The structure and composition of the sea includes: Water

 The structure and composition of the sea includes:

The composition of seawater is relatively stable across the world's oceans, but it can vary due to a number of factors: 

Salinity

The average salinity of seawater is 3.5% (35 grams of dissolved salts per liter), but it can vary between seas. For example, the Mediterranean Sea is slightly saltier at 38 parts per thousand (‰), while the northern Red Sea can be as salty as 41‰. 

Composition

The relative composition of dissolved salts is stable, but the composition can be modified by factors such as rivers, sedimentary material, and the formation and melting of sea ice. 

pH

The pH of seawater is typically between 7.5 and 8.4, but there is no universally accepted reference pH scale for seawater. 

Density

Seawater is denser than fresh water because the dissolved salts increase the mass by a larger proportion than the volume. 

Freezing point

The freezing point of seawater decreases as salt concentration increases. 

Water

Seawater is made up of 96.5% water and 3.5% salts. The salts include sodium chloride, magnesium chloride, and calcium chloride. 

Salinity

The salinity of seawater is the ratio of salt (in grams) to a liter of water, expressed as parts per thousand (ppt). Salinity varies by location, with lower salinity at the mouth of rivers where fresh water mixes with ocean water, and higher salinity in areas with high evaporation and little water circulation. 

Density

Seawater is denser than fresh water because of the dissolved substances in it. Warm water is less dense, while colder water is denser. These differences in density create ocean currents that transport water from the surface to greater depths. 

Carbon

The ocean contains a large amount of carbon, with the surface layer containing dissolved organic carbon and the deep layer containing dissolved inorganic carbon. Carbon enters the ocean from the atmosphere and from rivers. 

Chemical composition

The elemental composition of seawater is roughly 86% oxygen, 11% hydrogen, 2% chlorine, 1% sodium, 1% magnesium, 0.1% sulfur, 0.04% calcium, 0.04% potassium, 0.007% bromine, and 0.003% carbon. 

Economic importance

Seawater is a source of many commercially important chemical elements, including magnesium and bromine. It can also be desalted to provide a limitless supply of drinking water.