Figure 15. Composition of Earth’s CrustThe composition of air varies slightly from place to place because air is a
mixture of gases. The composition by volume is approximately as follows:
nitrogen, 78%; oxygen, 21%; argon, 1%. There are also small amounts of carbon
dioxide, water vapor, and trace gases.
PREPARATION OF OXYGEN. In 1774, an English scientist named Joseph Priestley
discovered oxygen by heating mercuric oxide in an enclosed container with a
magnifying glass. That mercuric oxide decomposes into oxygen and mercury can
be expressed in an equation: 2HgO → 2Hg + O 2. After his discovery, Priestley
visited one of the greatest of all scientists, Antoine Lavoisier, in Paris. As early
as 1773 Lavoisier had carried on experiments concerning burning, and they had
caused him to doubt the phlogiston theory (that a substance called phlogiston was
released when a substance burned; the theory went through several modifications
before it was finally abandoned). By 1775, Lavoisier had demonstrated the true
nature of burning and called the resulting gas “oxygen.”
Today oxygen is usually prepared in the lab by heating an easily decomposed
oxygen compound such as potassium chlorate (KClO 3 ). The equation for this
reaction is:
2KClO 3 + MnO 2 → 2KCl + 3O 2 (g) + MnO 2A possible laboratory setup is shown in Figure 16.