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Why is ocean water salty?

Why is ocean water salty?

Answer: 4 billion years ago, the acidic ocean dissolved the sodium on land.

Sea water is salty because sodium chloride, the main component of salt, is dissolved in it. This is something that most people know. So, do you know why sodium chloride is dissolved in seawater? Where exactly did sea salt come from?

The answer can be traced back to the birth of the Earth approximately 4.6 billion years ago1). The newly formed Earth was covered with a series of colliding "micro-planets" ranging in diameter from a few kilometers to 10 kilometers, and its surface was covered with sludgy magma. The asteroids contained hydrogen, water vapor, carbon dioxide, nitrogen, chlorine gas, hydrochloric acid, sulfur, and other gases that gradually accumulated above the Earth, eventually creating the Earth's atmosphere2).

Later, as the temperature of the earth's surface cools to about 100°C over millions of years, the water vapor in the atmosphere falls in large amounts as rain and accumulates on the earth's surface. This is the beginning of the oceans. The rain that fell at that time contained chlorine gas from the atmosphere, so the seawater was acidic. The seawater dissolved and combined with the sodium contained in the rocks that had cooled and hardened earlier to form sodium chloride.

The salinity of the Earth's oceans, where we live today, is about 3.4%, but it varies depending on the ocean area and depth (see related article, "How salty is seawater?"). ). (See related article, "How salty is seawater?") However, the composition of salt in seawater (the ratio of the components contained in seawater) has remained almost the same. This was first discovered in the late 19th century. The British warship Challenger collected seawater samples from around the world, and Professor Dittmar of the University of Edinburgh analyzed them. As a result, he found that while the concentration of seawater varied from region to region, the composition of salt was constant (Dittmar's Principle). The percentage of sodium chloride in seawater is 77.9%. Other than that, it contains 9.6% magnesium chloride, 6.1% magnesium sulfate, 4.0% calcium sulfate, and 2.1% potassium chloride, which is the component of bittern used to harden tofu3).

Article published: March 2022

reference data

(1) Eiichi Tajika / Supervisor, Dictionary of Earth Sciences. 2021. Newton Press

(2) Kantaro Fujioka, "How the Sea Formed: A History of Global Evolution on a Spectacular Scale". 2013. Kodansha Ltd.

(3) Tobacco and Salt Museum, "World Salt, Japanese Salt World Salt Resources (Composition of Sea Water, Distribution of World Salt Resources)," Tobacco and Salt This and That: https://www.tabashio.jp/collection/salt/s4/index.html

Supervisor: Mitsuharu Oyama

Born in Tokyo in 1957. Completed a master's degree at Tokyo Institute of Technology. After working as a physics teacher at a high school, a chief instructor at Chiba Prefectural Board of Education, and principal of Chiba Prefectural Chousei High School, he is currently a professor at Shumei University School Teachers' College, where he teaches lectures and exercises on teaching methods for "Science and Mathematics Exploration" and "Integrated Learning Time". He has appeared in many science experiment classes and TV experiment programs. He is also a project advisor for the Chiba City Science Museum, an executive director of the Japanese Society of Physics Education, a member of the Japanese Society for Science Education and the Japanese Society for Science Education, and a member of the editorial board of the monthly magazine "Science Education.

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