Seawater

Seawater is the water found in the world's oceans and seas. It is distinct from freshwater because it contains high amounts of dissolved salts. On average, seawater has a salinity of about 3.5%. This means there are 35 grams of salt in every liter of water.

Most of these salts are made of sodium and chloride ions. This chemical makeup is vital because it changes how water behaves physically. These properties influence how the ocean moves, how it freezes, and how life survives within it.

Salinity, or the salt concentration, is the main driver of seawater's unique properties. When salt dissolves in water, it increases the mass more than it increases the volume. This makes seawater denser than pure water. The average density at the surface is about 1.025 kg/L. At the surface, density usually ranges from 1020 to 1029 kg/m3. Deep in the ocean, high pressure can push density to 1050 kg/m3 or higher.

This density also affects how sound travels. Sound moves through seawater at about 1,500 m/s. This is much faster than the 330 m/s speed of sound in the air.

Salt also changes the temperature at which water turns to ice. As salt concentration increases, the freezing point of the water decreases. At typical salinity, seawater freezes at approximately -2 °C. Scientists found a record of liquid seawater at -2 °C in 2010. This was discovered in a stream under an Antarctic glacier.

This shows how salt allows water to remain liquid even in extreme cold.

Salinity is not the same in every part of the ocean. Most seawater stays between 3.1% and 3.8% salinity. However, different environments create different levels. Near river mouths, melting glaciers, or heavy monsoon rains, the water becomes less saline due to freshwater runoff. In contrast, the Red Sea is the saltiest open sea. This is caused by high evaporation, low precipitation, and low river runoff. Some isolated bodies are even more extreme. The Dead Sea is about ten times saltier than the average ocean.

Scientists use specific scales to measure these changes. In the past, the Practical Salinity Scale used units called PSU. Today, the standard is the Reference Salinity scale, which uses grams per kilogram (g/kg). Researchers also monitor the pH, which measures how acidic or basic the water is. The pH of seawater is typically between 7.5 and 8.4. Before 1850, the surface pH was about 8.2. Between 1950 and 2020, the average surface pH fell from 8.15 to 8.05. This decline is part of a process called ocean acidification caused by carbon dioxide emissions.

Seawater is also a massive home for microscopic life. A single liter of seawater can hold more than 20,000 different species of microbes. These include bacteria, which can be aerobic or anaerobic. It also includes Archaea, which are single-celled organisms that thrive in extreme environments like hydrothermal vents.

Some Archaea even break down methane, a greenhouse gas. Other microbes, like cyanobacteria, were important in the evolution of our atmosphere. Scientists even study deep-sea organisms to find new antibiotics for medicine.

However, human activity can disrupt these delicate systems. Pollution from sewage and chemicals can cause population explosions of certain organisms. These are called blooms or red tides. Such blooms can produce biotoxins that move up the food chain. Large vessels also carry pathogens in their ballast water. This can spread diseases like cholera to new areas. Even though the ocean is vast, these changes show how connected the water's chemistry is to all life.