Science Puzzle
Why Is the Sea Salty but the Rain Is Not?
Rain falls from the same clouds that formed over the salty sea, yet rainwater is fresh enough to drink. The sea, meanwhile, is far too salty to drink. And the sea does not seem to get any less salty over time.
Where does the sea's salt come from, and why does rain have none of it?
The Answer
The sun is a distiller. When it evaporates water from the sea, only the water molecules float up as vapour; the salt is far too heavy and stays behind. So the clouds, and the rain they drop, are fresh. Every drop of rain is effectively distilled seawater with the salt stripped out.
That rain lands on hills and soaks through rock and soil, dissolving tiny traces of mineral salt as it goes. Rivers then carry that faintly salty water back to the sea. The amount in any one river is minuscule, too little to taste. But here is the key: when seawater evaporates again, the salt cannot leave. So over hundreds of millions of years, river after river, the salt keeps arriving and never departs. It has been concentrating in the oceans for almost the entire history of the planet.
The principle: The water cycle plus accumulation. Evaporation carries pure water but leaves salt behind, and rivers steadily deliver dissolved minerals to the sea, where they concentrate because only water can evaporate away.