Salt water freezes at a lower temperature than fresh water, and exactly how much lower depends on how salty it is. Ordinary seawater, with a salinity of about 35 grams per kilogram, starts to freeze at roughly −1.8°C (about 28.4°F). Increase the salt concentration and the freezing point drops further, down to about −21°C (−6°F) for a saturated sodium chloride solution. That simple relationship between salt and freezing point has consequences that stretch from icy roads to polar oceans to the moons of distant planets.
How Salt Lowers the Freezing Point
When you dissolve salt in water, the dissolved ions get in the way of ice crystal formation. Water molecules need to arrange themselves into an orderly lattice to freeze, and salt ions disrupt that process. Traditionally, scientists described this as a colligative effect driven by the increase in disorder that dissolved particles create. Recent molecular-dynamics research has refined this picture, showing that the mobility of water molecules in the hydration shells around ions plays a bigger role than the raw disruption of hydrogen-bond networks.1PubMed. Molecular Insights into Anion-Specific Freezing Point Depression in Lithium Salt Solutions In plain terms, how freely water moves around each dissolved ion matters more than how many hydrogen bonds the ion breaks.
The relationship between salt concentration and freezing point is roughly linear at low concentrations. Each additional gram of salt per liter pushes the freezing point down by a small, predictable amount. But the curve flattens and eventually hits a wall called the eutectic point, where both ice and salt crystals precipitate out together and no further depression is possible. For plain table salt (sodium chloride) dissolved in water, that floor sits near −21.1°C. Natural waters contain a mix of ions beyond just sodium and chloride, including potassium, calcium, magnesium, and sulfate. Laboratory work on these mixed-ion solutions shows the eutectic process still operates at temperatures no colder than about −25°C.2Desalination. Temperatures required for eutectic freezing of natural waters In practice, that means virtually all naturally occurring salt water on Earth will freeze if you get it cold enough, even the brines pooled at the bottom of the Dead Sea.
What Happens When Seawater Actually Freezes
Seawater does not freeze the way a tray of tap water does. When ocean water cools to its freezing point, ice crystals begin forming as nearly pure water. The salt that was dissolved in the water gets excluded from the growing ice lattice, concentrating into pockets of cold, dense brine trapped between crystal grains. As freezing continues, those brine pockets drain downward through tiny channels in the ice, eventually dripping out the bottom of the ice sheet into the ocean below. The result is that sea ice is far less salty than the water it formed from, typically containing only a few parts per thousand of salt compared to the ocean’s roughly 35.
This brine rejection process is not just a curiosity. It drives some of the most important circulation patterns in the world’s oceans. When brine drains out of newly formed sea ice on continental shelves, the surrounding water becomes saltier and denser, and it sinks. That sinking feeds deep-water formation, which is a key driver of global ocean circulation. In the Arctic, brine rejection on the shelves ventilates deeper layers of the ocean, a process first proposed over a century ago.3Oceanography. Arctic Ocean Water Mass Structure and Circulation Direct measurements in the Okhotsk Sea have captured this happening in real time: moored instruments recorded a steady, month-long increase in salinity driven entirely by local ice formation, with the resulting dense brine sinking to ventilate North Pacific Intermediate Water.4PubMed. Direct observations of North Pacific ventilation: brine rejection in the Okhotsk Sea In other words, the freezing of salt water at the surface rearranges the ocean from top to bottom.
Road Salt and the Limits of Deicing
The same freezing-point depression that shapes polar oceans is at work every winter on salted roads. Spreading sodium chloride on pavement melts ice because the salt dissolves into the thin film of liquid water on the surface, lowering the freezing point of that film so the ice beneath it can no longer stay frozen. Road salt works well down to about −10°C (14°F) and can still have some effect a few degrees below that. Field testing over more than 70 snow events found that the key factors influencing how quickly salt clears a road are the application rate, the air temperature, and the amount of snow on the ground.5Transportation Research Record. Deicing Performance of Road Salt As temperatures fall further toward −15°C or below, standard road salt becomes increasingly ineffective because the brine it creates is approaching its own freezing point. That is why highway departments in extremely cold regions sometimes switch to calcium chloride or magnesium chloride, which depress the freezing point further than sodium chloride can.
There is a historical footnote worth knowing here. Daniel Gabriel Fahrenheit, the inventor of the Fahrenheit scale, set his zero point at the lowest temperature he could produce using a mixture of ice, water, and salt.6Research Starter. Daniel Gabriel Fahrenheit That cold mixture was essentially the eutectic point of a salt-ice-water slurry. So 0°F was originally defined by the freezing behavior of salt water, which makes the Fahrenheit scale, in a roundabout way, a monument to freezing-point depression.
Supercooled Salt Water in Polar Oceans
Under certain conditions, seawater can remain liquid below its expected freezing point, a state called supercooling. This sounds counterintuitive, but it happens regularly beneath Antarctic ice shelves. When water at depth melts ice from the underside of a thick ice shelf, that meltwater mixes with ambient seawater and rises. As it ascends, the pressure drops, and the freezing point for seawater actually rises with decreasing pressure. The water can find itself colder than its new, slightly higher freezing point before it has time to form ice crystals. Researchers have measured supercooling of 20 to 160 millikelvins near major Antarctic ice shelves, including the Filchner, Ronne, Ross, and Amery shelves.7Cold Regions Science and Technology. Oceanographic observations in supercooled water: Protocols for mitigation of measurement errors in profiling and moored sampling
Those are tiny temperature differences, fractions of a degree, but they have real consequences. Supercooled seawater can nucleate ice crystals on anything it touches: the seabed, mooring lines, scientific instruments. This ice, called anchor ice, can grow thick enough to lift rocks off the seafloor and damage underwater equipment. Supercooled water is a prerequisite for anchor ice formation, with frazil crystals (small, suspended ice discs) sticking to submerged surfaces as the most common initial growth mechanism. In Antarctica, this process appears to operate at substantial depths, with no clear physical limit, as long as the ice-shelf-driven supercooling reaches deep enough.8Progress in Physical Geography: Earth and Environment. Anchor ice in polar oceans For oceanographers, supercooled brine is both a scientific target and a logistical headache, since instruments lowered into these waters can come back encased in ice.
How Marine Fish Survive Freezing Salt Water
Seawater near the poles hovers right around its freezing point for months at a time, and the fish that live there face a constant risk of ice forming inside their bodies. Their blood, being less salty than seawater, has a higher freezing point than the water they swim in. Without protection, ice crystals that enter through the gills could propagate through their tissues. Many polar fish have evolved specialized antifreeze proteins and antifreeze glycoproteins that solve this problem in a way that has nothing to do with salt concentration. These proteins adsorb onto the surfaces of tiny ice crystals, preventing the crystals from growing.9PubMed. The importance of dissolved salts to the in vivo efficacy of antifreeze proteins The mechanism is non-colligative, meaning it works by physically blocking ice growth rather than by lowering the bulk freezing point of the blood the way dissolved salt does.
The dissolved salts already in the fish’s blood do contribute a small amount of freezing-point depression on their own, and the antifreeze proteins build on top of that. The two systems work together: salt provides a modest baseline depression, and the proteins widen the gap further by stopping any stray ice crystals from propagating. Without both layers of defense, Antarctic notothenioid fishes and Arctic cod would not survive in water that is perpetually at or below the freezing point of their own body fluids.
When Road Salt Ends Up in Fresh Water
The millions of tons of road salt applied each winter do not simply vanish when the snow melts. Runoff carries dissolved chloride into streams, lakes, and groundwater, and the consequences persist well beyond winter. Continuous monitoring in urban streams in Milwaukee found elevated chloride levels lasting through the summer months, fed by contaminated groundwater slowly releasing stored salt.10PubMed Central. A Fresh Look at Road Salt: Aquatic Toxicity and Water-Quality Impacts on Local, Regional, and National Scales Studies in Massachusetts confirmed the same pattern: chloride from highway deicing persisted in groundwater and surfaced in streams even in warm weather.
The ecological stakes are real. Road salt runoff is now a leading cause of what researchers call secondary freshwater salinization in north temperate climates. Rising chloride concentrations in freshwater can be directly toxic to aquatic organisms, alter their behavior, reduce biodiversity, and reshape food webs.11FACETS. Salty summertime streams—road salt contaminated watersheds and estimates of the proportion of impacted species This is an ironic twist: our exploitation of salt’s ability to lower the freezing point on roads is gradually making freshwater ecosystems saltier, with long-term effects that are still being tallied. Some municipalities are experimenting with brine pre-wetting (spraying salt with brine before application) or switching to alternatives like beet-juice blends to reduce the total amount of chloride applied, though none of these solutions eliminate the problem entirely.
How Seawater Crystals Actually Grow
If you could watch seawater freeze under a microscope, you would see something quite different from the solid sheet that forms when you freeze a glass of tap water. Ice crystals in freezing seawater start as tiny spherical nuclei. As they grow, each nucleus develops six primary arms, reflecting the hexagonal symmetry of the ice crystal lattice. Secondary branches, called dendrites, then sprout off each arm. Research using high-resolution imaging of solidifying seawater has shown that increasing the driving force behind crystal growth (the degree of undercooling or supersaturation) makes the crystal tips grow faster while simultaneously becoming thinner and more needle-like.12Desalination. Unveiling the dynamics of ice crystal growth and evolution in solidifying seawater The salt ions concentrated between the growing dendrite arms slow local freezing in an uneven way, creating the complex, porous microstructure that makes sea ice so different from freshwater ice. That porous structure is also what allows brine to drain through the ice over time, gradually desalinating the ice from the inside out.
Salt Water Freezing on Other Worlds
The physics of freezing-point depression is not unique to Earth. Wherever water and dissolved salts coexist in the solar system, the same principles apply, and they shape some of the most intriguing places in astrobiology.
Saturn’s moon Enceladus harbors a global, saline subsurface ocean beneath an outer shell of ice. Scientists investigating how that ocean’s salinity affects circulation and ice shell geometry found that the salt content influences the direction and strength of the ocean’s overturning circulation, which in turn determines how heat is distributed and how thick the ice grows at different latitudes.13PubMed Central. How does salinity shape ocean circulation and ice geometry on Enceladus and other icy satellites? When tiny droplets from that ocean are ejected through Enceladus’s famous south-polar geysers, they freeze rapidly in the vacuum of space. Laboratory simulations of that process found supercooling of roughly 25 to 30 kelvins before freezing occurs, producing ice grains that are mostly crystalline but contain up to five percent glass. Salts in the grains crystallize in a specific sequence: phosphates first, then carbonates, then chlorides.14The Planetary Science Journal. Supercooling, Glass Formation, and Mineral Assemblages upon Freezing of Salty Ice Grains from Enceladus’s Ocean The glassy fraction is of particular interest because it could potentially preserve organic molecules or even cells, if any exist in that hidden ocean.
Mars presents a different scenario. Its surface is too cold and its atmosphere too thin for pure liquid water to last, but perchlorate salts in the Martian soil can depress the freezing point enough for salty brines to exist temporarily. Laboratory experiments showed that when perchlorate salts come into contact with water ice under Mars-like conditions, liquid brine forms within minutes.15Geophysical Research Letters. Experimental evidence for the formation of liquid saline water on Mars These brines would be far too salty and cold for any known Earth life, but they demonstrate that the freezing-point depression of salt water is not just a terrestrial phenomenon. Wherever salts and water ice coexist in the solar system, liquid solutions can persist at temperatures well below 0°C, opening questions about chemistry and habitability that researchers are only beginning to explore.
Common Misconceptions About Freezing Salt Water
A few misunderstandings circulate widely about this topic and are worth clearing up. One is the belief that salt water simply “cannot freeze.” It can. Every salt solution has a freezing point; you just have to reach it. Even the Dead Sea, with salinity around 340 grams per liter, would freeze if you cooled it enough, though you would need temperatures far colder than any natural climate on Earth’s surface provides.
Another misconception is that adding salt to water always melts ice. It only works if the temperature is above the freezing point of the resulting salt solution. If you dump table salt on ice when it is −25°C outside, the salt dissolves into whatever thin film of liquid exists on the ice surface, but the resulting brine freezes right back because −25°C is below the eutectic point for sodium chloride. This is why road salt has a practical lower limit and why highway crews in places like interior Alaska or northern Canada often rely on sand for traction instead of salt for melting.
A third confusion involves the idea that sea ice is salty. New sea ice does contain some trapped brine, but it loses salt steadily over time as brine channels drain. Multi-year sea ice in the Arctic can be fresh enough to melt and drink. Explorers and indigenous peoples in the Arctic have long known that old sea ice, identifiable by its blue tint and rounded surfaces, is a reliable source of fresh water. The freezing process itself acts as a natural desalination mechanism, which is one reason freeze desalination has been explored as an engineering approach for producing fresh water, though it has yet to compete economically with reverse osmosis at scale.