Could an expert please take a look and tell me if this passage is correct or not? 【Regarding critical pressure】

After reading your description, I feel this passage is incorrect! “If the pressure is below its critical pressure, the gas cannot be liquefied no matter how low the temperature is. ”The sentence in red font of yours is incorrect. Below the critical temperature, increasing the pressure appropriately can liquefy the gas. Applying even higher pressure once again will surely liquefy the gas more effectively. The prerequisite for determining whether a gas can be pressurized into a liquid state is whether its temperature is below the critical temperature, rather than whether its pressure is below the critical pressure. So it’s wrong. There’s also the issue of the gas cylinders you mentioned later; if they are not low-temperature cylinders, that is, if the temperature inside the cylinders is at ambient level rather than low, then whether the substance inside the cylinders is in a gaseous or liquid state depends on the relationship between its critical temperature and the temperature inside the cylinders. If the critical temperature is lower than the temperature of the medium inside the cylinder, then there must be a gas, a compressed gas, inside the steel cylinder. Conversely, if the critical temperature is higher than the temperature of the medium inside the bottle, it is likely to be a liquid. So the second half of this passage is quite confusing. It doesn’t seem entirely correct.

If the gas temperature is lower than the critical temperature, then it can be liquefied at pressures below the critical pressure; is that how it can be understood? ?

If the gas temperature is lower than the critical temperature, then it can be liquefied at pressures below the critical pressure; is that how it can be understood? ?

If the gas temperature is lower than the critical temperature, then it can be liquefied at pressures below the critical pressure; is that how it can be understood? ?

This post was last edited by arpcd on 2017-8-8 17:53. All that physical chemistry stuff has been given back to the teacher. . The part in red should be taken from some article on the safety aspects of CO2 gas cylinders; I don’t know which organization’s technical expert wrote it. Are you really going to take that seriously? ? The critical properties of a single medium, the most important of which is the critical temperature, are understood by all to be such that above this temperature, a gas cannot be liquefied, no matter how high the pressure applied; thus, this critical temperature represents the \"highest\" temperature at which a gas can be liquefied. Note that it is the highest temperature. But what is the critical pressure? It corresponds to the minimum pressure at which the gas liquefies at this highest temperature; there is a corresponding relationship. The biggest flaw in the original poster’s understanding is that the critical point is merely a special point on the phase diagram of a medium, namely the endpoint of the saturated liquid line/gas line. . . On a phase diagram, in addition to points, there are also surfaces, and these exist in both physical chemistry. What are the points, lines, and surfaces in a phase diagram? ? ? ? Take water as an example: its critical temperature is 374.15°C, and its critical pressure is 22 MPa. To liquefy water vapor at 374.15°C, the pressure must be at least 22 MPa; this critical pressure represents the lowest pressure corresponding to that critical temperature of 374.15°C. That’s how subcritical boilers work. Conversely, to vaporize water at 22 MPa, you need to heat the water to 374.15°C. . These are industrial examples of critical temperature and critical pressure. Today’s supercritical and ultra-supercritical boilers operate at pressures above 374 and 22 respectively; as a result, such boilers do not have a drum – instead, there is directly a straight vaporization section. This is because in a supercritical state, there is no distinction between liquid and gas anymore, as everything exists as a supercritical fluid. Medium-temperature and medium-pressure boilers, such as those that produce steam at 9.8 MPa, do have a drum; the presence of an interface between saturated water and saturated steam is what gives the drum its purpose. As for water at room temperature, for example at 20°C under normal pressure, the state point of such water on the P-T diagram lies neither on the saturated liquid line nor on the saturated vapor line, but rather in the subcooled region – which is referred to as a \"region\" on the phase diagram – and it is far away from the critical point. In the subcooled region, the water is obviously in a liquid state, and this is determined by the properties of water. The same applies to CO2: you can’t just look at a single point and discuss liquefaction; surely you can’t say that to liquefy water vapor at room temperature of 20°C, one needs a pressure higher than 22 MPa, right? Just look at the PT plot of water to understand. . . The red circle represents the critical point. Show me where the state point is at 20°C and under normal pressure To the northeast of the triple point, it falls in the “liquid” region; would you still use critical properties to describe it at this point? The difference is too great. . .

My expertise in the field of chemistry is really poor; I need to study and research more

It doesn’t matter whether it’s right or wrong; at critical pressure, a phenomenon of non-liquefaction does indeed occur.

The graph above is quite complex and difficult to understand. A simpler saturation vapor pressure curve is available; one can find it in vapor pressure tables. If the point corresponding to the state of the substance lies below the curve, it is in a liquid state; if it is above the curve, it is in a gas state. The line itself represents saturated vapor

Come and learn*learn*. . . . . . .

Is this reversed? There should be a critical temperature, right? . .