Lavoisier's Demonstrations of the Compound Nature of Water
Until the late 1700's, it was thought water was elemental. This had been consistent with Plato's
four elements: Fire, Air, Water and Earth. Then, in the late 1700's, Lavoisier demonstrated the
compound nature of water via two dramatic experiments.
In the first experiment, Lavoisier combined hydrogen and oxygen, and ignited the mixture. The
result was a liquid, which had all the properties of water, and which was of equal mass to the
combined gases.
In the second experiment, he decomposed water by strongly heating a mixture of water and iron
fillings. The decomposition of water into oxygen and hydrogen was followed by collecting the
hydrogen gas over mercury. The oxygen reacted with the iron forming a solid oxide or rust. The
increased mass of the fillings, along with the mass of the collected gas, equaled the mass of the
water introduced into the original mixture.
Expt. #1: "... prepared a sort of double-tubed lamp for inflammable air [Hydrogen], one tube carrying
inflammable air and the other dephlogisticated air [Oxygen]. The two orifices through which the airs
passed were severely restricted, to make the combustion very slow, and they were proportioned in such
a way as to supply the amounts of the respective airs needed for combustion. The glass bell into which
the double tube led was immersed in mercury, and had no communication with the exterior air. ... The
quantity of inflammable air burned in this experiment was about thirty pints and that of dephlogisitcated
air from fifteen to eighteen."
"As soon as the two airs had been lit, the wall of the vessel in which the combustion took place visibly
darkened and became covered by a large number of droplets of water. Little by little the drops grew in
volume. Many coalesced together and collected in the bottom of the apparatus, where they formed a
layer on the surface of the mercury."
"After the experiment, nearly all the water was collected by means of a funnel, and its weight was found
to be about 5 gros, which corresponded fairly closely to the weight of the two airs combined. This
water was as pure as distilled water."
Inflammable Air + DePhlogisticated Air
Water
Modern Interpretation:
Hydrogen + Oxygen
Water
Expt. #2: " ... he filled a crystal bowl with mercury, inverted it in a vessel filled with mercury, and introduced a
small portion of water and of iron fillings, very pure and not rusted. From the first day, the iron began
to lose a part of its metallic luster; it was calcined and converted in part to rust. At the same time it
released a quantity of inflammable air in proportion to the quantity of dephlogisitcated air which had
been absorbed by the iron, as judged by the increase in weight which the fillings had acquired after
being dried. Thus water, in this experiment, is decomposed into two distinct substances,
dephlogisticated air which unites with the iron and converts it to a calx, and inflammable air which
remains separate. On the other hand, when one reunites and recombines these same two substances, one
recomposes water. Thus one is led still more nearly inevitably to conclude that water is not a simple
substance at all, not properly called an element, as had always been thought."
Water
Inflamable Air + DePhlogisticated Air
DePhlogisticated Air + Iron
Calx of Iron
Modern Interpretation:
Water
Oxygen + Iron
Hydrogen + Oxygen
Iron Oxide
Apparatus for Decomposing Water and Producing Hydrogen
As an alternative, water can be dripped into an iron gun barrel which is placed in a strongly heated
furnace. The water is converted to steam, which then decomposes into oxygen and hydrogen. The
oxygen then reacts with the iron of the gun barrel.
Taken from Observations sur la Physique 23, 452-5 (1783); translation by Carmen Giunta.