Question

Water can be broken down into smaller substances: two hydrogen atoms and one oxygen atom. Therefore, water is not an element. Who is responsible for this discovery?

A.
Bohr

B.
Boyle

C.
Goldstein

D.
Rutherford

Answer 1

Elements and Atoms: Chapter 6
Water is not an element: Lavoisier

Of the four elements of the ancients, water is the only one which is a pure chemical substance, albeit a compound and not an element. It should not be surprising, then, that it was the last of the four to be shown not to be elementary. After all, water is quite stable thermodynamically, and therefore rather difficult to decompose. And water is so common that its formation in many processes would be quite easy to overlook, especially if its formation was unanticipated or if other possible sources of moisture were present.
The discovery that water is a compound of hydrogen and oxygen (or, in the terminology of the day, of inflammable air and dephlogisticated air) was made in the early 1780s. Credit for the discovery has been given to or claimed on behalf of no less than four individuals: Henry Cavendish (View portrait at the Edgar Fahs Smith collection, University of Pennsylvania.) [Cavendish 1784], Antoine Lavoisier [la Place & Lavoisier 1781], Gaspard Monge (View portrait Centre International de Mathématiques Gaspard Monge.) [Monge 1783], and James Watt (View portrait at the Edgar Fahs Smith collection, University of Pennsylvania.) [Watt 1784][1]. The "water controversy" has engaged historians of chemistry and partisans of the protagonists since the early 19th century. Sidney Edelstein makes a persuasive case for Watt's priority [Edelstein 1948]; however, despite the title of his article "Priestley Settles the Water Controversy," the controversy apparently has not been settled, nor will I attempt to do so.

Even though it is apparent that Lavoisier was not the first to realize the compound nature of water, and was indeed aware of the work of his English contemporaries, I have chosen a selection from his work for the present chapter. The selection, a report of a paper (not even his first on the subject) read to the French Academy late in 1783, has several advantages over those of Watt and Cavendish: it is focused on the nature of water; it addresses experiments beyond the burning of hydrogen to produce water; and it is not couched in the terminology of the phlogiston theory (except in calling oxygen "dephlogisticated air").
https://web.lemoyne.edu/giunta/EA/LAVEAUann.HTML