Why is the sea salty?
Quick answer from Eartha
Rain falls on rocks and picks up tiny bits of salt and minerals. Rivers carry that salty water down to the sea. The sun warms the ocean and water evaporates into clouds, but the salt stays behind. Over millions of years, the sea got saltier and saltier. That's why it tastes salty today!
The longer story
Rain isn't pure water. As it falls and flows over rocks and soil, it dissolves tiny amounts of salt and minerals hiding inside them. Rivers and streams pick up these dissolved bits and carry them all the way to the ocean.
The sun then heats the ocean's surface, turning water into vapour that rises up and forms clouds. This is called evaporation. But here's the clever part: the salt is too heavy to evaporate, so it stays behind in the sea.
This has been happening for hundreds of millions of years, with rivers constantly topping up the ocean's salt. Scientists estimate ocean water is about 3.5 percent salt, which is enough that if you dried out all the sea, you'd be left with a layer of salt thick enough to cover countries.
Freshwater lakes usually don't taste salty because water flows out of them into rivers, carrying the salt away before it can build up. The ocean is different: water leaves by evaporating, but the salt has nowhere else to go.
Try this at home: Make your own mini evaporation experiment
- Stir a spoonful of salt into a cup of warm water until it disappears.
- Pour a little into a shallow dish and leave it on a sunny windowsill for a few days.
- Check back and see the salt crystals left behind once the water has evaporated.
Words to know
- Evaporation
- When water turns into invisible vapour and floats up into the air.
- Minerals
- Tiny natural bits found in rocks and soil, like salt.
- Dissolve
- When something mixes completely into water so you can't see it anymore.
For grown-ups: what is my child learning here?
Age band: 5-8
Curriculum link: Australian Curriculum v9 Science, AC9SFI06 - properties of water
The concept: Introduces the water cycle and dissolving as everyday science concepts.
More classroom ideas are on our teachers page, and safety notes for families are on for parents.
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