According to Baidu Encyclopedia, the method of separating liquid air is an industrial method to prepare oxygen, which belongs to a physical method. The principle is that the boiling point of nitrogen in air is lower than that of oxygen. The industrial production method of oxygen: the boiling point of nitrogen in air is lower than that of oxygen, the air is pressurized and cooled into liquid, and then heated. Because the boiling point of liquid nitrogen (-196 degrees Celsius) is lower than that of oxygen (-183 degrees Celsius), nitrogen is evaporated from liquid air first, leaving liquid oxygen, which is also called liquid-air separation method.