What Is a Gas?
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DeYoung, Robert Joliet Twp. H. S. - West
401 North Larkin Ave.
Joliet, IL 60435
Students will: 1) answer the question: What is a gas?
2) develop an understanding about how gases behave at the molecular
3) develop a mental and physical model of gas behavior at the
Equipment and Materials
Various sizes and shapes of balloons
Gas less dense than air
Gas denser than air
1 florence flask
Rope (4 meters/group)
Marble sets (4 marbles/student plus 1 shooter/group)
String (50 cm length/group)
Molecular motion demonstrator with accessory kit (Purchased from scientific supply
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
one group set of marbles (4 marbles/student plus 1 shooter/group)
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.
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
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
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.