* Physical Changes: Alter 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 oxygen is liquefied, the following happens:

* State Change: Oxygen transitions from a gas to a liquid. This is due to a decrease in temperature and/or increase in pressure.

* Molecular Structure: The oxygen molecules (O2) remain the same. They are just packed more tightly together in the liquid state.

Since the chemical composition of oxygen (O2) remains unchanged, it's a physical change.