Here's why:
* Critical Point: Oxygen has a critical point, which is the temperature and pressure above which it cannot exist as a liquid, no matter how much pressure is applied. This point is at -118.6°C (154.5 K) and 50.4 atmospheres (5100 kPa).
* Below the Critical Point: At temperatures below the critical point, oxygen can be liquefied by increasing the pressure. However, the required pressure decreases as the temperature gets lower.
* Boiling Point: At atmospheric pressure (1 atmosphere), oxygen boils (turns from liquid to gas) at -183°C (-297°F).
So, to answer your question directly: at atmospheric pressure, oxygen turns to liquid at -183°C (-297°F). But at higher pressures, the temperature at which it liquefies will be higher.
