Pure water freezes at 32°F (0°C) at normal air pressure.1EasyTemperature.com. Salt Water Freezing Point: 28°F vs. 32°F for Pure Water That number marks the point where ice becomes possible, not the moment every drop turns solid. What’s dissolved in the water and what it touches can shift when freezing actually happens, which is why a salted sidewalk and a bottle in the freezer can behave so differently.
Why 32°F and 0°C Are the Magic Numbers
At 32°F, also written 273.15 kelvin, ice and liquid water can exist side by side without one taking over.2EasyTemperature.com. Salt Water Freezing Point: 28°F vs. 32°F for Pure Water Below that temperature water tends to freeze, and above it ice melts.3EasyTemperature.com. Salt Water Freezing Point: 28°F vs. 32°F for Pure Water
The Celsius scale was built around this balance. Anders Celsius used melting ice from pure water as a fixed point to mark thermometers.4EBSCOhost. Celsius Proposes a Temperature Scale His original scale ran upside down, with freezing at 100 and boiling at 0.5EBSCOhost. Celsius Proposes a Temperature Scale The scale was later flipped so freezing became 0 and boiling 100.6EBSCOhost. Celsius Proposes a Temperature Scale Fahrenheit’s scale, still used in the US, puts freezing at 32 and boiling at 212.
Because 32°F equals 0°C, any Celsius reading below zero means below freezing. To convert, multiply Celsius by 1.8 and add 32. A forecast of -5°C works out to 23°F, well below freezing. Scientists often use kelvin instead, which starts at absolute zero.
The definition says “pure water” and “normal pressure” because the number assumes nothing is dissolved in the water and that the air pressure is about what you’d find at sea level. Tap water’s small mineral content barely changes it in practice.
The balance also explains a glass of ice water. The liquid stays at about 32°F as long as ice remains, because added heat goes into melting ice instead of warming the water. Only when the last ice has melted does the water start to warm. That makes an ice-and-water mix a handy way to check a thermometer, which should read about 32°F (0°C) once it settles.
How Water Stays Liquid Below Freezing
Water cooled below 32°F without freezing is called supercooled. It happens because ice has to begin around a tiny starting point, such as a dust speck, an impurity, or a rough spot on the container. Very clean water has few of these starting points, so it can stay liquid surprisingly far below 32°F.7Wikipedia. Supercooling
Ice forms when water molecules lock into an orderly pattern, and a tiny solid surface gives them something to line up on. That’s why the same water can freeze at slightly different temperatures from one trial to the next: which starting point turns up first is partly luck.
In tiny droplets, water has been kept liquid down to about -30°F (-34.5°C).8Proceedings of the Physical Society Section B. The Freezing of Supercooled Water Purified water at normal pressure can stay liquid down to about -55°F (-48.3°C).9Wikipedia. Supercooling At that point ice forms on its own even with nothing to start it.10Wikipedia. Supercooling
You can sometimes see this at home. A sealed bottle of very clean water left undisturbed in a freezer can stay liquid even when the freezer is well below 32°F. Picking it up, shaking it, or twisting the cap can set off freezing within seconds, because the jolt or a bubble gives ice a place to begin. Tap water usually freezes normally since it carries enough specks and minerals to act as starting points, so supercooling at home is hit or miss. Freezing rain is the same thing outdoors: supercooled drops turning to ice when they hit cold roads or branches.
Once ice starts, crystals spread quickly through the water. Freezing releases some heat, which warms the mixture back toward 32°F. That’s why a supercooled bottle turns to slush rather than solid ice.
Some living things are excellent ice starters. Certain bacteria carry proteins that make water freeze at temperatures as warm as about 28°F (-2°C).11PubMed Central. Toward Understanding Bacterial Ice Nucleation
How Salt and Other Dissolved Substances Lower the Freezing Point
The more salt dissolved in water, the lower its freezing point.12The Homeschool Scientist. Effects Of Salt on the Freezing Point of Water Dissolved salt particles mix in among the water molecules, so the water has a harder time lining up into the orderly pattern of ice. It must get colder before ice can win out, and more salt means a bigger drop. There is a limit to how far ordinary table salt can push it, though.13The Homeschool Scientist. Effects Of Salt on the Freezing Point of Water Seawater, at about 3.5% salt, freezes near 28°F (-2°C). When seawater does freeze, the ice that forms is mostly fresh water, leaving saltier water behind. In soil, more salt lowers the freezing point no matter which salt it is.14Cold Regions Science and Technology. Laboratory investigation of the freezing point of saline soil
This is the idea behind salting roads and sidewalks. Salt melts ice by lowering the temperature water needs to freeze. It works well near freezing but loses power in very hard cold. Below roughly 15°F, rock salt works slowly, so crews switch to other chemicals or sand for traction.
Salt isn’t the only substance that does this. Sugar and alcohol also lower freezing points, which is why vodka stays liquid in a home freezer and why sweet sorbets and ice cream stay scoopable. Car antifreeze works on the same principle.
Living things use a different trick. Some fish and insects make antifreeze proteins that stick to tiny ice crystals.15PubMed Central. Antifreeze Proteins and Their Practical Utilization in Industry, Medicine, and Agriculture The crystals cannot grow, so the animal’s fluids stay liquid below normal freezing.16PubMed Central. Antifreeze Proteins and Their Practical Utilization in Industry, Medicine, and Agriculture
Does Pressure or Altitude Change It
Pushing harder on ordinary ice slightly lowers the temperature at which it melts.17The Engineering ToolBox. Ice and Water – Melting Points vs. Pressure At everyday pressures the change is tiny, so altitude barely affects freezing. Water freezes near 32°F in Denver just as at the beach.
Extreme pressure is another matter. It can force water into solid ice even at room temperature.18ScienceDaily. Extreme-pressure experiment reveals a strange new ice phase Squeezing water beyond about 9,500 times normal air pressure at room temperature turns it into a dense form of ice.19ScienceDaily. Extreme-pressure experiment reveals a strange new ice phase
Does Hot Water Freeze Faster Than Cold
Careful experiments found no meaningful sign of hot water freezing faster than cold.20PubMed Central. Questioning the Mpemba effect: hot water does not cool more quickly than cold Earlier reports showed only marginal differences in cooling time.21PubMed Central. Questioning the Mpemba effect: hot water does not cool more quickly than cold Other controlled experiments found hot water sometimes freezes first purely by chance, because the moment ice starts near the freezing point is unpredictable, so freezing times scatter widely.22Molecules. Accidental Origin of the Mpemba Effect in Freezing: Reversals in the Freezing Order Are Stochastic—Experimentation and Theory
That scatter helps explain why people swear they’ve seen it. Hot water loses some of its volume to evaporation, so there is less water left to freeze. Where a container sits matters too: a tray against a cold wall or frost layer cools faster than one in the middle of the shelf. And if the colder sample happens to supercool, it can wait to freeze while the hot one finishes first by luck.
For ice trays, starting with cold water is the reliable way to get ice sooner, since hot water just adds cooling time on average. Boiled water may make clearer ice because dissolved air is removed.
Other Surprises in How Water Freezes
As water cools, molecules start forming open, roomy ice-like patterns.23Water Calculator. Why Is Water Densest at 4°C? So below about 39°F (4°C) liquid water expands instead of shrinking.24Water Calculator. Why Is Water Densest at 4°C? That’s why lakes freeze from the top down. As surface water cools toward 39°F it grows denser and sinks, which mixes the lake. Below that point colder water becomes lighter and stays near the top, so the surface reaches 32°F first and ice forms there. The ice then acts as a lid that slows further cooling, which often leaves liquid water below for fish to survive in.
Ice also takes up about 9% more space than the water it came from. In a closed space that push has nowhere to go. Ice often blocks a pipe in one spot, and the trapped water between that plug and a closed tap gets squeezed until the pipe splits. The split can appear in a different place from the ice itself, so a pipe may only leak after thawing. On a very cold night, letting a faucet drip keeps water moving and relieves pressure that would otherwise build. For the same reason, never freeze a full sealed glass bottle.
Whether outdoor water freezes depends on more than the number on the thermometer. Air must stay below 32°F long enough to pull heat out of the water. Puddles freeze before ponds because they hold less heat, and wind and clear skies speed things up. Bridges ice first because cold air reaches them from below as well as above.
Freezing Without Ice Crystals
Under certain conditions water skips ice entirely and becomes a glass-like solid.25PubMed Central. Complexity of confined water vitrification and its glass transition temperature In a glassy solid the water molecules are stuck in place without lining up into crystals, like a liquid whose motion has simply stopped. With no crystals, there are no sharp ice shapes to cause damage.
That matters in medicine. Cells soaked in protective chemicals and plunged into liquid nitrogen turn glassy instead of icy.26PubMed Central. Cryopreservation and its clinical applications The reason is that ice crystals inside cells can tear them apart.27PubMed Central. Ice Inhibition for Cryopreservation: Materials, Strategies, and Challenges A gentler method cools cells about 1.8°F (1°C) per minute so water can leave them first.28PubMed Central. Cryopreservation and its clinical applications
The same logic applies to food. When food freezes slowly, water moves out of cells and builds a few large crystals that rupture cell walls. That’s why slowly frozen berries go mushy after thawing: the damaged cells leak their juice. Fast freezing produces many small crystals that cause less damage, so texture holds up better.
This approach doesn’t work for whole people. A body can’t cool fast enough to avoid ice, and the protective chemicals at the strengths needed are toxic in large amounts. Embryos and eggs work because they are tiny.