* Physical Changes: Involve changes in the form or appearance of a substance, but not its chemical composition.

* Chemical Changes: Result in the formation of new substances with different chemical properties.

When nitrogen boils, it transitions from a liquid state to a gaseous state. The nitrogen molecules themselves remain the same (N2). The only change is the spacing between the molecules and their movement. This is a physical change.

Key Point: If you were to chemically react nitrogen with another substance (like oxygen to form nitrogen oxides), then that would be a chemical change.