Critical Point Temperatures Pressures Gases Table You'll Reuse
The critical point of a gas is the specific temperature and pressure where liquid and vapor become indistinguishable; below that temperature a gas can be liquefied by pressure, but above it, pressure alone no longer works. A practical critical point table usually lists critical temperature $$T_c$$ and critical pressure $$P_c$$ for common gases such as nitrogen, oxygen, carbon dioxide, ammonia, and helium.
What the table shows
This article is a fast-decoding guide to a gas table of critical temperatures and pressures, with values for widely used substances and a short explanation of how to read them. For example, carbon dioxide has a critical temperature of about 304.2 K and a critical pressure of about 7.39 MPa, while nitrogen's critical temperature is about 126.0 K and its critical pressure is about 3.39 MPa.
| Gas | Critical Temperature, Tc | Critical Pressure, Pc | Practical takeaway |
|---|---|---|---|
| Nitrogen | 126.0 K | 3.39 MPa | Requires cryogenic cooling to liquefy |
| Oxygen | 154.6 K | 5.08 MPa | Liquefies at very low temperature and elevated pressure |
| Carbon dioxide | 304.2 K | 7.39 MPa | Can exist as a supercritical fluid above room temperature near high pressure |
| Ammonia | 405.5 K | 11.29 MPa | High critical temperature makes condensation easier than for many gases |
| Hydrogen | 33.2 K | 1.30 MPa | Extremely hard to liquefy without deep cooling |
| Helium | 5.2 K | 0.23 MPa | One of the hardest substances to liquefy |
How to read it
A critical temperature is the highest temperature at which a substance can still be liquefied by pressure alone, while the critical pressure is the pressure required at that temperature to reach the critical point. If the gas is above its critical temperature, compressing it may make it denser, but it will not form a normal liquid phase. That is why refrigeration engineers, chemical engineers, and process designers rely on these values when choosing compressors, separators, and storage conditions.
- Find the gas in the table.
- Compare the operating temperature to the gas's critical temperature.
- If the temperature is below Tc, pressure can potentially condense the gas.
- If the temperature is above Tc, the gas will not liquefy by pressure alone.
- Use Pc to estimate the minimum pressure needed at Tc for liquefaction.
Why the values matter
The biggest operational lesson from a critical constant table is that different gases behave very differently under compression. Carbon dioxide is a classic example because its relatively high critical temperature means it can become supercritical at conditions that are not extremely cold, which is why CO2 is central in supercritical extraction and certain refrigeration systems. In contrast, helium's extremely low critical temperature explains why it is used in cryogenics and why liquefaction requires specialized cooling hardware.
Scientists and engineers also use these numbers to estimate phase behavior, design pipelines, and avoid unstable two-phase regions. The critical point is not just textbook theory; it directly affects separation columns, carbon capture systems, LNG processes, and industrial gas supply chains.
"At the critical point, liquid and vapor phases merge into a single supercritical state, and the distinction between the two phases disappears."
Representative table
The table below collects a compact set of commonly referenced gases with critical temperatures and pressures in consistent units, making it easier to compare how easily each substance can be liquefied.
| Gas | Tc (K) | Tc (°C) | Pc (MPa) | Pc (atm) |
|---|---|---|---|---|
| Hydrogen | 33.2 | -239.9 | 1.30 | 12.8 |
| Helium | 5.2 | -268 | 0.23 | 2.3 |
| Nitrogen | 126.0 | -147 | 3.39 | 33.5 |
| Oxygen | 154.6 | -118.6 | 5.08 | 50.1 |
| Carbon dioxide | 304.2 | 31.0 | 7.39 | 73.0 |
| Ammonia | 405.5 | 132.4 | 11.29 | 111.5 |
| Propane | 370.0 | 96.9 | 4.23 | 41.8 |
| Water | 647.1 | 374.0 | 22.03 | 217.5 |
Engineering context
In process engineering, the most important part of a phase diagram is often the region near the critical point because small changes in pressure or temperature can produce large changes in density and transport properties. A 2025 engineering reference table lists critical-point properties alongside gas constants precisely because those values are used in calculations for compressors, refrigeration cycles, and high-pressure vessel design.
That same practical need explains why tables often show both kelvin and degrees Celsius, plus MPa and atmospheres. Engineers move between unit systems constantly, and a usable table saves time while reducing conversion errors, especially when comparing natural gas components such as methane, ethane, propane, and carbon dioxide.
What stands out
- Helium and hydrogen have very low critical temperatures, so they need extreme cooling before liquefaction.
- Carbon dioxide is unusual because its critical temperature is above room temperature, which makes supercritical CO2 commercially useful.
- Ammonia has a relatively high critical temperature, which is one reason it has long been useful in refrigeration systems.
- Water's critical pressure is very high, which is why its supercritical state appears only in specialized high-temperature, high-pressure equipment.
Quick interpretation
A lower Tc means a gas is harder to liquefy and generally requires deeper refrigeration before compression can produce a liquid phase. A higher Pc means more pressure is required even after the temperature is brought to the critical point, which affects equipment sizing and operating cost.
For casual readers, the shortest mental rule is simple: if the gas is above its critical temperature, pressure alone will not turn it into a normal liquid. That one rule explains why helium, hydrogen, and nitrogen are handled so differently from ammonia, propane, and carbon dioxide in industrial systems.
Useful takeaway
If you need a fast rule for a gases table, focus on the pairing of Tc and Pc, not just one number. Tc tells you whether liquefaction is even possible by pressure alone, and Pc tells you the threshold pressure at that limiting temperature.
For most practical work, the most useful entries are nitrogen, oxygen, carbon dioxide, ammonia, hydrogen, and helium because they show the widest spread in critical behavior and cover the most common industrial use cases.
Helpful tips and tricks for Critical Point Temperatures Pressures Gases Table Youll Reuse
What is the critical point of a gas?
The critical point is the combination of temperature and pressure where the liquid and vapor phases become indistinguishable, ending the liquid-vapor boundary.
Why do some gases need such low temperatures?
Gases with weak intermolecular forces, such as helium and hydrogen, have very low critical temperatures, so they must be cooled dramatically before pressure can liquefy them.
Is carbon dioxide a gas or a liquid at room temperature?
At ordinary room temperature, carbon dioxide is above its critical temperature, so it cannot be liquefied by pressure alone; under sufficiently high pressure it can become a supercritical fluid rather than a normal liquid.
Why do engineers care about critical pressure?
Critical pressure tells engineers how much pressure is needed at the critical temperature to reach the critical point, which affects separation, storage, and refrigeration design.
Which gas in the table is hardest to liquefy?
Helium is among the hardest because it has the lowest critical temperature and a very low critical pressure, which makes cryogenic handling essential.