Malverne Union Free Public Schools

Summer 2001

The Gas State: Pressure

Performance objectives:

1.       Define pressure

2.       Describe the uses and operation of mercury barometer

3.       List the 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, mercury barometer

Motivation :

·        An 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.

1.     Water 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.

2.     Advance preparation: Fill the beaker or other container about three-fourths full with the crushed ice and water.

Procedure :1.  Place  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 explanation.

Result. 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.

Strategies:

·        After the “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)

Area

2.     Discuss 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.

·         Show students a large Styrofoam block and a small steel ball . (This equipment  is part of a Scientific  Attitude Kit and can be ordered from                           )

1.     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.

·         Ask, “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.

 Steel Ball Styrofoam Block area 1 cm2 100 cm2 mass(weight) 5 g 10g pressure 5 g/1cm2 = 5 g/cm2 10 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).

·         Ask: 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.

·         Fill 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.

·        Bring a 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  barometer.)

1.     Barometers 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.

2.     Two 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).

Challenge the class to explain  the illustration below.

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.

Summary:

1.     Explain how barometer is used

2.     Convert 2,000 Pa to Kpa

3.     What are the units for pressure

Adapted from  Prentice Hall Chemistry The Study of Matter and Prentice  Hall  Connections to our Changing World

4.     How are units of pressure related to each other

5.     Why 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 might explode.

6.     But 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.

7.     Why would you rather have a person wearing shoes step on your hand than a person wearing ice skates. Explain in terms of pressure.

Portfolio Assignments:

·        You may 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 aids.

·         Divers 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.

·        Marine 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.

·        Library 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

Learning Standards:

The following New York State Learning Standards are addressed:

Standard 1 Scientific  Inquiry

Standard 4 Understanding Concepts

Standard 7 Strategies and Connections

The following National “New” Standards are addressed:

Standard 1 Physical Science

1b Properties of matter

1d Motions and forces

Standard  4  Scientific Connections

4a cause effect, models

4d impact of technology

Standard 5 Scientific Thinking

5a cause and effect

5c use evidence

5d proposes explanations

5e proposes solutions

5f works individually

Standard 6 Scientific Tools

6a Use technology and tools

6d acquires information

Standard 7 Scientific Communication

7a represents results

7b argues from evidence

7d explain a scientific concept