As water from clouds falls and passes through the atmosphere, it interacts with the carbon dioxide present therein, triggering a chemical reaction that results in the formation of small amounts of carbonic acid. This chemical compound is ultimately responsible for rainwater's acidic pH, which, in turn, causes it to carry minerals to the sea when it comes into contact with rocks, aided by the force of the river. In seawater, some of these minerals precipitate and settle to the seabed; others are used by living organisms, and the rest react with each other. In addition to rainwater, there are two other important sources of oceanic salts: underwater volcanic eruptions, hydrothermal fluids, and salt domes—subterranean salt domes found beneath the surface of the Earth's crust. Up to 80 chemical elements have been identified in seawater, most of them in trace amounts. Virtually all mineral salts are made up of chlorine, sodium, magnesium, sulfur, calcium, potassium, bromine, strontium, boron, fluorine, and so on. Furthermore, dissolved gold can be found in seawater, albeit in very small quantities. Together, these chemical elements represent more than 991 parts per thousand (TP3T) of the mass of solutes dissolved in seawater. Focusing 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 forty-story building. However, not all of our planet's seas have the same salinity. For example, the Baltic Sea's salinity ranges between 6 and 18 grams per liter, making it brackish, not salty; while the Dead Sea's salinity 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 on latitude (tropical areas have higher salinity), freshwater flows, ocean depth, and ocean currents and waves. Sodium chloride does not evaporate. The sun causes liquid water to turn into vapor, initiating the hydrological cycle, the process of water circulation between the different compartments of the hydrosphere. This is a biogeochemical cycle in which water moves from one place to another and changes its physical state. Within the vastness of our planet, 71% of the surface is water, the majority of which is 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, even though rainwater is not salty. To understand why it doesn't contain sodium chloride, we just have to perform a simple experiment. If we take a glass of tap water and leave it in the sun to allow the water to evaporate, a thin layer of minerals dissolved in the water will remain at the bottom of the glass, since they haven't evaporated. Well, something similar happens in the sea: only the water evaporates; the dissolved minerals precipitate and remain in the sea. MORE INFORMATION news No Astronauts on the International Space Station remain in danger: NASA is also studying their rescue news No China finds never-before-seen rocks on the Moon Just because rainwater isn't salty doesn't mean it's fit for human consumption. Because precipitation removes particles and gases, it could contain microorganisms and heavy metals, such as lead, cadmium, or aluminum. But as Kipling would say, that's another story. ABOUT THE AUTHOR Pedro Gargantilla is an internal medicine physician at El Escorial Hospital (Madrid) and the author of several popular science books.
