Here's why:

* Compound: A substance formed when two or more different elements are chemically bonded together in a fixed ratio.

* Mixture: A combination of two or more substances that are not chemically bonded and can be separated by physical means.

In the case of H2O (water):

* It's formed from two elements: hydrogen (H) and oxygen (O).

* These elements are chemically bonded in a fixed ratio of 2:1 (two hydrogen atoms for every one oxygen atom).

* The properties of water are distinct from those of hydrogen and oxygen alone.

Therefore, H2O meets the definition of a compound.