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For example, the critical temperature of oxygen is -118.4°C, and its critical pressure is 5.04 MPa. When the temperature is below this critical value and the pressure is also below it, why is oxygen still liquefied even though the temperature is not above the critical value?
When the temperature exceeds a certain value, it cannot be liquefied no matter how high the pressure is increased. This temperature is called the critical temperature, and the corresponding liquefaction pressure is called the critical pressure. The critical temperature of oxygen is –118°C. The critical pressure at 4℃ is 5. 079 MPa. This means that at this pressure, the liquefaction temperature cannot be increased any further; liquefaction can only occur at a temperature lower than or equal to this value. When the liquefaction temperature is below the critical temperature, the corresponding liquefaction pressure can also be reduced.
The higher the pressure, the higher the liquefaction temperature, but there is a limit value, namely the critical temperature. Once the temperature exceeds this critical value, it cannot be liquefied no matter how high the pressure is increased. One temperature corresponds to one pressure; the liquefaction temperature below the critical temperature corresponds to the liquefaction pressure below the critical pressure