Pure water freezes at 0 degrees Celsius, which equals 32 degrees Fahrenheit. That number is one of the most recognized facts in science, and it served as the anchor point for the Celsius scale itself. But the neat, round answer hides a surprising amount of complexity: under real-world conditions, water routinely stays liquid well below its official freezing point, and dissolved substances can push that threshold lower still.
Why the Two Numbers Look So Different
The Celsius and Fahrenheit scales were built around different reference points. Anders Celsius designed his scale in the 1740s so that the freezing point of water sat at 0 and the boiling point at 100, giving a clean 100-degree span between the two. Daniel Fahrenheit, working a couple of decades earlier, chose a different pair of anchors: the coldest temperature he could reliably produce in his lab (a salt-ice-water mixture, set at 0°F) and roughly human body temperature (which he placed at 96°F, later revised). On Fahrenheit’s scale, water’s freezing point landed at 32 and its boiling point at 212, an awkward-looking 180 degrees apart.
Neither scale is more “correct” than the other. Celsius dominates in science and in most of the world’s daily life. Fahrenheit persists in the United States and a handful of other countries. Converting between them is straightforward: multiply the Celsius value by 1.8 and add 32 to get Fahrenheit, or reverse the process going the other way. For freezing, 0 × 1.8 + 32 = 32°F. The two scales also intersect at one point: −40°C and −40°F are the same temperature.
Freezing Isn’t Always Instant at 0°C
The textbook freezing point assumes pure water at standard atmospheric pressure, sitting in a container with enough imperfections or particles to give ice crystals a surface to start forming on. Remove those conditions and things get strange. Water can remain liquid at temperatures well below 0°C through a process called supercooling.
In laboratory experiments, very pure water cooled gently in a smooth container can drop to around −7°C and stay liquid for hours. One set of experiments showed supercooled water holding at that temperature in liquid form for more than five hours without any disturbance triggering crystallization.1Journal of Electronics Cooling and Thermal Control. A Study of Water Supercooling A tiny vibration, a speck of dust, or even a sharp tap on the container can break the spell, and the water rapidly crystallizes. This is not some exotic lab trick. Bottled water left in a freezer sometimes supercools if it is very still, and you can watch it freeze in seconds when you open the bottle and jostle it.
The reason supercooling happens is that ice formation requires a starting template. Water molecules need something to organize around, whether that is a microscopic particle, a rough spot on the container wall, or a cluster of water molecules that spontaneously arrange into an ice-like pattern. Without that seed, the molecules keep tumbling past one another even though thermodynamics says they “should” be solid.
What Happens in Clouds
Supercooling is not just a curiosity. It plays a major role in weather. Cloud droplets are tiny, extremely pure, and suspended in air with no container walls to help ice crystals start. As a result, liquid water droplets in clouds regularly exist at temperatures far below 0°C.
Research on atmospheric ice formation shows that without ice-nucleating particles, freezing of cloud droplets does not become widespread until temperatures drop to roughly −35°C to −40°C.2Quarterly Journal of the Royal Meteorological Society. The formation of atmospheric ice crystals by the freezing of droplets At temperatures above about −20°C, natural ice-nucleating particles like mineral dust and certain bacteria play the dominant role in triggering freezing. Below that range, droplets freeze on their own even without nuclei. Studies of this process suggest that at around −40°C, ice nucleation at the air-water surface of cloud droplets becomes efficient enough that almost no supercooled liquid water remains.3PubMed Central. Surface crystallization of supercooled water in clouds
This matters for everything from rainfall to aviation. Supercooled droplets that strike an aircraft surface instantly freeze on contact, building up ice on wings and engines. Understanding the temperature zones where supercooled water is most likely to persist helps pilots and forecasters assess icing risk.
When Dissolved Substances Push the Freezing Point Down
Pure water freezes at 0°C, but the water you encounter every day is rarely pure. Dissolve salt, sugar, alcohol, or almost any other substance in water and the freezing point drops. This is why you spread salt on icy sidewalks and why the ocean, at roughly 3.5 percent salt by weight, freezes at about −2°C instead of 0°C.
The effect is a general physical property: dissolved particles interfere with the orderly arrangement that water molecules need to lock into a crystal. The more particles you add, the lower the freezing point goes. Modeling this behavior for different types of dissolved substances, including salts that split into multiple charged particles, has been a longstanding challenge in chemistry and engineering.4Industrial & Engineering Chemistry Research. Estimation of Freezing Point Depression, Boiling Point Elevation, and Vaporization Enthalpies of Electrolyte Solutions
In practical terms, this is why antifreeze exists. The coolant in a car engine is typically a mixture of water and ethylene glycol, a substance widely used as a secondary fluid in refrigeration systems and heat pumps.5International Journal of Refrigeration. Experimental investigation of thermophysical properties of ethylene glycol based secondary fluids A 50/50 blend of ethylene glycol and water can remain liquid down to about −37°C, far below what pure water could handle. Road salt works on a similar principle at a much cruder level: a saturated sodium chloride solution freezes at around −21°C, which is why salt stops working once temperatures plunge into the deep negatives during extreme cold snaps.
Why Freezing Water Can Burst Pipes
One of the most practically important things about freezing is that water expands by about 9 percent when it turns to ice. Most substances contract as they solidify, but water does the opposite because its crystal structure spaces the molecules farther apart than they sit in the liquid state. That expansion generates enormous force.
When water freezes inside a closed pipe or container, the pressure can become extreme. Analysis of this process shows that even the strongest steels cannot resist the expansion force if all the water inside undergoes a complete phase change in a fixed volume.6European Journal of Physics. How can freezing water burst pipes and containers? In practice, plumbing failures from freezing are one of the most common and expensive forms of home damage in cold climates. The burst usually does not happen at the ice plug itself but downstream, where the expanding ice has pushed still-liquid water against a closed section of pipe, raising the pressure until the pipe gives way.
This expansion also explains why ice floats. Because solid water is less dense than liquid water, ice forms on the surface of lakes and ponds rather than sinking to the bottom. That floating ice layer insulates the water below, allowing aquatic life to survive through winter. If water behaved like most other substances and sank when it froze, lakes would freeze from the bottom up, with catastrophic consequences for ecosystems.
How Living Things Cope With Freezing
For organisms that live in freezing environments, 0°C is not just a number on a thermometer. It is a survival threshold. Ice crystals forming inside a cell can shred membranes and destroy tissue. To deal with this, many cold-adapted organisms produce antifreeze proteins that bind to the surface of tiny ice crystals and prevent them from growing larger.7PubMed Central. Bacterial ice crystal controlling proteins
These proteins work by physically attaching to ice crystal faces and blocking the addition of new water molecules. Different classes of antifreeze proteins use different strategies: some target the fast-growing faces of ice crystals, while others work on slower-growing surfaces. The distinction matters because blocking rapid growth requires the protein to adsorb quickly to the crystal, whereas blocking slow growth is more about long-term stability.8PubMed Central. Blocking rapid ice crystal growth through nonbasal plane adsorption of antifreeze proteins
Interestingly, some bacteria take the opposite approach. Rather than preventing ice, they produce ice-nucleation proteins that encourage ice crystals to form at relatively warm sub-zero temperatures. The bacterium Pseudomonas syringae, for example, uses these proteins to damage plant tissue, creating wounds that the bacterium can colonize. Both ice-inhibiting and ice-promoting proteins appear to use similar binding mechanisms despite their opposite functions.7PubMed Central. Bacterial ice crystal controlling proteins Snow-making machines at ski resorts have even used preparations of these ice-nucleating bacteria to help artificial snow form at warmer temperatures than it otherwise would.
Vitrification and Avoiding Ice Entirely
Medicine and biotechnology face their own version of the freezing problem. When you freeze cells, embryos, or tissue samples for long-term storage, ice crystals can destroy the very material you are trying to preserve. One way around this is vitrification: cooling a liquid so quickly, and with the right mix of protective chemicals, that it solidifies into a glass-like amorphous state without forming any ice crystals at all.9Cryobiology. Avoiding the problems of ice in tissues via vitrification
Vitrification has become a cornerstone of fertility medicine, where it is used to preserve eggs and embryos. The technique involves adding high concentrations of cryoprotectant chemicals and then plunging the sample into liquid nitrogen so fast that water molecules never get the chance to line up into crystals. They just lock in place as a disordered solid. Getting this right is still a challenge: the cryoprotectants themselves can be toxic at high concentrations, and the cooling rate has to be fast enough throughout the entire sample to avoid pockets of ice forming.10PubMed Central. Technologies for Vitrification Based Cryopreservation Researchers are actively working on ways to scale vitrification up to larger tissues and even whole organs, which would transform transplant medicine.
How Your Body Senses Cold
Your skin does not have a thermometer built in, but it has something functionally similar. The sensation of cold, including the sharp sting of touching something near freezing, is largely driven by a protein channel called TRPM8. This channel sits in the membranes of sensory nerve cells and opens when the temperature drops, allowing calcium and other ions to rush in and trigger a nerve signal.11PubMed Central. Regulation of TRPM8 channel activity
TRPM8 is also the reason menthol feels cold. Menthol activates the same channel chemically, fooling your nervous system into reporting a temperature drop that is not actually happening. This is why menthol cough drops, toothpaste, and muscle rubs produce that familiar cooling sensation. The channel starts responding at temperatures below about 26°C and becomes increasingly active as the temperature falls toward and below freezing, giving you a graded sense of how cold something is rather than a simple on-off alarm.
Where the Kelvin Scale Fits In
Scientists working in physics and engineering often skip Celsius and Fahrenheit altogether in favor of the Kelvin scale, which starts at absolute zero, the lowest temperature physically possible. On this scale, water freezes at 273.15 K. A degree on the Kelvin scale is the same size as a degree Celsius, so converting between them is just a matter of adding or subtracting 273.15.
The Kelvin scale was redefined in 2018 as part of a broader overhaul of the international measurement system. Rather than being anchored to the properties of water, the kelvin is now defined by fixing the value of the Boltzmann constant, a fundamental physical quantity that relates temperature to energy.12Annalen der Physik. The Boltzmann Constant for the Definition and Realization of the Kelvin Under this definition, one kelvin corresponds to the temperature change that shifts thermal energy by a precisely fixed tiny amount.13PubMed. Low uncertainty Boltzmann constant determinations and the kelvin redefinition For everyday purposes nothing changed: water still freezes at 273.15 K. But for scientists making extremely precise measurements, the new definition eliminated a dependence on a physical substance (water) that is subject to impurities and pressure variation.
Ice Under Extreme Pressure
At the pressures and temperatures you experience in daily life, ice takes a single familiar form: the hexagonal crystal structure that makes up snowflakes and ice cubes. But water has a remarkably rich phase diagram. Under very high pressures, ice rearranges into entirely different crystal structures. Scientists have identified more than a dozen distinct forms of ice, each stable under different combinations of pressure and temperature.
At the extreme end, computational studies have explored what happens to ice at pressures between 100 and 800 gigapascals, pressures found in the deep interiors of giant planets. At those conditions, several theoretically predicted ice phases collapse into a single structure below about 300 gigapascals, and only at higher pressures do distinct phases re-emerge.14PubMed Central. Thermodynamics of high-pressure ice phases explored with atomistic simulations None of this affects the ice in your drink, but it means the freezing behavior of water on, say, Uranus or Neptune is nothing like what happens on Earth. Water ice inside those planets may exist in exotic crystal forms that bear little resemblance to anything you have seen.
Even at more modest pressures, the freezing point of water shifts. Increasing pressure actually lowers the freezing point slightly for ordinary ice (one of water’s unusual properties), which is why the base of a thick glacier can contain liquid water even at temperatures just below 0°C. At much higher pressures the relationship reverses, and some high-pressure ice phases are stable at temperatures well above 0°C. This is not something you will encounter outside a laboratory or a planetary interior, but it underscores that “freezing temperature” is always a statement about specific conditions, not an absolute property of water itself.