7.5 Taking Compounds Apart
As you know, elements cannot be broken down into simpler
substances. Compounds, however, can be broken down. That is
what happened when electricity was passed through water in the
activity in Section 7.4.
Decomposition of Compounds
Decomposition is a chemical change in which compounds are
broken down into simpler substances. Many explosives work
because of decomposition. For example, nitroglycerin is the
explosive chemical inside sticks of dynamite. When dynamite is
detonated, each nitroglycerin molecule quickly decomposes into
many smaller gas molecules like nitrogen (N2) and carbon
dioxide, (CO2). The energy released in the explosion makes these
gases molecules move very quickly. These rapidly expanding
gases help give dynamite its destructive power.
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- decomposition: the breaking down of a compound into simpler
substances
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In general, compounds do not break down by themselves. They
need some help. Energy must be added for decomposition to take
place. Energy can be added in two ways:
- using electricity
- adding thermal energy to the compound
Electrolysis
During electrolysis, electricity is passed through a dissolved
compound, causing it to break down into simpler substances.
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- electrolysis: the process during which electricity is used to
break down a compound into simpler substances
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Electrolysis of Water
Water is a very stable compound. On its own, water will not break
down into its two elements. When electricity is passed through
water, some of the water molecules break down. The new
products formed are hydrogen gas (H2) and oxygen gas (O2). The
formula of water (H2O) tells us that a water molecule has twice as
many hydrogen atoms as oxygen atoms. Therefore, you might
predict that twice as much hydrogen as oxygen is produced. Look
closely at the electrolysis apparatus shown in Figure 1. Notice
that the amount of gas collected in each test tube shows that this
is true.
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Figure 1 Some water molecules decompose into hydrogen and
oxygen when electricity passes through water.
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Figure not reproduced.
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END FIGURE CAPTION.
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Refining Aluminum
Most metals in the periodic table are not found as elements in
nature. Instead, they exist only in compounds. This means that
useful metals such as aluminum and copper must first be
separated from their compounds before they can be used. This
can be done using electrolysis. For example, aluminum is the
light-weight metal used to make foil and auto parts. Aluminum is
never found in nature as an element, but minerals similar to the
compound aluminum oxide (A1203) are common. Pure aluminum
is made by passing electricity through liquid aluminum oxide.
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Did You Know?
Aluminum--More Precious than Gold
200 years ago, aluminum was rare. This made aluminum objects
such as cutlery more expensive than those made of gold. Only
kings were served with aluminum knives and forks at important
dinner parties. Less important nobles were served with gold
cutlery. Then, after scientists learned how to produce aluminum
using electrolysis, its price dropped to only a few cents per gram.
END SIDEBAR.
Heating and Decomposition
Sometimes, heating causes compounds to break down.
Lighting a Match
Decomposition occurs when you light a match. The match head
contains a mixture of chemicals including potassium chlorate
(KClO3). Rubbing the match head across the rough strip on the
matchbox releases thermal energy. This causes potassium
chlorate to break up into two simpler substances--oxygen gas
(O2) and potassium chloride (KCl). Oxygen, with the thermal
energy, causes other chemicals in the match to catch fire.
Baking Muffins
A baker uses decomposition when using baking soda to bake
muffins. The thermal energy of the oven causes sodium hydrogen
carbonate (NaHCO3), or baking soda, to decompose into three
simpler substances (Figure 2). Bubbles of carbon dioxide
produced in this process cause the muffin batter to rise, as shown
in Figure 3.
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Representation of three arrows not reproduced.
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Career Link
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Representation of three arrows not reproduced.
END PRODUCER’S NOTE.
Bakers use chemistry to make better cakes and pastries. To learn
more about being a baker,
GO TO NELSON SCIENCE
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BEGIN FIGURE CAPTION:
Figure 2 The thermal energy of an oven decomposes baking
soda into water (H2O), carbon dioxide (CO2), and sodium
carbonate (Na2CO3).
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Figure not reproduced.
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END FIGURE CAPTION.
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Figure 3 (a) NaHCO3 added to batter breaks down during
heating.
(b) Baking produces carbon dioxide (CO2) bubbles, making baked
goods rise.
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Figure not reproduced.
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Decomposition of Hydrogen Peroxide
Some compounds break down naturally at room temperature.
One such compound is hydrogen peroxide (H2O2) shown in
Figure 4. This compound does not break down into elements. It
decomposes into oxygen gas (O2) and water (H2O).
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Figure 4 A molecular model of hydrogen peroxide (H2O2)
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Figure not reproduced.
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Many factors, including light, make hydrogen peroxide break
down faster. That is why hydrogen peroxide is always packaged
in a dark bottle (Figure 5). Certain substances help speed up the
breakdown of hydrogen peroxide. These substances are called
catalysts. A catalyst speeds up a chemical change without being
used up or chemically changed itself.
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- catalyst: a substance that speeds up a chemical change
without being consumed or chemically changed itself
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BEGIN FIGURE CAPTION:
Figure 5 Hydrogen peroxide is kept in dark bottles because light
can cause the molecules to decompose.
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Figure not reproduced.
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END FIGURE CAPTION.
Have you ever used hydrogen peroxide to treat a cut? Your blood
contains catalysts, including one that speeds up the breakdown of
hydrogen peroxide, called catalase. This is why you see bubbles
form when hydrogen peroxide is placed on a cut. These bubbles
are oxygen gas.
Uses of Hydrogen Peroxide
Hydrogen peroxide is a toxic chemical that kills certain bacteria.
This makes it useful for disinfecting cuts and contact lenses.
In addition, the bubbling action of hydrogen peroxide helps to
physically remove dirt from a wound or a contact lens. Bleaches
such as hydrogen peroxide react with compounds that give things
their colour. The result is a new compound that has no colour.
This process can make stained teeth white (Figure 6) and dark
hair lighter in colour.
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Figure 6 A brighter smile is the result of a chemical process
between hydrogen peroxide and stains on your teeth.
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Figure not reproduced.
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7.5 Wrap Up
- Compounds can break down, or decompose, into simpler
substances.
- Energy is needed to decompose a compound. Common sources
of energy are heating and electricity.
- Some substances can speed up chemical changes without being
used up. These substances are known as catalysts.
CHECK YOUR LEARNING
1. (a) Compare and contrast the chemical formulas of water and
hydrogen peroxide.
(b) How do the products of their decomposition compare? K/U
2. (a) What property of hydrogen peroxide makes it suitable for
use in changing hair colour?
(b) Is this a chemical or physical property? How do you know?
K/U
3. Find two examples of household substances that are stored in
opaque plastic bottles. For each substance, describe its purpose
and usefulness. What are the hazards associated with its handling
and disposal? A
4. Consumer products such as hydrogen peroxide have a limited
shelf-life. T/I, A
(a) What do you think the term "shelf-life" means? (b) Why do
school labs often store hydrogen peroxide in dark bottles and in
the refrigerator?
5. Carbonated soft drinks contain carbonic acid (H2CO3). When
this compound decomposes, it releases carbon dioxide gas and
liquid water. What would you see in a soft drink as this process
happens? How could you make this process occur faster? K/U, C,
A