Saltwater and freshwater are the two primary categories of water, distinguished by their dissolved salt content. Saltwater, found predominantly in oceans, contains a significant concentration of dissolved salts. Freshwater, present in rivers, lakes, and glaciers, has a much lower salt content.
Core Physical and Chemical Distinctions
The most apparent difference between saltwater and freshwater lies in their salinity, the concentration of dissolved salts. Seawater typically contains about 3.5% (35 parts per thousand) of dissolved salts, primarily sodium chloride, along with other minerals like magnesium and calcium. Freshwater, by contrast, generally has a salinity of less than 0.05% (0.5 parts per thousand). Saltwater is denser than freshwater due to dissolved salts, which also lower its freezing point to approximately -2 degrees Celsius, colder than freshwater’s 0 degrees Celsius. Dissolved salts also enhance water’s ability to conduct electricity, making saltwater a better electrical conductor.
Life Adapted to Different Aquatic Environments
The varying chemical compositions of saltwater and freshwater necessitate distinct biological adaptations for organisms. Organisms maintain an internal balance of salt and water through osmoregulation. Freshwater fish, for example, excrete large volumes of dilute urine and actively absorb salts through their gills to counter water entering their bodies. Conversely, saltwater fish drink large amounts of seawater and excrete concentrated salts through specialized gill cells, producing minimal urine. This physiological distinction explains why marine organisms like sharks, tuna, corals, and jellyfish thrive in oceans, while freshwater habitats support species like trout, bass, amphibians, aquatic insects, and plants like water lilies.
Global Presence and Human Uses
Water covers approximately 71% of Earth’s surface. The vast majority, about 96.5% to 97%, is saltwater found in oceans and seas. Freshwater accounts for only 2.5% to 3.5% of the total global water supply. Most freshwater is not readily accessible, with 68% to 69% locked away in glaciers and ice caps, and 30% existing as groundwater. Less than 1% of total freshwater is found in accessible forms like rivers and lakes. Saltwater serves as a medium for global transportation, supports extensive fishing industries, and is utilized for energy generation like tidal and wave power. Desalination technologies also convert saltwater into potable water. Freshwater is indispensable for drinking, agriculture (accounting for 70% of global use), industrial processes, domestic purposes, and recreation.