CHEMISTRY 30S – MODULE 2
GASES AND THE ATMOSPHERE
LESSON 1 Atmosphere
OUTCOMES: The student will be able to:
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Identify the abundances of the naturally occurring gases in the atmosphere and
examine how these abundances have changed over geologic time.
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Research Canadian and global initiatives to improve air quality.
Hot air balloons, SCUBA diving, and baking all have one thing in common – they
involve gases…. oh and so does breathing, and driving a car. Gases are the state
of matter that is most affected by changes in surrounding temperature and
pressures. We will learn about how gases behave under changing conditions and
in some cases how that affects our lives.
Present Composition of the Atmosphere
Earth’s air is composed of two types of gases: permanent and variable. The gases are called
permanent because their amounts have not significantly changed in recent history. The
permanent gases in the atmosphere by percentage are:
 Nitrogen 78.1%
 Oxygen 20.9%
These two gases comprise 99% of the Earth’s lower atmosphere.
Some of the other permanent gases are:
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Argon 0.9%
Neon 0.002%
Helium 0.0005%
Krypton 0.0001%
Hydrogen 0.00005%
As their name suggests, variable gases
 Water vapor 0 to 4%
 Carbon Dioxide 0.035%
 Methane 0.0002%
 Ozone 0.000004%
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The Origins of Earth’s Atmosphere
Scientists believe that before life began on the earth, the composition of the atmosphere
was dramatically different than it is today. Billions of years ago the atmosphere consisted
mainly of helium, hydrogen, ammonia and methane. It is believed that little free oxygen
existed.
Assuming that volcanic eruptions have the same composition as today, it is believed that
volcanoes released gases into the developing atmosphere. This volcanic outgassing
released nitrogen, carbon dioxide and water vapour. Volcanic eruptions contain about 85%
water vapour. This water vapour accumulated in the atmosphere and eventually returned to
the Earth in the form of rain. The rain collected and created lakes, rivers and oceans.
Nitrogen gas is not very chemically reactive so it continued to accumulate in the
atmosphere.
Scientists believe that ultraviolet radiation from the sun penetrated the relatively dense
atmosphere and sparked chemical reactions that eventually led to life on Earth. Origin-oflife models have generally proposed that about 1 billion years after the first primitive
organisms emerged, blue-green algae appeared on the Earth. These algae converted the
existing carbon dioxide and water to free oxygen gas and glucose through the process of
photosynthesis.
Another important source of oxygen was the photodecomposition of water vapour by
ultraviolet light according to the equation below:
2 H2O(g)  2 H2(g) + O2(g)
As the amount of free oxygen increased, an ozone layer began to form filtering out
ultraviolet radiation and allowing for the development of more complex species.
In previous science courses you may have learned about the nitrogen cycle. This cycle
maintains the present amount of nitrogen in the atmosphere. Nitrogen is removed from the
atmosphere by nitrogen-fixing bacteria and lightning. The nitrogen is returned to the
atmosphere through the decomposition of biological matter.
The Variable Gases
Water on the earth is present as solid, liquid and gas. Water plays several roles in the
atmosphere. It distributes heat, provides fresh water for plant and animal life as well as
functioning as a greenhouse gas. Greenhouse gases trap heat energy within the
atmosphere. Water vapour is present in largest amounts, about 4%, between the tropics and
is in its lowest amounts, as low as 0%, in deserts and at the poles.
In previous science courses you likely studied the carbon cycle and learned that carbon
dioxide is removed from the atmosphere by photosynthesis and returned to the atmosphere
by respiration, decay of biological material, volcanic activity and burning fossil fuels.
Some scientists believe that carbon dioxide is a greenhouse gas and increased amount of
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atmospheric CO2 is responsible for global warming. This is a topic of intense debate
among scientists, environmentalists and politicians. Between 1840 and the year 2000, the
average amount of atmospheric carbon dioxide steadily increased by 25%. The increase in
atmospheric CO2 is believed to be largely due to the increased burning of fossil fuels and
deforestation.
Methane is also considered to be a greenhouse gas. Its levels have increased dramatically
over the last 200 years due to the increased amounts of rice paddies, grazing animals and
landfills.
Lesson Summary
In this lesson we have learned

99% of the Earth’s atmosphere is nitrogen and oxygen.
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Gases present in variable amounts are water vapour, carbon doxide, methane and
ozone.
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The Earth’s early atmosphere was composed of largely helium, hydrogen, ammonia
and methane. Volcanoes and the advent of life changed the composition of the
atmosphere to where it is today.
LESSON 2 Gases & Pressure
If you listen to the weather reports, you will hear units such as kilopascals and millibars. In
this lesson you will learn how these units were developed and what they actually mean.
OUTCOMES:
When you have completed this lesson, you will be able to:

Describe the historical development of the measurement of pressure.

Describe the various units used to measure pressure.
Defining Gas Pressure
Recall from Module 1 that according to the kinetic molecular theory, gas particles are
constantly moving. As they move, they collide with the sides of their container. The force
per unit area due to these collisions is called gas pressure.
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History of Measuring Pressure
Galileo Galilei (1564-1642) developed the suction pump. He used air to draw underground
water up a column, similar to how a syringe draws water. He was perplexed as to why
there was a limit to what height the water could be raised. That limit was 32 feet or about
11 metres.
In 1643 Evangelista Torricelli (1608-1647) developed the first barometer. He carried on
Galileo’s work by determining the limit to the height with which Galileo’s pump could
draw water was due to atmospheric pressure. He inverted a closed-end tube filled with
mercury into a pan of mercury at sea level.
The height of the column of mercury in the tube (in mmHg) is
equal to the atmospheric pressure acting on the mercury in the
pan. He determined that the height of mercury supported by
atmospheric pressure at sea level is 760 mm or 76 cm. He did the
same experiment with water first, but found that the glass tube he
had to use was too long and fragile.
Between 1643 and 1645 Otto von Guericke (1602-1686) made a
pump that could create a vacuum so strong that a team of sixteen
horses could not pull two metal hemispheres apart. Otto von
Guericke reasoned that the hemispheres were held together by
the mechanical force of the atmospheric pressure rather than the
vacuum. The same experiment can be demonstrated by pushing
two plungers together.
In 1648, Blaise Pascal (1623-1662) used Torricelli’s
“barometer” and traveled up and down a mountain in southern
France. He discovered that the pressure of the atmosphere
increased as he moved down the mountain. Sometime later the SI unit of pressure, the
‘Pascal’, was named after him. He is also well known for his work in mathematics and
inventing the first calculator.
In 1661 Christiaan Huygens (1625-1695) developed the manometer to study the elastic
forces in gases. He also developed some of the first practical vacuum pumps. Huygens,
however, is better known for his work in mathematics and astronomy.
In 1801, a couple years before publishing his atomic theory, John Dalton (1766-1844)
stated that in a mixture of gases the total pressure is equal to the sum of the pressure of
each gas, if it were in a container alone. The pressure exerted by each gas is called its
partial pressure. This is known as Dalton’s Law of Partial Pressures. Another way of
stating this relationship is the total pressure of gases in a container is the sum of the
pressures of each gas.
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In 1808 Joseph Louis Gay-Lussac (1778-1850) observed the law of combining volumes.
He noticed that, for example, two volumes of hydrogen combined with one volume of
oxygen to form two volumes of water. He is also well known for his passion for hot air
ballooning.
Amadeo Avogardo (1776-1856), after studying the work of Gay-Lussac and others,
published what is known as Avogadro’s Hypothesis in 1811. His hypothesis stated that a
sample of any gas at the same temperature and pressure will contain the same number of
particles. His hypothesis also suggests that the more gas particles, the greater the pressure.
Unfortunately, because Avogadro did not do his own experiments, his hypothesis was
ignored for about 50 years.
UNITS OF PRESSURE
The atmosphere is a common unit for expressing pressure. Two atmospheres of
pressure is equal to twice the pressure of the atmosphere at sea level.
1 atm = 760 mmHg
1 atm = 101.3 kPa
The SI unit for pressure is the Pascal (Pa), named after Blaise Pascal. The Pascal is
defined as 1 Newton of force per square metre. This is rather small unit, so when
measuring gas pressure we use kiloPascals, which is 1000 Pascals. You can watch an
animation in the first module on WebCT to see how the Pascal and the Newton are related.
Millimetres of mercury (mmHg)is not a common unit used outside the laboratory today,
however, many barometers found in the home use both mm of mercury as well as another
unit like kiloPascals. In the United States, air pressure is often reported in terms of inches
of mercury.
The millibar is a meteorological unit of atmospheric pressure. One bar is equal to standard
atmospheric pressure or 1 atmosphere.
One other common pressure unit is pounds per square inch (psi). This is an Imperial unit
of pressure. This unit is often encountered when filling car and bicycle tires. One
kiloPascal is equal to 0.145 psi and one atmosphere is equal to 14.7 psi.
Measuring Pressure
Pressure can be measured using several devices or instruments. Air pressure or
atmospheric pressure is measured with a barometer, whereas gas pressure is measured
with a monometer. A manometer usually has a bulb or glass container on one end and
can be open or closed on the other. A liquid, often mercury, is placed in a U-shaped
tube. The pressure is measured by finding the difference in height on both sides of the
tube (see figure below).
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In the open-ended manometer, the pressure of the gas is related to the height
difference, h, of the mercury (or other liquid) in both sides of the U-tube. It is called
open-ended because one end is exposed to the gas pressure and the other end is open
to the atmosphere. When the pressure of the trapped gas (Pgas) is equal to the
atmospheric pressure (Patm) the height on both sides of the U-tube are equal (a in the
figure below). When the pressure of the trapped gas is greater than the atmospheric
pressure, the height of the liquid in the open side of the U-tube will be greater than the
closed side (b in the figure below). The opposite is true if the atmospheric pressure is
greater than the gas pressure (c in the figure below).
To calculate the pressure of the trapped gas in (b), when the gas pressure is greater
than the atmospheric pressure
Pgas = Patm + h
In (c) above, when atmospheric pressure is greater than gas pressure
Pgas = Patm – h
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Example 1. Calculate the gas pressure in kilopascals (kPa) in the manometer shown
below.
Solution.
The gas pressure is greater than the atmospheric pressure so we use the equation
Pgas = Patm + h
Pgas = 762 mm + 15 mm = 777 mm Hg
We must convert mmHg to the SI unit kiloPascals.
If 760 mmHg = 101.3 kPa, then
Lesson Summary
In this lesson you have learned,

Gas pressure is due to the force of gaseous particles colliding with their
container.

Evangelista Torricelli invented the barometer to measure atmospheric pressure.

Among the units used to describe gas pressure are: millimetres of mercury
(mmHg), atmospheres (atm), kilopascals (kPa), millibars and pounds per
square inch (psi).

Christiaan Huygens developed the manometer to measure the pressure of a
sample of a gas.
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When using a manometer, when gas pressure exceeds air pressure Pgas = Patm +
h or when airpressure exceeds the gas pressure Pgas = Patm – h.
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