TIP
Graphic representation of how a catalyst lowers the required acti-vation energyPROPERTIES OF OXYGEN. Oxygen is a gas under ordinary conditions of temperature
and pressure, and it is a gas that is colorless, odorless, tasteless, and slightly
heavier than air; all these physical properties are characteristic of this element.
Oxygen is only slightly soluble in water, thus making it possible to collect the gas
over water, as shown in Figure 16.
Although oxygen will support combustion, it will not burn. This is one of its
chemical properties. The usual test for oxygen is to lower a glowing splint into
the gas and see if the oxidation increases in its rate to reignite the splint. (Note:
This is not the only gas that does this. N 2 O reacts the same.)
TIP
The ozone layer protects us from UV rays from the sun.OZONE. Ozone is another form of oxygen and contains three atoms in its molecular
structure (O 3 ). Since ordinary oxygen and ozone differ in energy content and form,
they have slightly different properties. They are called allotropic forms of oxygen.
Ozone occurs in small quantities in the upper layers of Earth’s atmosphere, and
can be formed in the lower atmosphere, where high-voltage electricity in lightning
passes through the air. This formation of ozone also occurs around machinery
using high voltage. The reaction can be shown by this equation:
3O 2 + elec. → 2O 3Because of its higher energy content, ozone is more reactive chemically than
oxygen.
The ozone layer prevents harmful wavelengths of ultraviolet (UV) light from
passing through Earth’s atmosphere. UV rays have been linked to biological
consequences such as skin cancer.
Hydrogen
PREPARATION OF HYDROGEN. Although there is evidence of the preparation of
hydrogen before 1766, Henry Cavendish was the first person to recognize this gas
as a separate substance. He observed that, whenever it burned, it produced water.
Lavoisier named it hydrogen, which means “water former.”
Electrolysis of water, which is the process of passing an electric current
through water to cause it to decompose, is one method of obtaining hydrogen. This