Water covers more than 70% of the Earth’s surface and about 97% of that is salt water. People who have been to the beach know that sea water is salty. And we all know that fresh water in rivers and rain doesn’t taste salty. But why? It is because of what happens to rainwater as it travels down streams and rivers and into the sea; as well as what happens to this water when it reaches the sea.

When rain falls from the clouds it picks up carbon dioxide from the atmosphere. This mixes with water to form carbonic acid which makes rain water slightly acidic.

When rainwater hits the earth it seeps through rocks and passes through soil. When it does this it breaks down rocks and soil. The minerals and salts in the rocks are released into the rain water as it flows into streams and rivers. Rivers are in constant flow and lakes have water flowing in and out of them, so the minerals don’t accumulate and build up in this fresh water – the concentration remains low and we can’t taste it. But when the water reaches the sea it collects there and the minerals and salts become more concentrated.

The minerals and salts in the sea become even more concentrated when the heat from the sun causes the water to evaporate, leaving the salt particles behind. However, the amount of the salt in the sea is not the same everywhere. Seawater is not as salty towards the north and south poles because water is diluted by melting ice. The intense heat of the tropics makes water there denser and saltier as more water evaporates.

Isolated lakes such as the Great Salt Lake and the Dead Sea are extra salty. Lakes store water temporarily. While some streams and rivers carry water into lakes, other rivers carry water out of lakes. Great Salt Lake and The Dead Sea have no outlets. In these lakes all the water that flows in escapes only by evaporation. There are no rivers that carry the water out of these lakes. When the water evaporates under the scorching sun it leaves large salt deposits behind. Both these lakes are about 10 times saltier than seawater. People float in the Dead Sea more easily than in other seas.

The salts in the sea have built up over billions of years and the salts that flow in all the earth’s rivers would only equal the salt in the ocean in about 200 to 300 million years.

Tell us: Do you think the sea will continue to become saltier?