Maria "Mary-anne" Garcia Return to Chemistry Menu
Malverne Union Free Public Schools
Gas State: Pressure
uses and operation of mercury barometer
different units for pressure
Materials: empty soft-drink can; large beaker or
other container filled with crushed ice and water; beaker tongs; hot plate or
burner, scientific attitude kit, drinking glass, colored water, index card,
exciting way to begin this lesson is to perform demonstration , “Orange
Crush.” This demonstration illustrates the tremendous force that can be
exerted by our atmosphere. Ask the students why we are not particularly aware of
this atmospheric pressure although it is continuously pressing down on our
bodies to the extent of about 1 kilogram per square centimeter.
is placed into an empty soft-drink can that is then heated until the water
evaporates. Students observe that when the can is turned upside down and placed
into a container of crushed and water the can collapses.
preparation: Fill the beaker or other container about three-fourths full with
the crushed ice and water.
few milliliters of water into an empty soft-drink can. Using
a hot plate or burner, heat
the can until some water vapor escapes from it. 2. Remove the can with beaker
tongs and immediately plunge it, upside down, into the ice water. Continue to
hold the can under water until the result is evident. Ask students for an
The can will collapse because the vaporized water molecules slow down and
condense, causing the pressure to decrease inside the can and allowing normal
atmospheric pressure outside the can to crush it.
“Orange Crush” demonstration, brainstorm
students what do they think is the definition of
pressure. Explain and write
on the board the definition of
pressure as the force exerted on one unit area.
1. Pressure = Force(weight)
that the greater the force, the greater the pressure. The greater the
area, the lower the pressure. Reinforce the concepts directly
proportional and inversely proportional.
students a large Styrofoam block and a small steel ball . (This equipment
is part of a Scientific Attitude
Kit and can be ordered from
Have several students hold each in a hand and predict which is heavier.
They will choose the steel ball. Have
a student weigh them on a
double-pan balance. The Styrofoam block is heavier.
“Why did the steel ball feel
heavier ?” Brainstorm with students. Elicit:
The steel ball had a much smaller area. The pressure of the steel ball was
greater. Your senses mistook pressure for weight.
Illustrate the concept as shown below.
g/1cm2 = 5 g/cm2
g/100 cm2 = 0.1 g/cm2
The steel ball is lighter , but exerts 50 times more pressure than the
block. (Note: g/cm2 is not a unit of pressure we use. Units of pressure will be
discussed later in the lesson).
What are the different instruments used to measure pressure? Explain.
1. There are two types instrument
used to measure pressure. The first type is manometer , use to measure gas
pressure confined in a container. The second type is barometer, used for
accurate measurement of air pressure.
a tall glass with colored water. Place an index card over the mouth of the glass
and turn it upside down, on its side. The water does not spill out. Ask how can
they account for their observation. Elicit:
The air pressure against the card from outside is greater than the pressure of
the water from inside. In fact, the pressure of the atmospheric pressure can
support a column of water more than 30 feet tall. Use the demonstration to lead
into discussion of the mercury barometer.
small barometer to class and set it up before students come in. Demonstrate how
the barometer is used and read. Relate the reading on the barometer to the
content of atmospheric pressure. (Caution: Do not handle mercury. Use a mercury
barometer that is self-contained and will not spill. Be careful no to tip
are commonly used to measure atmospheric pressure. The SI unit of pressure is
the Pascal (Pa). Atmospheric pressure at sea level is about 101.3 kilopascal (Kpa).
Explain that 1,000 Pa= 1 Kpa.
older units of pressure are millimeters of mercury (mm Hg)and the atmosphere (atm).
One millimeter of Hg (mm Hg) is the pressure needed to support a column of Hg
,1mm high. This unit was developed from the early use of Hg barometers. One
standard atmosphere (1atm) is the pressure required to support 760 mm of Hg on a
mercury barometer at 25 oC. This is the average atmospheric pressure
at sea level. Thus 1 at equals 760 mmHg. Standard conditions when working with
gases are at temperature of O oC and a pressure of 1 atm. This is
standard temperature and pressure (STP).
the class to explain the
Normal atmospheric pressure pushing on a simple
mercury barometer supports a column of Hg about 760 mm Hg high. On top of Mt.
Everest ( at 9000-m altitude) the air exerts enough push to support a column of
Hg only 253 mm High.
students the following questions
how barometer is used
2,000 Pa to Kpa
are the units for pressure
Adapted from Prentice
Hall Chemistry The Study of Matter and Prentice
Hall Connections to our
are units of pressure related to each other
are spray cans labeled with bold warning sign that tells consumers not to
dispose of the empty cans in an incinerator? The answer is that the spray cans
why should an empty spray can explode ? Even an empty spray can contains some
amount of gas, which is at pressure equal to atmospheric pressure. This is
because the contents can be released only a soon as the pressure inside the can
is greater than the atmospheric pressure outside. Because the can is sealed,
this small amount of gas remains inside the can. When the can is thrown into an
incinerator, the high temperature causes the pressure inside the can to
increase. A temperature could eventually be reached when the can would rupture,
causing it to explode.
would you rather have a person wearing shoes step on your hand than a person
wearing ice skates. Explain in terms of pressure.
find a pressure cooker in the kitchen of many houses. But how do commercial
food-prepares use similar devices to pre-cook such canned foods as soups and
vegetables ? Have students prepare a written and oral report supported by visual
need to understand the gas laws. Have students research the effects
of temperature and pressure on divers. They have to summarize their
findings in the form of an article for a sports magazine. This project is
interdisciplinary with Biology.
mammals such as whales and dolphins face the same problems as deep-sea divers.
Have students research to determine what
built-in mechanisms help such animals to overcome these problems. This is once
again interdisciplinary with Biology.
research how are modern submarines
equipped to withstand high pressures underwater and to provide sufficient
oxygen for the crew members
Adapted from Prentice Hall Chemistry The Study of Matter and Prentice Hall Connections to our Changing World
following New York State Learning Standards are addressed:
1 Scientific Inquiry
4 Understanding Concepts
7 Strategies and Connections
following National “New” Standards are addressed:
1 Physical Science
1b Properties of matter
1d Motions and forces
4 Scientific Connections
4a cause effect, models
4d impact of technology
5 Scientific Thinking
5a cause and effect
5c use evidence
5d proposes explanations
5e proposes solutions
5f works individually
6 Scientific Tools
6a Use technology and tools
6d acquires information
7 Scientific Communication
7a represents results
7b argues from evidence
7d explain a scientific concept