Ask A Scientist

How is salt water made in the ocean?

Asked by: Emma Taylor
School: Seton Catholic at All Saints
Grade: 6

Hobbies/Interests: Drawing

Answer from Sree Naresh Koneru

Research Associate, Clinical Science and Engineering Research Center and PhD Candidate

Research area: Bio Electromagnetics
Interests/hobbies: Wellness, cricket, volleyball, tennis, cooking, video gaming and most outdoor activities

Emma, you’ve asked a question that has captured the imagination of many civilizations. Folklore stories from almost every culture have their own version of how the ocean became salty. Although they are fun to read, the truth is in fact best explained by simple science. The very short answer to your question is, salt found in ocean water comes from land. Strange, but true! Let us see how that happens.

The first thing we should ask ourselves is "what is salt?" Salt, found as Sodium Chloride (NaCl) is composed of Sodium (Na) and Chloride (Cl) and accounts for up to 85% of the solids in ocean water. It is essential for all life to survive and in fact, 97% of all water found on Earth is salt water! An estimate made by the US Geological Survey says that if we took out all of this salt and laid it evenly on all the land on Earth, it would be around 500 feet high! So where did all this salt come from and how did it get into the water?

We know that the atmosphere consists of Oxygen and Carbon Dioxide (CO2) besides many other gases. Similar to how CO2 is dissolved in soda to create a fizz, atmospheric CO2 dissolves with rain (very little, so we do not see a fizz) and results in the formation of carbonic acid. This leads to rainwater being slightly acidic so when it falls on land, it physically and chemically erodes rocks by carrying salts and other constituents like calcium, magnesium and bicarbonate with it into water bodies such as streams and rivers. The water from these bodies eventually ends up in the ocean where all the salts and minerals dissolve into ocean water. Processes like evaporation and consumption by microorganisms leads to many of the other constituents being taken out of the water, but the salt remains. As the processes of evaporation and condensation take over, the amount of salt in the ocean keeps getting higher and higher. It is safe to assume that millions, or even billions of years ago ocean water was much less salty than what it is today.

Here is a fun experiment you can try under the supervision of an adult. Mix 1 tablespoon of salt in 200 ml of water and once it dissolves, put a drop in your mouth. Now boil the mixture on a stove until you see that no or very little water remains. To this, add another 200 ml of water and 1 tablespoon of salt and let it cool completely. Now put another drop in your mouth. Is it more or less salty than the first time you tasted it? Is the amount of water different or the same?

Last Updated: 3/1/17