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C03_032A
Cathode ( )
Anode (
+
)
Hydrogen
bubbles
Oxygen
bubbles
Oxygen gas
Hydrogen gas
Power supply
Get It?
Define
element
and
compound
.
Get It?
Explain
the process of electrolysis.
Compounds
Many pure substances can be classified as compounds. A
compound
is made up of two
or more different elements that are combined chemically in a fixed ratio. Most matter in
the universe exists in the form of compounds. Today, there are more than 50 million
known compounds, and new compounds continue to be developed and discovered at
the rate of about 100,000 per year. There appears to be no limit to the number of com-
pounds that can be made or that will be discovered. Considering this virtually limitless
potential, several organizations have assumed the task of collecting data and indexing
the known chemical compounds. The information is stored in databases.
Separating compounds into components
As you have read earlier in this module,
elements can never be separated into simpler substances. However, compounds can be
broken down into simpler substances by chemical means. In general, compounds that
occur naturally are more stable than the individual component elements. Separating a
compound into its elements often requires external energy, such as heat or electricity.
Figure 15
shows the setup used to produce the chemical change of water into its
component elements—hydrogen and oxygen—through a process called electrolysis.
During electrolysis, one end of a long platinum electrode is exposed to the water in a
tube and the other end is attached to a power source. An electric current splits water
into hydrogen gas in the compartment on the right and oxygen gas in the compartment
on the left. Because water is composed of two parts hydrogen and one part oxygen,
there is twice as much hydrogen gas as there is oxygen gas.
The chemical symbols of the periodic table make
it easy to write the formulas for chemical com-
pounds. For example, table salt, which is called
sodium chloride, is composed of one part sodium
(Na) and one part chlorine (Cl), and its chemical
formula is NaCl. Water is composed of two parts
hydrogen (H) and one part oxygen (O), and its
chemical formula is H
2
O. The subscript 2 indicates
that two hydrogen atoms combine with one
oxygen atom to form water.
Figure 15
An electric current breaks down water into its
components, oxygen and hydrogen.
Determine
What is the ratio between the amount of hydrogen
and the amount of oxygen released during electrolysis?
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Module 2 • Matter—Properties and Changes