Sea water is a very pure substance. Over ninety six per cent of it is pure water. Almost three per cent is common salt, and the remaining one per cent is made up of small traces of lots of other elements. All these elements are washed into the oceans by rivers and stay there. Water evaporates from the surface but the salt remains dissolved. So the seas are gradually getting saltier. Some seas are much saltier than others, depending on how much water evaporates from them. The Red Sea, lying between Africa and Arabia, basks in hot sunshine all year round. It loses a lot of water through evaporation, which makes it six times saltier than the Baltic Sea high up in northern Europe. The Baltic does not get anything like as much sunshine. Therefore less water evaporates from it and the salt solution in the water is weaker.
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