The solid that separates when air is cooled and pressurized to liquefy it is carbon dioxide (CO2).

Here's why:

* Liquefaction: When air is compressed and cooled, the gases within it condense into liquids. Nitrogen and oxygen, the most abundant components of air, liquefy at relatively low temperatures (-196°C for nitrogen and -183°C for oxygen).

* Carbon Dioxide's Properties: Carbon dioxide has a much higher liquefaction temperature (-78.5°C). As the air is cooled, the CO2 will solidify into a solid called "dry ice" before the other components of air liquefy.

Let me know if you have more questions about this process!