* Physical Changes: Alter the form or appearance of a substance but don't change its chemical composition. Boiling is a physical change because it only changes the state of matter (from liquid to gas) without altering the chemical makeup of the air.

* Chemical Reactions: Involve the formation of new substances with different chemical compositions. Boiling air doesn't create new molecules; it simply separates the existing molecules (nitrogen, oxygen, etc.) further apart.

In summary: Boiling liquid air is a physical change because it only changes the state of matter and doesn't create any new chemical compounds.