The short answer starts on land
Seawater contains dissolved salts supplied partly by the weathering of rocks. Rainwater interacts with minerals, and streams and rivers carry dissolved ions towards the sea. A freshwater river therefore does not mean water with absolutely no salts: its concentration is usually much lower than the ocean’s. Clear water can still contain dissolved substances that you cannot see.
Why evaporation leaves salts behind
During evaporation, water enters the atmosphere while most dissolved salts remain. The water vapour then participates in cloud formation and precipitation. This helps explain salt accumulation, but the ocean is not a saucepan that simply boils down forever. Water supply, mixing, chemical reactions and removal of substances into sediments all occur alongside evaporation.
How much salt is in seawater
Average ocean salinity is roughly 35 grams of dissolved salts per kilogram of seawater, or about 3.5 percent by mass. That is a reference value, not a reading for every beach. Sodium and chloride dominate, but other dissolved substances are present too. Real seawater is therefore more chemically complex than water mixed with ordinary table salt alone.
Why the Baltic is less salty
The Baltic receives substantial freshwater from rivers and has restricted exchange with saltier North Sea water. Its water is consequently described as brackish. Salinity changes from the western entrance towards northern and eastern areas and also varies with depth. A single number for the whole Baltic conceals those differences, including important changes in conditions for aquatic life.
Why some waters become extra salty
Strong evaporation with limited water exchange can raise salt concentrations. The Dead Sea, geographically a lake, is a familiar example. However, the ocean does not have to grow saltier every year: salts also leave the water and become part of bottom sediments. Seafloor processes, including hydrothermal activity, contribute to the exchange of substances as well as inputs from land.
How to compare salinity readings
Check the location, depth and date attached to a measurement. Do not compare an open-ocean average with a river-mouth sample as if the conditions were identical. For a small home observation, leave some table-salt solution on a separate saucer: a solid residue remains after the water evaporates. This illustrates a principle, rather than reproducing the complex chemistry of a real sea.
A general educational overview, not a substitute for professional guidance. Check current local rules and trusted primary sources before making consequential decisions.
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