Ammonia boils at −33.34 °C (−28.01 °F, or 239.81 K) when measured at standard atmospheric pressure. That is cold enough to make your freezer feel balmy by comparison, yet warm enough that ammonia exists as a gas under everyday conditions on Earth. The reason ammonia sits at this particular temperature, and why that number matters far beyond a chemistry classroom, touches on everything from industrial cooling to the moons of Saturn.
How the Boiling Point Compares to Related Molecules
Ammonia’s boiling point looks unremarkable at first glance, but it becomes interesting the moment you compare it to the other hydrogen compounds in its neighborhood on the periodic table. Phosphine (PH₃), the hydrogen compound of the element directly below nitrogen, boils at about −87 °C. Arsine (AsH₃) boils at −55 °C. If you draw a line through those heavier compounds and extrapolate upward, ammonia “should” boil somewhere around −77 °C based on molecular weight alone. Instead, it boils roughly 44 degrees higher than that prediction. The culprit is hydrogen bonding between ammonia molecules, which adds an extra stickiness that pure molecular weight cannot account for.
Water shows the same anomaly in even more dramatic fashion: it “should” boil far below 0 °C based on the trend set by hydrogen sulfide and hydrogen selenide, yet it boils at 100 °C. The reason water’s deviation is so much larger than ammonia’s comes down to hydrogen-bonding geometry. Quantum molecular dynamics simulations show that each nitrogen atom in liquid ammonia forms only about one hydrogen bond on average, at a distance of roughly 2.24 Å, and that bond is fleeting, lasting around 0.1 picoseconds. Water, by contrast, forms about two hydrogen bonds per oxygen atom and holds onto them longer.1PubMed Central. Hydrogen Bonding in Liquid Ammonia Fewer and shorter-lived hydrogen bonds mean less energy is needed to pull ammonia molecules apart from each other and push them into the gas phase, which is why ammonia’s boiling point sits so much lower than water’s.
The Energy Needed to Boil Ammonia
A substance’s boiling point tells you the temperature at which it vaporizes, but the heat of vaporization tells you how much energy you actually have to pump in to make that happen. For ammonia near its boiling point (around 239 K), the enthalpy of vaporization is about 23.4 to 23.5 kJ per mole.2NIST Chemistry WebBook. Ammonia – Section: Phase change data At higher temperatures, closer to 308 K, that figure drops to about 22.7 kJ per mole, because the liquid is already closer to its critical point and the molecules need less of a push to escape into the vapor.2NIST Chemistry WebBook. Ammonia – Section: Phase change data
For comparison, water’s heat of vaporization is about 40.7 kJ per mole, nearly double ammonia’s value. That difference traces directly back to the hydrogen-bonding story: water’s network is denser and more persistent, so you need more energy to disassemble it. But ammonia’s heat of vaporization is still high relative to many other refrigerants, and that turns out to be one of its most useful industrial properties.
Ammonia’s Full Phase Portrait
The boiling point is just one landmark on ammonia’s phase diagram. NIST reference data fill in the rest of the picture. Ammonia’s melting point is 194.95 K (about −78.2 °C), with an uncertainty of just 0.3 K.2NIST Chemistry WebBook. Ammonia – Section: Phase change data That is close to dry ice territory, so solid ammonia is genuinely frigid stuff. The triple point, where solid, liquid, and gas all coexist in equilibrium, occurs at a pressure of just 0.06060 bar, far below normal atmospheric pressure.2NIST Chemistry WebBook. Ammonia – Section: Phase change data
At the other extreme, ammonia’s critical temperature is 405.4 K (about 132 °C).2NIST Chemistry WebBook. Ammonia – Section: Phase change data Above this temperature, no amount of pressure can squeeze ammonia into a distinct liquid phase; it becomes a supercritical fluid where the boundary between liquid and gas disappears. The fact that ammonia’s critical temperature sits well above room temperature is practically important: it means you can liquefy ammonia at moderate pressures without exotic cooling, which is one reason it has been a favorite working fluid in refrigeration systems for more than a century.
Why Refrigeration Engineers Love Ammonia’s Boiling Point
Ammonia was one of the first substances ever used in mechanical refrigeration, dating back to the 1870s, and it remains the dominant refrigerant in large industrial cold-storage facilities, ice rinks, and food-processing plants. Its boiling point of −33 °C sits in a sweet spot for refrigeration: low enough to cool spaces well below freezing, but not so low that the system needs extreme pressures to operate. A refrigerant works by evaporating at low pressure (absorbing heat from the space you want to cool) and then condensing at high pressure (dumping that heat outside). Ammonia’s combination of a relatively high heat of vaporization and a low boiling point gives it an outstanding coefficient of performance compared to synthetic hydrofluorocarbon (HFC) refrigerants.3ALAM Journal of Maritime Studies. Ammonia Reliability as Primary Refrigeration for Air Conditioning System
In plain terms, ammonia can move more heat per kilogram of fluid than most alternatives, so a system using ammonia can be smaller or run more efficiently for the same cooling capacity. It also has zero ozone-depletion potential and zero global-warming potential as a refrigerant, which makes it increasingly attractive as environmental regulations tighten around synthetic alternatives. The trade-off is toxicity and flammability: ammonia is acutely toxic at concentrations above about 300 parts per million, and it can form flammable mixtures in air at concentrations between roughly 15% and 28%. That safety profile is why ammonia refrigeration is common in large, well-ventilated industrial facilities but rare in household appliances, where even a small leak would be dangerous in a confined space.
Pressure Changes Everything
The −33 °C figure assumes you are at standard atmospheric pressure, about 1.013 bar. Change the pressure and the boiling point shifts. This is the principle behind every ammonia refrigeration system: compress ammonia gas to a few bars of pressure and it condenses into a liquid at a temperature your system can manage, then let it expand to lower pressure and it boils, absorbing heat as it does. The NIST Antoine equation coefficients for ammonia cover a temperature range from 164 K all the way up to 371.5 K, mapping out the vapor pressure curve across that span.2NIST Chemistry WebBook. Ammonia – Section: Phase change data In practical terms, at about 10 bar of pressure, ammonia’s boiling point climbs to roughly 25 °C, meaning it is a liquid at room temperature if you keep it in a pressurized tank. This is exactly how anhydrous ammonia is shipped and stored: as a liquid under modest pressure, typically in steel cylinders or large refrigerated tanks.
This pressure sensitivity also matters for altitude and weather. At very high altitudes where atmospheric pressure drops, ammonia would boil at a slightly lower temperature. The shift is not dramatic for the atmospheric pressure changes you encounter on Earth’s surface, but it becomes relevant in the extreme environments of other worlds.
Ammonia on Other Worlds
Ammonia is one of the most abundant nitrogen-bearing molecules in the solar system, showing up in the atmospheres of Jupiter and Saturn and in the subsurface oceans hypothesized to exist beneath the icy shells of moons like Enceladus and Titan. Its low boiling point and ability to act as an antifreeze when dissolved in water make it a molecule of intense interest in astrobiology.
On Titan, Saturn’s largest moon, researchers have proposed that ammonia-water mixtures play a role in cryovolcanism. In one model, cracks form at the base of Titan’s ice shell, allowing pockets of ammonia-water liquid to develop. As these pockets partially freeze, the ammonia concentration in the remaining liquid increases, lowering the mixture’s density and potentially allowing it to rise toward the surface.4Icarus. Resurfacing of Titan by ammonia-water cryomagma This ammonia-enriched slurry could erupt onto the surface in a process analogous to volcanism on Earth, but with water-ice “lava” instead of molten rock.
When ammonia is dissolved in water at low concentrations, the boiling point of the mixture stays close to water’s 100 °C at standard pressure, depressed by only a few degrees for a 1% ammonia solution. At elevated pressures, like those expected beneath tens of kilometers of ice on an ocean moon, both the boiling and freezing points shift further.5FEMS Microbes. Ammonia as a parameter shaping habitability on icy moons The presence of ammonia expands the range of conditions under which liquid water can exist, which is why astrobiologists treat it as a potentially key ingredient for habitability on icy worlds.
Ammonia as a Green Fuel and Hydrogen Carrier
In recent years, ammonia has attracted attention as a carbon-free fuel and a convenient way to transport hydrogen. The logic is straightforward: hydrogen gas is difficult to store and ship because it is extremely light and must be either compressed to very high pressures or cooled to −253 °C to liquefy. Ammonia, by contrast, liquefies at −33 °C at atmospheric pressure or at room temperature under about 10 bar, as noted earlier. Each molecule of ammonia contains three hydrogen atoms, so you can ship hydrogen in a much denser, more manageable form and then “crack” the ammonia back into nitrogen and hydrogen at the destination.
Retrofitting existing floating storage and regasification units (originally designed for liquefied natural gas) to handle ammonia is already being studied. One analysis found that a stainless-steel ammonia storage tank would experience a boil-off gas rate of about 0.02% by the fifth day, and that repurposing that boil-off through fuel cells could generate around 30.6 MWh of electricity from the ammonia fraction alone.6International Journal of Hydrogen Energy. Retrofit of floating storage and regasification units for ammonia and hydrogen: Material selection, thermal analysis, and boil-off gas utilization The low boiling point is both an asset and a complication here: it makes ammonia easier to liquefy than hydrogen by a wide margin, but it still requires insulated tanks and careful thermal management to minimize losses during long-duration storage at sea.
Whether ammonia ultimately becomes a major shipping fuel depends on solving several problems beyond just storage temperature. Combustion of ammonia in marine engines is still being optimized, and the toxicity concerns that limit its use in household refrigeration apply even more in confined shipboard environments. Still, the fact that ammonia can be produced renewably (using green hydrogen and atmospheric nitrogen) and burned without direct carbon emissions keeps it near the top of the list for decarbonizing heavy transport.
Ammonia Volatilization from Soil and Its Climate Connection
Ammonia’s tendency to evaporate at relatively modest temperatures has environmental consequences that go far beyond the lab. Agriculture is the largest source of ammonia emissions worldwide, primarily from livestock waste and synthetic fertilizers. When ammonia-based fertilizer is applied to warm soil, the ammonia volatilizes into the atmosphere, where it can form fine particulate matter, contribute to haze, and deposit nitrogen in ecosystems that do not need it.
The rate of this volatilization is strongly temperature-dependent. Higher soil surface temperatures push more ammonia from the liquid or dissolved phase into the gas phase, following the same basic thermodynamics that govern ammonia’s boiling point in a flask. Climate models project rising soil temperatures across much of Europe’s agricultural belt, suggesting that ammonia emissions from farming could increase in a warming world even if fertilizer application rates stay the same.7Atmospheric Chemistry and Physics. A roadmap to estimating agricultural ammonia volatilization over Europe using satellite observations and simulation data Satellite observations are now being paired with surface temperature simulations to map ammonia emission hotspots and forecast how they might shift under future climate scenarios. For farmers, the practical takeaway is that applying ammonia-based fertilizers during cooler parts of the day or incorporating them into the soil rather than leaving them on the surface can significantly reduce losses to the atmosphere.
How Ammonia’s Weak Hydrogen Bonds Shape Its Behavior as a Solvent
Ammonia’s boiling point is closely tied to its hydrogen-bonding network, and that same network makes liquid ammonia an interesting solvent in its own right. Because ammonia’s hydrogen bonds are weaker and fewer than water’s, liquid ammonia dissolves many substances that water cannot handle well. Alkali metals like sodium and potassium, for example, dissolve in liquid ammonia to produce vivid blue solutions, something that would cause a violent reaction in water. The weaker intermolecular forces in ammonia mean it is a “gentler” solvent for certain reactive materials.
Liquid ammonia also has a lower viscosity and surface tension than water, again because of its less extensive hydrogen-bonding network. The crystalline form of ammonia is even more striking: simulations suggest that hydrogen bonding is practically nonexistent in crystalline ammonia-I, the solid phase that forms when you cool ammonia below its melting point at normal pressure.1PubMed Central. Hydrogen Bonding in Liquid Ammonia This stands in sharp contrast to water ice, which is held together by an elaborate hydrogen-bonded lattice. The absence of hydrogen bonding in solid ammonia helps explain why ammonia ice is softer and mechanically weaker than water ice, a property that matters when planetary scientists model the behavior of ice shells on moons like Titan and Enceladus.
Understanding ammonia’s boiling point, then, is not just a matter of memorizing a number. The −33 °C figure encodes information about the molecule’s shape, its intermolecular forces, and the compromises it makes between wanting to stick together and wanting to fly apart. That single temperature connects to refrigerator coils, cryovolcanic eruptions on distant moons, and the fertilizer evaporating from a farmer’s field on a warm afternoon.