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Yes, ammonia can become solid as pressure rises—but only when its temperature is already close to the pressure-dependent solid–liquid boundary. Near ordinary pressures, pure ammonia melts at 194.95 K (about −78.2 °C); NOAA’s CAMEO data sheet rounds the freezing point to −77.7 °C. In this low-pressure region, compression generally raises the melting temperature, so a sufficiently cold liquid can cross into the solid region.
Pressure is not a stand-alone freezing rule. Compression can also heat ammonia, the material may be an ammonia–water mixture rather than pure NH3, and the observed transition can lag equilibrium because of nucleation and flow conditions. State identification requires both pressure and temperature, plus composition and a validated property source.
What “freezing under pressure” actually means
Freezing is a phase change from liquid to solid. For ammonia, the boundary between those phases is a curve on a pressure–temperature diagram, not a single pressure value. A pressure gauge tells you only one coordinate. Without the corresponding temperature, it cannot tell you whether ammonia is liquid, vapor, two-phase, or solid.
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Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →At a fixed temperature near ammonia’s normal melting range, raising pressure can move the operating point across that boundary. The result is pressure-induced solidification. At a different starting temperature, the same pressure increase may leave the material liquid. In a real line, compression can also increase temperature, which pushes the state in the opposite direction, and a phase change may be delayed by the time needed to form solid crystals.
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Reference points for pure ammonia
| Property | Value | Qualification and source |
|---|---|---|
| Fusion temperature | 194.95 K (approximately −78.2 °C) | NIST compiled thermodynamic data, based on Timmermans (1921) |
| Freezing point listed for general chemical-safety use | −77.7 °C | NOAA CAMEO Chemicals data sheet, accessed 2026 |
| Triple-point pressure | 0.06060 bar | NIST compiled data, based on Fonseca and Lobo (1989) |
| Critical temperature | 405.4 K | NIST compiled thermodynamic data, based on Brunner (1988) |
| Critical pressure | 11.27 MPa | NOAA CAMEO Chemicals data sheet, accessed 2026 |
The two quoted freezing temperatures differ by about half a degree because they come from different compilations and rounding conventions. They describe the normal-pressure reference point, not a universal value valid at every pressure.
Why compression can raise ammonia’s melting temperature
The slope of a solid–liquid equilibrium line follows the Clapeyron relation:
dP/dT = ΔS/ΔV = L/(TΔV)
Here, L is the latent heat of fusion, T is absolute temperature, and ΔV is the liquid volume minus the solid volume for the phases being compared. The sign and size of ΔV determine whether pressure raises or lowers the equilibrium melting temperature.
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Published low-pressure ammonia data are consistent with a positive initial slope. In that range, increasing pressure requires a higher temperature to keep liquid and solid in equilibrium. A liquid that was just stable at its original pressure can therefore become supersaturated with respect to the solid when compressed, especially if heat removal keeps it very cold.
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This thermodynamic statement describes an equilibrium boundary. It does not say that every pressure rise instantly creates ice-like ammonia. Nucleation barriers, residence time, mixing, wall effects and heat transfer determine when a visible or flow-blocking solid actually appears.
Is there one pressure that freezes ammonia?
No. The required pressure depends on temperature and on which solid phase is involved. A question such as “What pressure freezes ammonia?” is incomplete unless it also supplies the starting temperature, the pressure range and the composition.
At ordinary refrigeration pressures
Use a pressure–temperature saturation table or pressure–enthalpy chart for the specific ammonia formulation. Locate the measured pressure and temperature, then determine whether the point lies in the liquid, vapor, two-phase or solid region. If a compression path crosses the solid–liquid line, freezing is thermodynamically possible; if it does not, pressure alone cannot produce equilibrium solidification.
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NIST lists ammonia’s triple-point pressure as 0.06060 bar. The triple point is where solid, liquid and vapor coexist. Below that pressure, equilibrium liquid ammonia is not a stable phase, so cooling does not proceed through an ordinary liquid-to-solid transition at equilibrium. A system that reaches such a low pressure follows vapor–solid (sublimation/deposition) behavior instead of the usual liquid–solid path.
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Why the simple rule fails at high pressure
Ammonia has more than one solid phase. High-pressure experiments have mapped melting behavior to at least 9 GPa and 900 K, demonstrating that the low-pressure trend cannot be extended indefinitely.
A separate study by Ojwang and colleagues in the Journal of Chemical Physics (2011) reported a melting-curve maximum near 37 ± 6 GPa and 2140 ± 200 K. Above and around portions of that range, the melting curve has a negative slope: increasing pressure can lower the melting temperature for the relevant phase. The exact boundary depends on the solid structure and the pressure–temperature path.
These extreme conditions are far outside normal refrigeration operation, but they matter when interpreting broad claims such as “pressure always raises ammonia’s freezing point.” The defensible rule is limited to the low-pressure regime represented by the applicable property data.
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How to diagnose a suspected ammonia freeze-up in a refrigeration system
A blockage that operators call a freeze-up can have several causes. Treat it as a process and safety event, not as proof that a particular pressure caused solid ammonia.
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- Make the system safe under the site procedure. Keep personnel away from a suspected release or blocked section, use the facility’s ammonia emergency controls, and involve qualified refrigeration and safety staff. Do not loosen fittings or vent ammonia as an improvised diagnostic.
- Confirm the measurements. Record pressure and temperature at the same location and time. Check instrument isolation valves, impulse lines, calibration status and sensor placement; a pressure reading and a remote temperature reading may describe different parts of the circuit.
- Check composition. Establish whether the charge is specified as essentially pure ammonia or contains water or another contaminant. A pure-NH3 chart is not valid for an ammonia–water mixture.
- Use a validated property source. Compare the measured state with ammonia thermodynamic tables or a pressure–enthalpy diagram. The OSHA IIAR reference page identifies the IIAR Ammonia Data Book as containing thermodynamic property tables, pressure–enthalpy diagrams, safety data and regulatory material.
- Map the flow path. Compare upstream and downstream pressure, temperature and flow indications to locate a restriction. A pressure drop, loss of flow or abnormal heat-transfer pattern can identify where the state changed, but none of those signs alone proves solid ammonia.
- Evaluate the heat balance. Determine whether expansion, evaporation, ambient heat loss or a control change cooled the fluid faster than expected. Conversely, compression may have heated the bulk fluid while a cold wall or stagnant pocket remained near the solid boundary.
- Recover only through the approved method. Follow the manufacturer’s and facility’s procedures for isolation, warming, draining, clearing restrictions and returning the circuit to service. Document the measured conditions so the corrective action can be checked against the property data.
The key diagnostic output is a pressure–temperature–composition record tied to a specific point in the circuit. A single gauge value is not enough to classify the phase.
Pure ammonia and ammonia–water mixtures are not interchangeable
Water changes ammonia’s phase behavior. A mixture has its own bubble and dew relationships, concentration effects and possible solid phases, so the pure-ammonia freezing line cannot be substituted into a mixture calculation.
| Decision axis | Essentially pure NH3 | Ammonia–water mixture |
|---|---|---|
| Composition needed for a calculation | Ammonia purity and any known impurities | Ammonia and water concentrations, plus other components if present |
| Phase boundary | Use a pure-ammonia pressure–temperature or pressure–enthalpy source | Use mixture-specific equilibrium data; pure-NH3 boundaries do not apply |
| Pressure and temperature relation | Defined by the selected pure-fluid formulation and range | Composition-dependent; a single saturation temperature at a pressure may not represent the whole mixture |
| Operating temperature range | Must remain within the selected property model’s validated range | Must remain within mixture data coverage for the actual concentration |
| Exposure and equipment controls | Ammonia toxicity, materials compatibility and site rules apply | Ammonia controls still apply, with additional composition and corrosion considerations determined by the installation |
The available data does not establish one universal mixture freezing pressure, so any such number would be misleading without a specified composition and model.
Common mistakes and the correct interpretation
“Higher pressure always means a higher freezing point.”
That is a useful low-pressure approximation for ammonia’s initial melting curve, not an all-pressures law. Multiple solid phases and the high-pressure maximum reported near 37 ± 6 GPa invalidate a single straight-line extrapolation.
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“The gauge crossed a limit, so the ammonia must be solid.”
Pressure can correspond to liquid, vapor, two-phase or solid material depending on temperature and enthalpy. Verify the complete state with a property chart and independent measurements.
“Compression automatically freezes the charge.”
Compression often heats a fluid. Whether the final state is colder or warmer depends on the compression process, heat transfer and flow restriction. A cold starting state and a path that reaches the solid region are required for pressure-induced freezing.
“A published normal freezing point works for every installation.”
The 194.95 K and −77.7 °C figures are reference values near ordinary pressure. They are not substitutes for the operating pressure, temperature and composition of a particular circuit.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsWhat operators should record for a defensible phase determination
- Pressure, temperature and timestamp at each relevant measurement point.
- Ammonia concentration, water content and known contaminants.
- Flow direction, valve positions, compressor or pump status and recent control changes.
- Upstream and downstream conditions around the suspected restriction.
- The exact property table or pressure–enthalpy chart, including its pressure and temperature validity range.
- Whether the condition was steady, transient, stagnant or undergoing compression or expansion.
Keeping these records separates an equilibrium calculation from an operational observation. It also allows qualified personnel to distinguish a genuine solid-phase event from sensor error, two-phase flashing, hydrate-like deposits, blocked strainers or another flow problem.
Bottom line for the “does ammonia freeze under pressure?” question
Pure ammonia can freeze during compression when its temperature and pressure path cross the low-pressure solid–liquid boundary. Its normal fusion temperature is 194.95 K (about −78.2 °C), and pressure generally raises that boundary in the ordinary range. There is no single freezing pressure: below 0.06060 bar the equilibrium liquid phase is unavailable, while at extreme pressures ammonia’s multiple solid phases produce nonmonotonic melting behavior. For refrigeration work, determine the state from measured pressure, temperature, composition and validated ammonia property data—not from pressure alone.
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