Question

with respect to composition, how is the compound made from hydrogen and oxygen different from mixtures of these two elements?

Answer 1

Each substance in the mixture keeps its own properties and can be easily separated from the mixture. In a mixture of hydrogen and oxygen, molecules of both the gases will exist independently.
The compound on the other hand has properties different from the elements it contains. In water as a compound of hydrogen and oxygen what you will have is molecules of H20.