If you have ever got a splash of sea water in your mouth on a beach in Chennai or Mumbai, you know it tastes very salty. But the rivers that flow into the sea taste fresh. So where does all that salt come from?
It starts with rain
As rain falls, it picks up a little carbon dioxide from the air. This makes rainwater very slightly acidic (an acid is a substance that can slowly wear away certain materials). Over a long time, this rain breaks down rocks on land. Tiny bits of minerals from the rocks dissolve in the water.
Rivers carry, the sea collects
Streams and rivers carry these dissolved minerals to the sea. River water has so little salt that it still tastes fresh. But at sea, the Sun’s heat makes water evaporate (turn into vapour and rise into the air). The water leaves, but the salt stays behind. Over a very long time, salt has built up and made the sea salty.
Salt also enters the sea from below. On the ocean floor are cracks called hydrothermal vents. Sea water seeps into the rocks there, gets very hot, dissolves minerals and flows back out.
How salty is the sea?
On average, about 3.5 per cent of the weight of sea water is dissolved salt. Most of it is made of sodium and chloride, the same pair that makes up the salt in your kitchen. If all the salt in the oceans were spread evenly over the land, it would make a layer more than 150 metres thick, about as tall as a 40-storey building!
Salt and India
India is the third-largest salt producer in the world, and Gujarat makes almost three-quarters of it. Salt makers let salty water dry in wide, shallow pans under the Sun until only white crystals are left.
Salt is also part of India’s freedom story. Under British rule, a law stopped Indians from making their own salt and put a tax on it. In 1930, Mahatma Gandhi walked about 387 kilometres in 24 days, from Sabarmati Ashram to Dandi on the Gujarat coast. There, he broke the law by making salt from the sea.
Try this: Stir two spoons of salt into half a cup of water. Pour a little into a shallow plate and leave it in a sunny spot for a few days. What is left when the water disappears?