When the water from the clouds falls and passes through the atmosphere, it interacts with the carbon dioxide present in it, causing a chemical reaction that gives rise to the formation of small amounts of carbonic acid.
This chemical compound is ultimately responsible for the rain water It has an acidic pH, which, in turn, causes it to drag minerals into the sea when it comes into contact with the rocks, aided by the force of the river.
In seawater some of these minerals precipitate and settle on the bottom, others are used by living organisms and the rest react among themselves.
In addition to rainwater, there are two other important sources of oceanic salts: submarine volcanic eruptions, hydrothermal fluids, and salt domes—subterranean salt domes that lie beneath the surface of the Earth’s crust.
Up to eighty chemical elements
Eighty different chemical elements have been identified in seawater, most of them in trace amounts. Almost all of the mineral salts are made up of chlorine, sodium, magnesium, sulfur, calcium, potassium, bromine, strontium, boron, fluorine… Furthermore, in seawater it is possible to find dissolved gold, yes, in very small amounts.
Taken together, all these chemical elements represent more than 99% of the mass of dissolved solutes in seawater.
If we focus on salt, the concentration in seawater is approximately 35 parts per thousand -35 grams per liter-, which means that if we could extract and store all the salt on our planet, we would be able to build a building forty stories high.
Now, not all the seas on our planet have the same salinity, for example, that of the Baltic Sea is between 6 and 18 grams per liter, which makes it brackish, not salty; while the salinity of the Dead Sea ranges between 350 and 370 grams per liter.
The salinity of the sea depends on several factors, including the evaporation of seawater caused by solar energy, but also latitude – tropical areas have higher salinity, freshwater flows, ocean depth and of sea currents and waves.
Sodium chloride does not evaporate
The sun causes liquid water to turn into steam, starting the hydrological cycle, the process of water circulation between the different compartments of the hydrosphere. It is a biogeochemical cycle in which water moves from one place to another and changes its physical state.
Within the immensity of our planet, 71% of the surface is water, the majority being salty –around 97%-. From this data it can be deduced that most of the water that evaporates and therefore precipitates as rain comes from the oceans, despite the fact that rainwater is not salty.
To understand why it does not contain sodium chloride we only have to carry out a simple experiment. If we take a glass of tap water and leave it in the sun for the water to evaporate, at the end of the glass there will be a thin layer of minerals dissolved in the water, since they have not been evaporated.
Well, something similar happens in the seas, only the water evaporates, the minerals that were dissolved precipitate and remain in the sea.
The fact that rainwater is not salty does not mean that it is suitable for human consumption, since precipitation removes particles and gases, it could contain microorganisms and heavy metals, such as lead, cadmium or aluminum. But as Kipling would say, that is another story.