How much seawater does it take to make salt?
― Three buckets for one kilo. The real job is the heat needed to drive off the water
Making one kilogram of salt takes about 30 litres of seawater — roughly three buckets, less than you might expect. But the real work lies in the nearly 29 kilograms of that water you have to boil away as steam.
Walking home after a swim in the sea, you rub your dry arm and white powder flakes off. There really is salt in seawater.
So how much salt would you get if you boiled seawater down in your kitchen? Say you simmer a full pot of it for tens of minutes.
What's left on the bottom of the pot is just a few teaspoons of white grains. Thinking about the gas bill, it starts to feel like a bad trade.
Why is getting salt from seawater so much work? Just two reasons
The salt dissolved in seawater makes up only about 3.4% of its weight. To get 1 kilogram of salt, you need about 29 kilograms of seawater. In other words, you're throwing away about 28 times as much water as the salt you want to keep.
The way to get rid of that water is to turn it into steam. Turning 1 kilogram of water into steam takes about 6.7 times more heat than warming the same water from 20°C to 100°C. Making salt is really an exercise in "driving off water."
Put these two facts together, and you can see how much heat 1 kilogram of salt really demands — and why people on beaches have long done what they do.
How much water has to be driven off for 1 kilogram of salt?
Out of 29 kilograms of seawater, only 1 kilogram is the salt you want to keep. The remaining roughly 28 kilograms of water all have to be turned to steam. It's close to boiling three buckets of water dry in a pot.
Doing this entirely with gas or electricity takes about 64 megajoules of heat. Converted to electricity, that's about 18 kilowatt-hours — roughly two days' worth of power for an ordinary household. Yet a kilogram of salt costs only about a hundred yen at the supermarket. Simply boiling down raw seawater simply doesn't pay.
That's why coastal salt-making has long been done in two stages. First, sun and wind are left to drive off water and make a concentrated brine (kansui). Only after that does it go over the fire. Try the slider on Figure 1 to change how concentrated the brine is.
A salt farm (enden) is a device for collecting this "free heat." Seawater is spread thinly on sand and left to dry, then the salt-coated sand is gathered and rinsed with seawater. This produces a brine many times more concentrated than seawater. Fuel is only used for the last step: boiling down this brine.
What happens inside the pot as it boils down?
Seawater doesn't just hold table salt. It also contains bitter-tasting magnesium salts and calcium salts that form gypsum. As it boils down, these come out in a fixed order (Figure 2).
Once the volume shrinks to about a fifth, gypsum is said to appear first, clouding the liquid white. By the time it's down to around a tenth, salt crystals finally appear. The bitter component left in the liquid to the very end is nigari, used to set tofu.
Salt-makers never let the pot run completely dry. Once enough salt has formed, they skim off the crystals and leave behind the nigari-rich liquid. Drying everything out would leave a harshly bitter salt.
An adult's daily salt intake is said to be around 10 grams. Converted back to seawater, that's about 290 grams — about a cup and a half. Every day, we're effectively eating the end result of sun or fuel drying out that much seawater.
Japan's rainy climate isn't well suited to large open salt farms. Domestic salt production today mainly uses electricity to concentrate the salt from seawater through membranes, producing a concentrated brine. The idea of concentrating before boiling is the same as with salt farms.
In short
One kilogram of salt takes about 30 litres of seawater. Of that, more than 28 kilograms is water that simply has to be boiled off as steam. Because turning water into steam takes so much heat, doing it entirely with fuel would use about two days' worth of household electricity. That's why people have long let the sun and wind do the first part of the job.
Making salt isn't really about collecting salt — it's about driving out water.
And for most of that heat, people have long let the sun and wind foot the bill.
How much heat it takes to turn water into steam is also the star of why a lid makes a pot boil faster. Why the sea is salty in the first place is covered in why seawater is salty, and how salt clumps up in humidity is explained in why sugar and salt clump together.
- If you don't have seawater, dissolve 3.4 grams of table salt in 100 grams of water instead.
- Spread it thinly on a flat plate and set it by a sunny window. Compare it with the same amount simmered on low heat in a small pot.
- Watch how long it takes for white crystals to appear, and how much salt is left at the end. Both should leave only a little over 3 grams.
With real seawater, a slightly damp, bitter patch remains just before it dries out completely — that's the nigari. If you use a flame, always watch it closely so the pot doesn't boil dry.
Want to go deeper? ― Terms, formulas, and textbook linksWe flag the level of each section, from junior-high science up to university specialist courses
- JHSCovered in junior high school science
- HSCovered in high school chemistry/physics
- HS+Advanced high-school content, or textbook sidebar material
- Univ.Not covered in high school — university specialist content (chemical engineering, marine chemistry)
- ResearchNot yet settled even at university level — an active research question
JHSTerms: this phenomenon has names
- Mass percent concentration: the weight of dissolved substance as a percentage of the total liquid's weight. Seawater's salinity is about 3.4%.
- Latent heat of evaporation (heat of vaporization): the heat needed to turn a liquid into a gas at the same temperature. For water, this is about 2.26 megajoules per kilogram.
- Recrystallization: when a dissolved substance that can no longer stay dissolved comes out of the liquid as crystals. Salt appearing as seawater boils down is an example of this.
JHSHSCheck with a formula: how much seawater and heat for 1 kilogram of salt?
We derive the seawater needed from the definition of concentration, then work out the water to be driven off and the heat that takes. Finally, we compare this with starting from pre-concentrated brine.
| In symbols | m = M ÷ s, mw = m − M, Q = L × mw |
| In words | Seawater needed = salt to make ÷ salt concentration. Water driven off = seawater − salt. Heat needed = heat of evaporation × water driven off |
| Where each comes from | The first is simply the definition of concentration. The second is conservation of mass — the salt doesn't evaporate, only the water leaves. The third is conservation of energy: a fixed amount of heat is needed for every kilogram of water turned to steam. |
| Symbol | Meaning and unit |
|---|---|
| M | Weight of salt to make (kg) |
| s | Salt concentration (as a fraction of weight; 3.4% = 0.034) |
| m | Weight of seawater (or brine) needed (kg) |
| mw | Weight of water driven off as steam (kg) |
| L | Heat of evaporation of water (MJ/kg); about 2.26 at 100°C |
| Q | Heat needed to drive off the water (MJ) |
| Salt to make M | 1 kg |
| Seawater salt concentration s | 0.034 (3.4%) |
| Heat of evaporation of water L | 2.26 MJ/kg |
| Electrical conversion | 1 kWh = 3.6 MJ |
| Brine concentrated on a salt farm (example) | 0.2 (20%) |
| Seawater needed m (kg) | 1 ÷ 0.034 ≒ 29.4 |
| Water driven off mw (kg) | 29.4 − 1 = 28.4 |
| Heat needed Q (MJ) | 28.4 × 2.26 ≒ 64.2 |
| Converted to electricity (kWh) | 64.2 ÷ 3.6 ≒ 17.8 |
| Amount at 20% brine (kg) | 1 ÷ 0.2 = 5 |
| Water driven off at 20% brine (kg) | 5 − 1 = 4 |
| Heat at 20% brine (MJ) | 4 × 2.26 ≒ 9.04 |
| Fuel fraction | 64.2 ÷ 9.04 ≒ 7.1 |
| Seawater equivalent of 10g daily salt (g) | 10 ÷ 0.034 ≒ 294 |
Boiling down raw seawater takes about 64 megajoules (about 18 kilowatt-hours) of heat per kilogram of salt. Pre-concentrating to 20% cuts the fuel to about a seventh. This leaves out the heat needed to warm the water to 100°C and heat lost from the pot, so real fuel use would be higher.
HSHS+Why crystals appear "in order"
HSEvery substance has a fixed limit (solubility) on how much can dissolve in a given amount of water. Table salt can dissolve up to about 36 grams per 100 grams of water at 20°C, and seawater reaches this limit once boiled down to about a tenth of its volume. Gypsum (calcium sulfate) has a much lower solubility, so it reaches its limit first and appears earlier.
HS+Magnesium chloride, the main component of nigari, has very high solubility, so it stays in the liquid until the very end. Separating substances by exploiting differences in solubility is called fractional crystallization (purification by recrystallization). Table salt's solubility barely changes with temperature, so cooling doesn't bring much of it out — driving off the water is the only way to extract it.
Univ.Salt farms as "solar evaporators" by design
In chemical engineering, the efficiency of a water-removal device is judged by how much latent heat of evaporation it can save. Industrial salt plants use multi-effect evaporators, which reuse the heat from steam produced in one stage to heat the next, cutting fuel use to a fraction. A salt farm's evaporation rate is set by sunlight, wind speed, and humidity, and is estimated using meteorological evaporation formulas such as the Penman equation.
📖 For the derivation of the formulas and more: Heat of evaporation (Japanese Wikipedia) / Salt farms (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How much does nigari affect flavour? One theory holds that the trace magnesium and potassium left in salt create a "roundness" of flavour, but how much is needed before people notice the difference varies a lot between individuals, and it isn't settled.
- Is the sea's salinity changing with warming? Regions with heavy evaporation are reportedly becoming saltier, and rainier regions more diluted. How this feeds back into ocean currents and climate is still being studied.
- What decides crystal shape? On still water, hollow pyramid-shaped salt crystals can form. Exactly how evaporation speed and liquid flow determine this shape is still under investigation.
In other words, this article too is an explanation based on what's currently understood. The numbers are representative values that vary somewhat by sea region and season.
Links to textbooks, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | 1st-year science, "aqueous solutions" (mass percent concentration, recrystallization) | Seawater's 3.4%, salt appearing on boiling down |
| HS | Basic physics "heat", basic chemistry "solubility" | Scale of heat of evaporation, order crystals appear |
| HS+ | Chemistry: "solid solubility and fractional crystallization" | Why nigari is left to the end |
| Univ. | Chemical engineering (evaporation operations), meteorology (evaporation rate) | Multi-effect evaporators, estimating a salt farm's evaporation rate |
| Research | Marine chemistry, food science | Changes in ocean salinity, differences in salt's flavour |
| ― | Everyday connections | Nigari and tofu, sun-dried vs. refined salt, the feel of a household energy bill |
- Salt Industry Center of Japan (塩事業センター), "Salt Encyclopedia" (塩百科)
- Seawater (海水) - Japanese Wikipedia
- Salt farms (塩田) - Japanese Wikipedia
- Salt Industry Act (塩事業法, enacted 1996; the law governing salt production and distribution in Japan)
- National Astronomical Observatory of Japan (ed.), Chronological Scientific Tables (理科年表), Maruzen Publishing (heat of evaporation of water, solubility of substances)
※This article is a general-interest science explainer. The figures given are approximations meant to aid understanding. Seawater's salinity varies by sea region and season. If you use a flame in the experiment, have an adult nearby, and check that the shore is safe before collecting seawater.