Salt from Sea Water and the Ocean's Resources

What is in sea water?

Sea water is a solution. The water is the solvent and many salts are dissolved in it. Common salt (sodium chloride) is the largest part. On average, every 100 g of sea water holds about 3.5 g of dissolved salts. That is why the sea tastes salty and why we cannot drink it.

Rivers wash tiny amounts of minerals from rocks into the sea. The water evaporates but the minerals stay, so over millions of years the sea became salty.

Making salt in salt pans

This method needs only sunshine, wind and wide flat land near the sea.

1. Let in sea water. Gates let sea water flow into shallow ponds.
2. Evaporate. Sun and wind turn water into vapour. A shallow pan has a big surface for its small volume, so water leaves quickly.
3. Move the brine. The water is passed from pan to pan. Each pan holds a stronger brine (very salty water).
4. Crystallise. When the brine reaches about 26% salt it is saturated: it cannot dissolve any more salt. Extra salt now appears as white crystals.
5. Harvest. The salt is raked, washed with strong brine to remove mud, and dried in heaps.

Important: only the water evaporates. The salt cannot turn into vapour at these temperatures, so it all stays behind.

Why does the salt percentage rise?

Take 100 g of sea water with 3.5 g of salt. If 50 g of water leaves, 50 g of solution is left, but the salt is still 3.5 g. The percentage is now 3.5 / 50 x 100 = 7%. The salt did not increase; the water decreased.

At 26% the brine is full. To get there from 100 g of sea water, you must lose about 86.5 g of water. After that, each extra bit of water that leaves makes some salt crystallise.

Purifying raw salt

Raw salt from pans or from rocks has mud, sand and other salts. To clean it:

1. Dissolve it in water.
2. Filter the liquid. Sand and mud stay on the filter paper.
3. Evaporate the water gently. Pure salt crystals form.

Some salts of magnesium dissolve more easily than common salt. They stay in the liquid left behind, called bittern, which is used to get magnesium and other chemicals.

Other resources from the sea

The sea is more than salt:

• Food: fish, prawns and seaweed.
• Minerals: magnesium, bromine and sand; salt is the raw material for washing soda, caustic soda and chlorine.
• Energy: oil and gas under the sea floor, and clean power from tides, waves and offshore wind.
• Fresh water: removing salt from sea water (desalination) gives drinking water where rivers are scarce.

Using them wisely: take only as much fish as can grow back, stop plastic and oil from reaching the water, and return strong brine from desalination slowly so sea life is not harmed.

Try it: salt from salty water at home

Stir 3 spoons of salt into a cup of warm water. Pour a thin layer into a flat plate and keep it in a sunny, safe place. Mark the water level with a marker on the outside if the plate is clear. After a day or two, watch the level fall and white crystals appear at the edge. In the 3D, predict the percent first, then move the slider to check. Never taste anything made in a lab.

Key formulas and definitions

• Sea water: about 3.5 g salt per 100 g
• Salt in brine (%) = salt mass / solution mass x 100
• Salt starts to crystallise at about 26% (saturated brine)
• Steps to purify: dissolve, filter, evaporate

Worked examples

Common mistakes

• Thinking the salt evaporates too. Only the water leaves; salt stays behind.
• Thinking the salt increases as water evaporates. The amount of salt stays the same; its percentage rises.
• Forgetting that crystals appear only when the brine is saturated, near the end.
• Calling filtration enough to separate dissolved salt from water. Dissolved salt passes through filter paper; use evaporation.