What Is a Gas?

DeYoung, Robert                   Joliet Twp. H. S. - West
                                  401 North Larkin Ave.
                                  Joliet, IL 60435
                                  1-815-727-6905

Objectives
                                                                   
Students will:  1) answer the question:  What is a gas? 
                2) develop an understanding about how gases behave at the molecular 
                   level. 
                3) develop a mental and physical model of gas behavior at the 
                   molecular level. 
              
Equipment and Materials

Various sizes and shapes of balloons
Dry ice
Gas less dense than air
Gas denser than air
1 florence flask
Poster paper  
Rope (4 meters/group)                        
Marble sets (4 marbles/student plus 1 shooter/group)
String (50 cm length/group)
Metric Rulers
Scissors
Markers
Molecular motion demonstrator with accessory kit (Purchased from scientific supply 
 companies) 

Recommended Strategies

Demonstration. Fill several balloons with a gas less dense than air.  Fill one 
balloon with a gas denser than air.  Place dry ice in a florence flask and cap the 
top of the flask with a balloon end. 

Observe and discuss demonstration.  End the discussion by limiting the lesson to 
developing a scientific model of gas behavior at the molecular level. 

Review the concept of a scientific model. Review the concept of kinetic theory and 
the assumptions of kinetic theory. (The marbles will be used in this activity to 
represent gas particles.) 

Divide the class into groups of four students.  Each group should receive the 
following:  
            poster paper
            one group set of marbles (4 marbles/student plus 1 shooter/group) 
            string
            one marker
            rules for marble activity (See below.) with observation section 

After the students have used the marbles to develop the model of a gas, write all of 
their observations on the chalk board.  (Observations may include the following:  
straight line movement, some marbles move faster than others, collisions, marbles 
have mass, velocity, kinetic energy transferred.) 
          
Discuss how this model of a gas may be improved.  (When using the marbles  only the 
shooter and marbles that were hit moved.  To improve this model all of the marbles 
should be in constant motion.)  

Use the molecular motion demonstrator on the overhead projector to show all marbles 
in constant motion.  Add observations from the overhead projector to the previous 
observations.  (Observations may include the following:  random movement, collisions 
appear elastic, particles occupy shape and volume of container, particles in constant 
motion, particles far apart, no apparent attraction between particles.)  
                                
Homework:  Use the model of a gas developed in this activity to explain what changes, 
if any, will occur at the molecular level when    
   1) the temperature of a gas is  a) increased; b) decreased. 
   2) the pressure of a gas is  a) increased; b) decreased. 

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Rules for marble activity

1.  Place poster paper on the floor.
2.  Use string and a marker to draw a circle with radius approximately 50 cm on the 
    paper. 
3.  Place 4 meters of rope around the circle at some distance from the circumference 
    to prevent marbles from rolling around the room.  The rope may be repositioned to 
    facilitate shooting. 
4.  Each student selects 4 marbles from the group set and places them anywhere within 
    the circumference of the circle. 
5.  For the first round each student will get one turn in succession. 
6.  At each student's turn the shooter is positioned outside the circumference of the 
    circle.  The shooter will be used to try to knock marbles out of the circle. 
7.  Marbles knocked out of the circle are collected by the student who is shooting. 
8.  Starting with the second round students who knock marbles out of the circle may 
    continue for another turn. 
9.  After each student's turn that student will give one observation which is to be 
    recorded by all students in their group observation section. 
10. If no marbles are knocked out of the circle the turn passes to the next student. 
11. When no marbles remain in the circle the student with the most marbles wins. 

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Group Observations: