Question

How will you prove that water is a compound?

Answer 1

Reason to prove water is a compound:
Water's melting point and boiling point varies according to external pressure, volume and temperature. Hence one of the physical parameters used to identity water varies. This implies that it is a compound and not an element as for the latter, all of it's properties remain the same under any condition.

Water is stable at normal conditions. This is unlike elements which tend to be reactive (unless they are noble gases, but they exist as gas under normal conditions while water is a liquid).