Pure ammonia freezes at about −77.73 °C (−107.91 °F, or 195.42 K) under normal atmospheric pressure. That makes it one of the lower-freezing common gases, staying liquid well below the point where water turns to ice. But the straightforward number hides a more complicated story once you factor in pressure, mixtures with water, and the exotic conditions found deep inside icy moons and giant planets.
How Cold Is That, Really
To put −78 °C in perspective, the coldest naturally recorded air temperature on Earth’s surface was about −89 °C in Antarctica, so ammonia’s freezing point sits just a handful of degrees warmer than the most extreme cold our planet has ever produced. Dry ice, which is solid carbon dioxide, sublimes at roughly −78.5 °C, nearly identical to ammonia’s freezing point. In a well-equipped chemistry lab, you could freeze ammonia using a dry-ice bath without much trouble. Outside of polar research stations and cryogenic facilities, though, you are unlikely to encounter temperatures low enough to solidify ammonia under ordinary pressure.
At its freezing point, ammonia transitions from a colorless liquid with a famously pungent smell into a white crystalline solid. The solid is less dense than you might expect and has a waxy, translucent appearance. Because ammonia’s boiling point sits at about −33 °C, there is a roughly 45-degree window between where ammonia boils and where it freezes. This liquid range matters for industrial applications, particularly in refrigeration, where ammonia needs to remain fluid while absorbing and releasing heat.
What Determines the Freezing Point
Ammonia is a small, polar molecule with three hydrogen atoms bonded to a central nitrogen atom in a pyramidal shape. The nitrogen carries a lone pair of electrons, which lets ammonia form hydrogen bonds with neighboring molecules, though those hydrogen bonds are weaker than the ones between water molecules. That weaker intermolecular attraction is the main reason ammonia freezes and boils at much lower temperatures than water does. Water’s extensive hydrogen bonding network keeps it liquid up to 100 °C and solid below 0 °C, while ammonia’s comparatively modest bonding network means it needs far less thermal energy to remain fluid.
When ammonia does freeze at standard pressure, it crystallizes into a structure known as phase I, a cubic arrangement where the molecules sit on a regular lattice but still rotate fairly freely. Researchers sometimes describe this as a “plastic crystal” because the molecules can spin in place even though they are locked into fixed positions. Cooling the solid further eventually restricts that rotation, leading to additional phase transitions at still lower temperatures.
Pressure Changes Everything
Under everyday conditions, ammonia has a single freezing point. But squeeze it hard enough and the picture gets dramatically more complex. Researchers studying ammonia at pressures reaching tens of gigapascals and temperatures up to hundreds of kelvins have mapped out a phase diagram with multiple distinct solid forms, each with different crystal structures and molecular arrangements.
At moderate pressures, solid ammonia exists in its familiar cubic phase I. Increase the pressure further and it transitions through phases II and III before reaching phase IV, an ordered orthorhombic structure. The transition between phase III and phase IV has been studied in detail up to 20 GPa and 500 K using diamond anvil cells, which can generate the crushing pressures needed to force ammonia into these exotic arrangements.1PubMed. High pressure-high temperature phase diagram of ammonia Phase III is a disordered “plastic” phase where hydrogen atoms move around freely, while phase IV locks those hydrogens into definite positions. Scientists have described the III-to-IV transition as a kind of pseudo-melting of the hydrogen sublattice, because it is the hydrogen atoms, not the overall molecular arrangement, that undergo the most dramatic change.
Raman spectroscopy has proven especially useful for mapping these transitions. By shining laser light on ammonia samples under extreme compression, researchers can detect sharp changes in the vibrational spectra that signal a shift from one crystal phase to another. Measurements at pressures up to 15 GPa and temperatures as low as 20 K have helped pin down the boundaries between phases I, II, III, and IV.2Journal of Raman Spectroscopy. Raman study of solid ammonia at high pressures and low temperatures These aren’t subtle differences visible only to specialists. Phase IV is birefringent, meaning it bends light differently depending on the viewing angle, which makes the transition visible to the naked eye through the diamond window of the pressure cell.
Ammonia-Water Mixtures Freeze at Different Temperatures
Pure ammonia freezing at −78 °C is only part of the story for most real-world and planetary scenarios, because ammonia rarely exists in pure form. It dissolves readily in water, and ammonia-water mixtures behave very differently from either pure substance alone. Adding ammonia to water depresses the freezing point of the mixture, much the way salt lowers the freezing point of road ice.
At standard atmospheric pressure, even a small concentration of ammonia shifts the freezing point below 0 °C. At about 1% ammonia by weight and normal pressure, the mixture freezes around −3 °C. Raise the pressure and the depression grows more dramatic. At 100 MPa (roughly a thousand times atmospheric pressure), that same 1% ammonia solution freezes at approximately −9 °C. Push to 300 MPa and the freezing point drops to around −33 °C.3PubMed Central. Ammonia as a parameter shaping habitability on icy moons At the highest ammonia concentrations, near the eutectic point of the ammonia-water system, the mixture can remain liquid down to roughly −100 °C under the right conditions. This antifreeze effect is one of the reasons ammonia gets so much attention in planetary science.
As these mixtures cool past their freezing points, they don’t simply turn into a single block of ice. Instead, specific hydrate compounds crystallize out. Ammonia monohydrate (NH₃·Hâ‚‚O) and ammonia dihydrate (NH₃·2Hâ‚‚O) are two of the best-studied. Modeling work on ammonia-ammonium chemistry across the solar system has extended solubility data down to 173 K, where these hydrate phases precipitate from solution.4Icarus. Modeling ammonia–ammonium aqueous chemistries in the Solar System’s icy bodies Each hydrate has its own crystal structure that shifts under pressure. Neutron diffraction experiments on deuterated ammonia monohydrate, for instance, have revealed a phase transformation from one crystal structure to another at about 351 MPa.5US EPA HERO. Equation of state and phase transition of deuterated ammonia monohydrate (ND3 center dot D2O) measured by high-resolution neutron powder diffraction up to 500 MPa
Frozen Ammonia on Other Worlds
Ammonia’s freezing behavior matters enormously for understanding planets and moons in the outer solar system, where temperatures routinely plunge far below −78 °C and pressures inside icy bodies can reach thousands of atmospheres. On the surfaces of objects like Saturn’s moon Enceladus or Jupiter’s moon Europa, temperatures hover in the range of 40–110 K, cold enough that ammonia would be frozen solid even without any pressure assistance.
Researchers studying the infrared properties of ammonia ice at 40 K, a characteristic surface temperature for these distant bodies, have measured its optical constants to help interpret data from spacecraft and telescopes.6The Planetary Science Journal. The Infrared Complex Refractive Index of Amorphous Ammonia Ice at 40 K (1.43–22.73 μm) and Its Relevance to Outer Solar System Bodies At these extremely low temperatures, ammonia can form amorphous ice rather than the crystalline phases seen at warmer conditions. Amorphous ice lacks the regular repeating lattice of a crystal, instead freezing into a disordered glassy state. The distinction matters because amorphous and crystalline ammonia ice absorb and reflect light differently, which changes how scientists identify ammonia on remote surfaces from spectral data.
Deep inside icy moons and the ice giants Uranus and Neptune, conditions get even more extreme. Pressures can reach hundreds of gigapascals, and temperatures climb to thousands of kelvins. Under those conditions, ammonia is predicted to enter a “superionic” state where the nitrogen atoms form a fixed lattice while hydrogen ions flow freely through it like a liquid. This bizarre state is neither fully solid nor fully liquid in the conventional sense, and understanding where it begins and ends on the phase diagram helps scientists model the internal structure and magnetic fields of these planets.
The antifreeze effect of ammonia dissolved in water ice is a leading candidate for explaining how subsurface oceans can persist on icy moons. Even a few percent ammonia mixed into an ice shell can lower the melting point enough to maintain pockets or layers of liquid water at depths where the pressure and modest geothermal heat combine to keep temperatures in the right range. This has direct implications for astrobiology, since liquid water is considered one of the prerequisites for life as we understand it. Ammonia-water mixtures at high pressures show freezing points that drop substantially below 0 °C, potentially keeping water liquid in environments that would otherwise be frozen solid.3PubMed Central. Ammonia as a parameter shaping habitability on icy moons
Ammonia in Industrial Refrigeration
Ammonia’s low boiling point of −33 °C and its favorable thermodynamic properties have made it one of the most widely used refrigerants in industrial cold-storage facilities, ice rinks, and food processing plants for well over a century. Its freezing point at −78 °C gives engineers a comfortable margin: the ammonia circulating through a refrigeration system stays well above its freezing temperature even when cooling environments to −40 °C or lower. This means there is essentially no risk of the refrigerant solidifying inside the equipment during normal operation.
In practice, industrial ammonia refrigeration systems operate at pressures and temperatures where ammonia exists as a liquid or gas, cycling between the two states to absorb heat from a cold space and release it elsewhere. The wide liquid range of ammonia, that 45-degree span between boiling and freezing, gives system designers flexibility. Unlike some synthetic refrigerants that have been phased out for environmental reasons, ammonia has zero ozone depletion potential and negligible global warming potential, which has helped sustain its popularity even as regulations have tightened around other coolants.
The tradeoff is toxicity. Ammonia is an irritant at low concentrations and dangerous at higher ones, so industrial ammonia systems require careful engineering, leak detection, and ventilation. The freezing point itself rarely comes into play in standard refrigeration, but it becomes relevant in cryogenic applications. Researchers working with ammonia at very low temperatures in laboratory settings need to account for the possibility of solidification, particularly when using ammonia as a solvent or reaction medium in cryochemistry experiments.
Common Misconceptions About Ammonia’s Freezing Point
One persistent confusion involves mixing up ammonia gas (NH₃) with aqueous ammonia, which is the household cleaning product most people encounter. The ammonia solution in a spray bottle is mostly water with a few percent dissolved ammonia, and it freezes close to 0 °C, just slightly below water’s freezing point. This is nothing like the −78 °C required to freeze pure ammonia. If someone puts a bottle of household ammonia cleaner in the freezer and it solidifies at −18 °C, that is the water freezing, not the ammonia.
Another misconception is that ammonia and dry ice have similar properties because their phase-transition temperatures nearly overlap. While the coincidence of dry ice sublimating at −78.5 °C and ammonia freezing at −77.7 °C is striking, the two substances are chemically unrelated and behave very differently. Dry ice skips the liquid phase entirely under normal pressure, going straight from solid to gas. Ammonia, by contrast, has a well-defined liquid phase and must be cooled well below its boiling point to reach the freezing point.
People sometimes also assume that because ammonia is a gas at room temperature, it must freeze at some extraordinarily low temperature requiring specialized equipment. In reality, −78 °C is cold but achievable with relatively simple lab setups. A mixture of dry ice and acetone, which is a standard cooling bath in chemistry labs, can easily reach this temperature range. Freezing ammonia is not the technical challenge that, say, liquefying helium is. Helium’s boiling point sits at −269 °C, making ammonia’s freezing point look downright balmy by comparison.
How Scientists Study Solid Ammonia
Pinning down the behavior of frozen ammonia under extreme conditions requires specialized tools. Diamond anvil cells, which squeeze tiny samples between the tips of two gem-quality diamonds, can generate pressures exceeding 100 GPa while still allowing scientists to probe the sample with light or X-rays through the transparent diamonds. Combined with resistive heating elements, these setups let researchers explore the full pressure-temperature phase diagram of ammonia from near absolute zero to hundreds of degrees above room temperature.
Raman spectroscopy is one of the primary techniques used in these studies. When laser light hits a solid sample, most photons bounce back at the same energy, but a small fraction lose or gain energy from molecular vibrations. The pattern of those energy shifts acts like a fingerprint for the crystal structure. In ammonia, the shapes of specific vibrational bands change dramatically between phases. Broad, smeared-out peaks in the disordered phase III sharpen into distinct, well-resolved lines in the ordered phase IV, providing a clear signal of when the transition occurs.2Journal of Raman Spectroscopy. Raman study of solid ammonia at high pressures and low temperatures
X-ray diffraction offers complementary information, revealing the precise arrangement of atoms in the crystal lattice. Phase III’s cubic structure and phase IV’s orthorhombic structure produce distinct diffraction patterns that are easy to distinguish.1PubMed. High pressure-high temperature phase diagram of ammonia Neutron diffraction is especially valuable for ammonia because neutrons interact strongly with hydrogen atoms, which are nearly invisible to X-rays. This makes neutron studies ideal for tracking what the hydrogen atoms are doing during phase transitions, such as the ordering that occurs at the III-IV boundary. Neutron powder diffraction has been used to study ammonia hydrates under pressure, revealing structural details that would be inaccessible through other methods.5US EPA HERO. Equation of state and phase transition of deuterated ammonia monohydrate (ND3 center dot D2O) measured by high-resolution neutron powder diffraction up to 500 MPa
These experimental techniques have been supplemented in recent years by computational methods that simulate the behavior of ammonia molecules at conditions too extreme even for diamond anvil cells. First-principles molecular dynamics calculations can predict where new phase boundaries should appear and what crystal structures might be stable at pressures found deep inside giant planets. The interplay between lab experiments and computer simulations has pushed the known phase diagram of ammonia to pressures and temperatures that would have been inaccessible a generation ago, turning what seems like a simple question about freezing into an active area of research that connects chemistry, physics, and planetary science.