Seawater

Sea water comes from the ocean.

It has salt in it. This salt makes the water heavy. It is heavier than fresh water.

The salt also keeps the water from freezing easily. Some parts of the sea are very salty. The Red Sea is one of them. Other parts have less salt from rivers. Many tiny living things live in the sea.

It is a busy world under the waves.

Seawater is the water found in our oceans. It is different from fresh water because it contains salt. Most seawater has a salinity of about 3.5%. Salinity is a way to measure how much salt is in the water.

The salt is mostly made of sodium and chloride. This salt makes seawater denser than fresh water. Density means how heavy a liquid is for its size. Because it is dense, seawater sinks more easily than fresh water.

Salt also changes when the water freezes. More salt means the water stays liquid at colder temperatures. In 2010, scientists found liquid seawater at -2 °C under a glacier.

Not all ocean water is the same. The Red Sea is very salty because water evaporates there. Near river mouths, the water is less salty. This is because rivers add fresh water. Some plants and tiny life forms even use the salt to live.

Desalination plants can even take salt out of seawater to make it fresh.

Seawater is the water found in our vast oceans and seas. It is different from the fresh water we drink because it contains dissolved salts. On average, seawater has a salinity of about 3.5%. This means that every kilogram of seawater contains about 35 grams of salt.

Most of this salt is made of sodium and chloride ions. This salt is important because it changes how the water behaves. Seawater is denser than fresh water. Density is a measure of how much mass is in a certain volume. Because of the salt, seawater has a higher mass for its size.

Salt changes the way seawater freezes and moves. When salt concentration increases, the freezing point of the water decreases. At typical salinity, seawater freezes at about -2 °C. In 2010, scientists found liquid seawater at -2 °C under an Antarctic glacier.

The density of the water also changes based on its temperature and salinity. At the surface, the density is often around 1.025 kg/L. Deep in the ocean, high pressure can push the density to 1050 kg/m3 or even higher. This density helps move water around the planet. It also affects how sound travels through the deep sea. Sound moves at about 1,500 m/s in seawater.

Salinity is not the same everywhere in the world. Most seawater has a salinity between 3.1% and 3.8%. However, some places are much saltier than others. The Red Sea is the saltiest open sea. This happens because of high evaporation and low rainfall. In other places, the water is less salty. This occurs near river mouths where fresh water flows in. It also happens near melting glaciers or during heavy monsoon rains. Some isolated bodies of water are even saltier. The Dead Sea is about ten times saltier than the average ocean.

Scientists use different scales to measure these things. In the past, people used the Practical Salinity Scale. This measured salinity in units called PSU. Today, the standard is the Reference Salinity scale. This scale uses grams per kilogram to show how much salt is present. Scientists also study the pH, which measures how acidic or basic the water is. The pH of seawater is usually between 7.5 and 8.4. Before the year 1850, the surface pH was around 8.2. Since then, the pH has been decreasing. This change is due to a process called ocean acidification. This process is related to carbon dioxide emissions.

Seawater is also home to a huge world of tiny living things. A single liter of seawater might hold more than 20,000 different species of microbes. These include bacteria and even tiny viruses. Some microbes, like cyanobacteria, helped create the oxygen in our atmosphere. Other tiny life forms, called Archaea, live in extreme places like hydrothermal vents.

Some people even look at deep sea trenches to find new medicines. They hope to find new antibiotics in the unique organisms there. Even though the ocean is huge, human waste can still hurt it. Pollution can cause tiny organisms to grow too fast in events called blooms. This shows how connected the salt, the water, and life truly are.

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.