Why does the ocean stay salty but rain is fresh?
After you watchWhy does the ocean stay salty but rain is fresh?
The short answer
The ocean is salty because the sun evaporates only pure water off the sea, leaving the salt behind. Rivers keep washing tiny amounts of dissolved salt into the ocean, but when water evaporates it leaves that salt in place, so over millions of years the salt built up while the rain that returns stays fresh.
Try this next
- What if you made the sun hotter so the sea evaporates faster? In the machine, lift more water off each year and predict before you fast-forward: does the salt that's left behind pile up quicker, or does the extra rain rinse it away?
- What if the rivers stopped flowing into the sea? Turn off the salt the rivers deliver, run a thousand years, and predict: does the ocean's saltiness keep climbing, hold steady, or slowly drop?
- What if salt could float up with the water vapor? Pick salt instead of pure water for the sun to lift, run it, and predict what the rain tastes like and whether the sea ever gets salty at all.
Now you — bend it
- What if Set the rule to 'Only water rises' and run a thousand years — watch the meter climb. Now hit run again and again without changing the rule. Does the meter ever stop climbing, or does it keep going up every single run?Predict first. The lab adds the same chunk of salt each run because rivers keep dripping it in and pure water keeps leaving. The model has no way to TAKE salt back out, so it can only climb. What's missing that the real ocean has?
- What if Thought experiment (no slider for this): the lab's meter only ever climbs under 'Only water rises.' But the real ocean has been roughly 3.5% salt for hundreds of millions of years. What's missing from the model that stops it climbing forever?Predict what kind of 'drain' the lab leaves out. Run the water rule a few times to see salt only ever pile up on the seabed — it never leaves. In the real sea, salt keeps arriving from rivers, so to hold steady the same amount must leave each year. Guess where it goes if it can't evaporate.
- What if Thought experiment (no slider for this): flip the rule to 'Water + salt rise' and run it — the rain falls salty and the meter stays flat. Now imagine evaporation that lifted just 1% of the salt with the vapor instead of all of it. Would that rain be drinkable?Predict the rain's saltiness. The 'Water + salt rise' rule is the real control that lets you watch salty rain fall. Seawater is ~35 grams of salt per liter; 1% carried up is still ~0.35 g/L. Compare that to the ~0.5 g/L limit where water starts tasting salty — does even a tiny leak survive the test?
Can you prove it?Evaporation leaves essentially ALL the salt behind — the vapor is salt-free, not just low-salt. — Half-fill a clear cup with salty water, mark the level, and cover it loosely so vapor can escape but nothing splashes in. Leave it in a warm window for days. As the water level drops, taste-test or measure: the water that's LEFT gets saltier and saltier, and a white crust forms at the old waterline — meaning the vapor that left carried none of the salt away. If evaporation took salt with it, the leftover water would stay the same saltiness instead of concentrating.
Design your own test:Before you run it: set the rule to 'Only water rises' and predict whether the saltiness meter climbs, holds steady, or falls when you fast-forward a thousand years. Then switch to 'Water + salt rise,' predict again, and run it. Which rule makes the salt pile up — and why does that match a real ocean that's salty but a rain that's fresh?
Explain it to a 6-year-old: When the sun sips water up off the sea, it leaves all the salt behind — so the sea keeps the salt and the rain comes back fresh.
The whole story
How it works
Water travels in a loop: the sun lifts it off the sea as vapor, it forms clouds, and it rains back down. Evaporation lifts only the water molecules, not the dissolved salt, so the vapor and the rain are fresh. Meanwhile rivers flow over rocks and soil, picking up tiny tasteless specks of dissolved salt and carrying them to the sea. Because pure water keeps leaving and salt keeps arriving but cannot evaporate away, the salt stays in the ocean and slowly accumulates.
What people get wrong
People often think the ocean is salty because someone added salt once, or because rain brings salt down into it. It is the opposite: rain and evaporated vapor are fresh, carrying no salt at all. The salt is what gets left behind every time water evaporates, and rivers keep delivering more, so it builds up in the sea instead of being carried away.
The catch
The real rule, only-water-rises, explains the salty sea, but the ocean is not getting saltier forever. Over millions of years salt is also removed: sea creatures use it and it settles onto the seafloor, so the ocean has leveled off near a steady saltiness of about 3.5 percent. The simpler-sounding idea, that water and salt rise together, would mean rain tastes salty and the sea never changes, and we never see either of those, so it cannot be how it works.
Questions kids ask
If rivers are fresh, how do they make the sea salty?
River water looks clean but isn't perfectly pure. Flowing over rocks and soil, it picks up tiny tasteless amounts of dissolved salt and carries them to the sea. Each river adds only a little, but they have been doing it for millions of years, and the salt has nowhere to go once it arrives.
Why doesn't the salt evaporate too?
When water evaporates, only the water molecules float up into the air; the salt is far heavier and stays dissolved in the sea. That is why clouds and rain are fresh. The salt gets left behind every single time water leaves, so it keeps building up.
Is the ocean going to keep getting saltier and saltier?
No. Salt is also removed from the sea: ocean creatures use it and tiny amounts settle onto the seafloor over long stretches of time. The amount arriving and the amount leaving have roughly balanced, so the ocean has leveled off at about 3.5 percent salt rather than turning solid.
Why is rain fresh enough to drink if it comes from the salty sea?
Because evaporation lifts only pure water off the salty sea, the way distilling cleans water. The salt is left behind, so the vapor that makes clouds, and the rain that falls, carry no salt at all.
Talk about it
- Rain falls fresh but the sea it came from is salty — guess where the salt goes when the water lifts off.
- Rivers taste fresh, yet they're what makes the ocean salty. How do you think that works?
- The ocean has been getting salt for millions of years but isn't solid salt. What might be carrying salt back out?
For grown-ups
Evaporation acts like a near-perfect distiller: water molecules enter the vapor phase but dissolved ions, mostly sodium and chloride, do not, so precipitation is essentially salt-free. Rivers continuously deliver small amounts of salts weathered from rock; with pure water leaving and salts staying, ocean salinity built up over geologic time. It now sits near a long-term steady state because salts are also removed by sea spray, biological uptake, and deposition into seafloor sediments and salt beds, so input and output have roughly balanced.
Keep going
What else makes you wonder?
- Where does the salt in rivers come from in the first place?
- Why is the Dead Sea so salty you can float on top of it?
- Could an ocean ever dry up and leave a desert made of salt?