* Compounds are formed when two or more different elements combine chemically.

Let's look at the options:

* Water (H₂O): A compound made of hydrogen and oxygen.

* Air: A mixture of gases, primarily nitrogen and oxygen. It's not a compound because the gases aren't chemically bonded.

* Calcium (Ca): An element, not a compound.

* Iodine (I₂): While iodine exists as a diatomic molecule (I₂), it's still considered an element because it's made of only one type of atom.

Therefore, the only compound in your list is water.