misconception is that seawater freezes at 0 degrees C; the salt lowers the freezing point. Another is that drinking seawater hydrates you; it dehydrates you because kidneys need extra water to remove salt.