[Reading Science] Why Is Seawater Salty?

Why is seawater salty? Is salt produced separately?
Photo by YouTube Screenshot

[Asia Economy Reporter Kim Jong-hwa] Earth is a planet with a shortage of water. Compared to other planets without water, it is better off, but still not abundant. More than 70% of the Earth's surface is covered by water, and including glaciers, about 80% of the Earth's surface is surrounded by water. So why is there a shortage of water?

The water that living organisms, including humans, can drink is freshwater. This is because salty seawater is undrinkable. The proportion of freshwater on Earth is only about 2%. Moreover, much of it exists as glaciers in high mountain areas, so the amount in land, rivers, and lakes is less than 1%. Of this 1%, only a very small portion is drinkable water.

The reason humans cannot drink seawater is due to dehydration. The human body must maintain a constant salt concentration. The salt concentration in human cell fluid is about 0.9%, whereas seawater has a salt concentration of about 3%. Water moves from areas of low concentration to high concentration. Therefore, when seawater is consumed, water inside the human cells is drawn out, causing dehydration.

In movies where shipwreck survivors drift at sea, there are scenes where a crew member dies after drinking seawater due to thirst and running out of water, and this is exactly why. It is said that to dilute 1 liter of seawater, 1.5 liters of freshwater is needed. So if you drink one cup of seawater, you need to drink at least one and a half cups of freshwater to prevent dehydration.

Then, why is seawater salty? Seawater contains various substances dissolved in it, such as salt. These substances are called salts. The various minerals dissolved in seawater are called salts, and usually, about 35 grams of salts are dissolved in 1 kilogram of seawater. This amount is not considered very salty to the human palate.

For comparison, Korean soy sauce used for seasoning contains 300 grams of salt per 1 kilogram of water, so the salt content in seawater is lower than that of soy sauce. Salinity is the amount of salts dissolved in 1 kilogram of seawater expressed in grams, but it is represented in parts per thousand (‰, per mille), not percentage (%). Therefore, the salinity of seawater can be expressed as 35‰.

Tourists experiencing floating effortlessly without life jackets due to the higher salinity compared to the sea.
[Photo by YouTube screen capture]

The total amount of minerals dissolved in seawater is called salts, and the most abundant substance among these salts is sodium chloride (NaCl), commonly known as salt. The next most abundant salt in seawater is magnesium chloride. This component gives seawater its salty and slightly bitter taste.

How were these salts formed? Scientists believe that as streams and rivers flow into the sea, they carry salts from the rocks on land that have been dissolved over long periods, and these combine with many substances released by underwater volcanic eruptions to form the salinity of seawater.

How do substances in seawater combine? Various compounds in water are divided into cations and anions. Even sodium chloride consists of the cation sodium (Na+) and the anion chloride (Cl-). Cations such as sodium (Na+), calcium (Ca++), and potassium (K+) dissolve from rocks into the water, while anions such as chloride and sulfate come from underwater volcanic activity.

The salinity of the Dead Sea, a lake located between Jordan and Israel, is about 300‰, which is similar to the salinity of Korean soy sauce. It would not be an exaggeration to call it a soy sauce sea. On the other hand, the Baltic Sea in Northern Europe is a mild sea, with salinity ranging from 6 to 8‰, which is very low compared to the typical seawater salinity of 35‰.

Therefore, drinking Baltic Sea water reportedly does not cause dehydration like typical seawater. Instead, it is said to replenish moisture. If a shipwreck survivor were drifting in the Baltic Sea, their chances of survival might be relatively higher.

The seas surrounding Korea have lower salinity compared to the world's oceans, with the West Sea having lower salinity than the East Sea. This is because many rivers from China supply water to the West Sea. Also, about 50% of Korea's annual rainfall occurs in summer, so the salinity in summer is lower than in winter.