The temperature at which a substance is in a critical state is not necessarily called "critical temperature". Cooling and pressurization are conditions for liquefied gas. However, pressurization alone does not necessarily liquefy the gas, which depends on whether the gas is below the critical temperature at that time. If the gas temperature exceeds the critical temperature, no matter how the pressure increases, the gaseous substance will not liquefy. For example, the critical temperature of water vapor is 374℃, which is much higher than normal temperature. So ordinary water vapor can be easily cooled into water. Others, such as ether, ammonia, carbon dioxide, etc. , has a critical temperature higher than or close to room temperature, and this substance is easily compressed into a liquid at room temperature. However, there are also some substances with very low critical temperatures, such as oxygen, air, hydrogen and helium. They are extremely difficult to liquefy. The critical temperature of helium is -268℃. In order to liquefy these gases, certain low-temperature technology and equipment are needed.