Basics and Discovery of Atmospheric Chemicals - Air Pollution and Global Warming: History, Science, and Solutions - page 14

separately known for making the first systematic study
of a chemical reaction and developing the concepts of
latent heat and specific heat.

1.2.2.7. Molecular Hydrogen Again -
Inflammable Air
In 1766, Henry Cavendish (1731-1810), an English
chemist and physicist, followed up Black's work by
producing a gas he called “ inflammable air ”(highly
flammable gas). This gas was obtained by diluting
either sulfuric acid [H 2 SO 4 (aq)] or hydrochloric acid
[HCl(aq)] with water and pouring the resulting solu-
tion on a metal, such as iron, zinc, or tin. This exper-
iment was similar to that of Paracelsus, who also
observed an inflammable vapor. Cavendish believed
that “inflammable air” was phlogiston, but this turned
out to be incorrect. Nevertheless, Cavendish isolated the
properties of the gas. In 1783, he found that exploding
amixture of the gas with air produced water. Subse-
quently, Lavoisier called the gas hydrogen ,the“water
producer.” More specifically, the gas was molecular
hydrogen [H 2 (g)].
In other experiments, Cavendish exposed mar-
ble, which contains CaCO 3 (s), to hydrochloric acid
[HCl(aq)] to produce CO 2 (g), as Van Helmont had done
previously. Cavendish took the further step of measur-
ing the properties of CO 2 (g). He is also known for study-
ing the weights of gases and the density of the Earth.
In 1783, after oxygen had been discovered, Cavendish
calculated that air contained 20.83 percent oxygen by
volume, close to the more accurate measurement today
of 20.95 percent.

Figure 1.15. Joseph Black (1728-1799). Edgar Fahs
Smith Collection, University of Pennsylvania Library.

He called the former acidum volatile and the latter
acidum fixum .Healsonoted that sulfuric acid is the
stronger acid.

1.2.2.6. Carbon Dioxide Again - Fixed Air
In 1756, Joseph Black (1728-1799; Figure 1.15), a
Scottish physician and chemist, performed an exper-
iment in which he heated magnesium carbonate
[MgCO 3 (s)], an odorless white powder called magne-
sia alba (“white magnesia”) at the time. On heating,
MgCO 3 (s) lost weight, producing a heavy gas that nei-
ther sustained a flame nor supported life. When the gas
wasexposed to quicklime [CaO(s), calcium oxide], a
white-gray crystal, the weight was reabsorbed. Black
called the gas “ fixed air ” because of its ability to attach
or “fix” to compounds exposed to it. The fixed air turned
out to be carbon dioxide [CO 2 (g)], and when it was
reabsorbed on exposure to CaO(s), it was really forming
calcium carbonate [CaCO 3 (s)]. Fixed air was renamed
to carbon dioxide in 1781 by Lavoisier. What Black did
not recognize was that fixed air, or CO 2 (g), had been dis-
covered by Van Helmont more than a century before.
In 1756, Black also isolated ammonia gas [NH 3 (g)],
previously observed by Van Helmont and later called
“ alkaline acid air ”byJoseph Priestley. Black is

1.2.2.8. Molecular Nitrogen (Gas) - Mephitic Air
In 1772, Daniel Rutherford (1749-1819; Figure 1.16),
astudent of Joseph Black, performed an experiment
by which he allowed an animal to breathe the air in
an enclosed space until the animal died, removing the
molecular oxygen, O 2 (g), which had not yet been dis-
covered. He then exposed the remaining air to the crys-
tal caustic potash [KOH(s), potassium hydroxide or
pot ashes], obtained by burning wood in a large iron
pot. CO 2 (g) in the remaining air reacted with caus-
tic potash, forming pearl ash or potash [K 2 CO 3 (s),
potassium carbonate]. The residue after CO 2 (g) was
removed could not sustain life; thus, Rutherford called
it “ mephitic (noxious or poisonous) air .” Mephitic air is
now known as molecular nitrogen gas [N 2 (g)], which
makes up nearly 80 percent of air by volume. Nitrogen
is tasteless, colorless, and odorless. The name nitrogen ,
the “nitre maker” was given in 1790 by Jean-Antoine