What is the chemical composition of sea water?

September 23, 2019 | Environment
Sea water: H2O with a few extras | Photo: Shutterstock

Sea water - or salt water - is basically water from the oceans and seas. It covers roughly 70 percent of our planet.

The chemical composition of sea water is relatively simple to interpret - it's H2O with a few extras.

If you collect a sample of ocean water and take it to the laboratory, a chemist will quickly identify six major ions.

He or she will detect chloride (Cl−), sodium (Na+), sulfate (SO24−), magnesium (Mg2+), calcium (Ca2+), and potassium (K+).

They represent over 99 percent of all sea salts, even though the concentration of each found in a sample could vary.

Seawater also contains other substances and constituents, even if in smaller scales.

Boron, bromide, fluoride, inorganic carbon, and strontium are some of the dissolved substances found; at an even smaller scale, inorganic nitrogen and inorganic phosphorus.

Sea water: the oceans of the world have a salinity level of only 3.5 percent | Photo: Shutterstock

The Salinity Levels

When you think about sea water, you can almost feel that taste in your mouth, especially on warm summer days, when your skin dries after a swim or a surf.

Interestingly, the world's oceans have, on average, a salinity level of only 3.5 percent, i.e., 35 grams of dissolved salts per liter of water.

This salinity level is relatively stable everywhere.

Nevertheless, the North Atlantic, South Atlantic, Indian Ocean, and South Pacific areas are saltier than average (3.6 percent).

On the opposite side of the spectrum, the oceanic regions around the Antarctic, Arctic Ocean, Southeast Asia and west coast of North and Central America show lower salinity levels than average (3.4 percent).

But the fact is that the oceans are salty.

To put things into perspective, let's just say that freshwater bodies like rivers and lakes have, on average, less than 0.5 percent of salt.

The saltiest sea on the planet is the Dead Sea with around 33.7 percent salinity, that is, almost ten times more the levels present in the oceans of the Earth.

Variables like the location and depth of the sample, precipitation, proximity to river mouths and icebergs, and temperature may interfere with the seawater salinity levels.

Sea water: denser than pure or fresh water | Photo: Shutterstock

The Density of Sea Water

It's understandable why seawater is denser than pure or fresh water - the salts increase the mass.

As a result, people, animals, and objects will float easier in salt water than in fresh water.

And, because it has higher density levels, the water from the oceans freezes at around -2°C (28°F).

Its viscosity - i.e., internal resistance to flow - is also higher than that of fresh water.

Sea water's average pH typically ranges between 7.5 and 8.4, making it slightly alkaline.

However, the negative impact of climate change and global warming are making the oceans of the world more acidic, resulting in, for example, the death of the corals.

Sea water: people and objects float easier in salt water than in fresh water | Photo: Shutterstock

The Chemical Composition of Sea Water

In conclusion, and because seawater is a solution of salts in water - H2O - you can't really elaborate a specific chemical formula.

Its elemental composition is, more or less, as follows:

Oxygen: 86%
Hydrogen: 11%
Chlorine: 2%
Sodium: 1%
Magnesium: 1%
Sulfur: 0.1%
Calcium: 0.04%
Potassium: 0.04%
Bromine: 0.007%
Carbon: 0.003%

Although drinking seawater could potentially cause death to humans, its health benefits are immense and well-known.

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