Nature of Matter Unit Overview

 

The Nature of Matter Unit is intended to engage students in myriad experiences with hands-on and computer-based materials that will help them modify their existing ideas and construct new ideas about the nature of matter-what makes up the things we see around us.  The first cycle (Cycle A) is an introductory cycle for students with little background in the variables we need to continue this unit.  Cycle I begins looking at observable phenomena involving air.  Cycle II begins to introduce a particulate theory for gases. Cycle III expands the theory to include solids and liquids.

The Unit consists of four cycles:

 

Cycle A:     Background Definitions

Cycle I:       How do Gases Behave?

Cycle II:     What is the Nature of Gases?

Cycle III:    What is the Nature of Solids and Liquids?

 

It is strongly recommended that you teach the Static Electricity and Magnetism unit before this unit, so that your students have practice in developing models.  This is an abstract unit, and very model-based, so the more comfortable your students are with models and abstract concepts, the easier this unit will be.

 

This unit is also very qualitative.  It is aimed more at an elementary education physics course rather than high school physics.  It would be very possible to expand this unit to go into more of the quantitative issues, but as it stands, it is very qualitative.

 

Target Ideas for Unit

 

The Nature of Matter Unit was designed to provide the opportunities for students to construct ideas, which are closely aligned with the ones listed below.  At the end of each activity in the development phase, students are asked to add or modify an idea in their Idea Journal, based on evidence gathered within that activity.  We have found this semi-structured approach for development of a common set of ideas to work well with high school students, prospective and in service elementary teachers.  Naturally, as part of their consensus discussion for each cycle, students will probably develop these ideas in their own words.  However, the conceptual content of their own ideas should be similar to these.  The Teacher Guide for each cycle provides examples of the kinds of statements students actually develop in the class. After the class agrees on a set of ideas the teacher should introduce appropriate terminology and conventions so that the students' are more closely aligned with the corresponding ideas they would find in textbooks or when they talk with other students.

 

 

Target Ideas for Cycle A

 

  1. Mass definition: The mass of an object tells us something about how much “stuff” is in the object.  Mass is measured by finding the number of standard masses that balance the object.
    The standard unit used most often by scientists to measure the mass of small objects is the gram.  For larger objects, the standard unit of mass is the kilogram, which is 1000 grams.

 

  1. Volume definition: The volume of an object tells us how much space it occupies. Volume is measured by finding the number of standard unit cubes that fit inside the object.

    When volume is measured with one-centimeter cubes, the unit of volume is the cubic centimeter (cm3). If 1000 1-cm unit cubes are piled together, they form a larger unit of volume called the liter (l). Since 1 cm3 is one-thousandth of a liter, it is also called a milliliter (ml). A milliliter and a cubic centimeter are two names for the same unit of volume.

 

  1. Density definition: The density of a (homogeneous) object tells us the mass of each unit cube of volume of the object.  Density is measured by finding the mass and volume of an object, then dividing the mass measurement by the volume measurement.  The unit of density used most often by scientist is grams/cm3. For gases, however, it is often more convenient to use grams/liter. For example, the density of air is 0.0012 grams/cm3. Since 1 liter is 1000 cm3, the density of air can also be reported as 1.2 grams/liter.

    The density of an object is characteristic of the kind of substance the object is made of—different substances have different densities.  The densities (in grams/liter) of some common gases at room temperature and 1 ATM. pressure are:

Chlorine        2.99

Carbon Dioxide 1.84

Nitrogen            1.17

Helium            0.17

Sulfur Dioxide 2.73

Oxygen           1.34

Carbon monoxide 1.16

Hydrogen       0.084

Butane          2.40

Air                  1.21

Methane            0.52

 

 

  1. Area definition: The area of a figure is the number of standard unit squares that fit inside the figure. The surface area of an object is the number of standard unit squares that cover the object.

    When area is measured with one-centimeter squares, the unit of area is the square centimeter (cm2). Larger units of area include the acre and the square mile.

 

  1. Ratio definition: A ratio is a quantity of one thing per quantity of another thing.  A unit ratio is the quantity of one thing for each one unit of a different thing.

 

6.      Pressure definition: Pressure is the push (force) one object exerts on each unit of area of a second object. Pressure is measured by finding the surface area and total force on an object, then dividing the total force by the surface area.
Note: It is not appropriate to say “pressure pushes.” Concepts cannot push—only substances or objects can push on each other.

 

7.      Unequal forces Idea: When two forces are exerted on an object (initially at rest) in opposite directions, the object will move in the same direction as the larger force (towards the smaller force).

 

Target Ideas for Cycle I

 

  1. Push of air Idea: Gases can only push on objects—they can’t pull. The push of a gas on each unit square area (pressure) is the same in all directions.

 

  1. Particles of air Idea: A gas occupies all the space of its container, and has a very small mass. Air particles are spread out randomly and far apart, with empty space between them.  Air particles do not clump together—they stay independent of each other. Air particles do not change size or shape when the air is compressed or removed.

 

  1. Movement of air Idea: Air particles are moving around.

 

  1. Air pressure Idea: Air is a mixture of gases (about 79% nitrogen, 20% oxygen, and 1% of many other gases, including carbon dioxide, water vapor, and rare gases).  Air pressure is about 14.7 pounds per square inch at sea level and 20 degrees Celsius.  Air pushes in all directions.

 

  1. Container Idea:  Containers can be classified by the type of container (open or closed) and the type of container walls:

 

·        Closed container with fixed, rigid walls (e.g. soda bottle with lid)

·        Closed container with at least one rigid, moveable wall (e.g. closed syringe with piston, soda bottle with unstretched balloon over top)

·        Closed container with at least one stretch (elastic), moveable wall (e.g. bottle with stretched balloon on top)

·        Open container with fixed, rigid walls (e.g. open soda bottle)

·        Open container with at least one rigid, moveable wall (e.g. open syringe with piston)

 

  1. Pressure and density Idea: When the density of a gas changes (increases or decreases), the pressure of the gas changes (increases or decreases)—at least in infinitesimal increments.
    At a constant temperature, there are two ways to change the density of a gas:

    (1) add or remove gas from a container, or
    (2) compress or expand the gas.

    What happens in each case depends on the type of container and container walls.

 

a. Adding Air to or Removing Air from a Container

 

(I). Open Container

 

When air is slowly pumped into (out of) an open container, the gas escapes (enters) the opening of the container at the same rate, so,

·        the volume stays the same,

·        the density stays the same,

·        the pressure stays the same.

 

(II) Fixed, Rigid Walls

 

When air is slowly pumped into (out of) a container with fixed, rigid walls at constant temperature, the

 

·        volume stays constant,

·        air density increases (decreases), and

·        air pressure increases (decreases).

(III) Moveable, Rigid Walls

 

When air is slowly pumped into (out of) a container with a moveable wall at constant temperature, the

 

·        volume increases (decreases),

·        air density stays constant, and

·        air pressure stays constant.

 

In this case, a tiny density change inside the container causes a tiny change in the air pressure in the container.  But the air pressure outside the container stays the same. So there is an unbalanced force on the piston, causing the piston to move a tiny distance until the densities and pressures are the same again.  This process occurs in such continuous and infinitesimal steps that the density and pressure do not appear to change. 

 

b. Compressing or Expanding a Gas in a Closed Container

 

When a gas is compressed (expanded) slowly in a closed container with rigid walls (so the temperature remains constant), the

 

·        volume decreases (increases),

·        so the gas density increases (decreases), and

·        the gas pressure increases.

 

 

Target Ideas for Cycle II

 

In Cycle II, the students are given 4 basic Small Particle Theory “rules”, and they are asked to provide evidence for them and make additions to the rules.  The rules and the additions they should come up with are below.

 

  1. Size of Particles Idea: Particles of matter are very, very small.  Additions: (a) Particles can not be seen through our most powerful optical microscopes.  A particle of air (e.g. nitrogen) is only about 0.00000001 cm in diameter. (b) Different substances are made of different particles.  The particles differ in both mass and size.

 

  1. Arrangement of Particles Idea: Gas particles are far apart with empty space between them.  Additions: (a) When a gas is heated (cooled) in a closed container with rigid walls, the gas does not expand or contract, so the spacing of the particles is about the same. When the container is open or has moveable walls, the faster gas particles push on each other harder when they collide, so they move further apart (and closer together when the gas is cooled).  (b) The number of particles in a unit volume (density of particles) is the same for different gases.

 

  1. Motion of Particles Idea: Gas particles move constantly and randomly in all directions, occasionally colliding with each other and the walls.  Additions: (a) Gas particles always move faster when the gas is heated (and slower when the gas is cooled).  (b) At a fixed temperature, heavier particles move slower (on the average) but push harder during each collision than lighter particles.  These effects balance, so the average pressure of different gases is the same. (c) Different gases spread out and mix with each other. This process is called diffusion. The rate of diffusion is different for different gases. The higher the density of the gas, the slower the rate of diffusion.

 

  1. Interaction of Particles Idea: When gas particles collide with a wall and bounce off, they push on the wall.  Additions: (a) Pressure is caused by particle collisions:

 

 

 

 
Target Ideas for Cycle III

 

As in Cycle II, for Cycle III the students are given several statements. They use the statements throughout the development cycle and are asked to provide evidence for them and additions to the statements.  The statements and the additions are below.

 

  1. Arrangement of Particles Idea: The particles of a liquid are close together, in no particular pattern, with empty space between them.

 

  1. Attraction of Particles Idea: Particles of a liquid are moderately attracted to each other.

 

  1. Motion of Particles Idea: Particles of a liquid are constantly in motion, bumping into and sliding past one another. Addition: The particles of a liquid move at different speeds.

 

  1. Effects of Heating Idea: The particles of a liquid always move faster when heated (and slower when cooled).

 

  1. Arrangement of Particles Idea: The particles of a solid are close together, in a specific pattern, with empty space between them.

 

  1. Attraction of Particles Idea: Particles of a solid are strongly attracted to each other.

 

  1. Motion of Particles Idea: Particles of a solid are constantly in motion, vibrating in their place in the pattern.

 

  1. Effects of Heating Idea: The particles of a solid always move faster when heated (and slower when cooled).

 

  1. Phase Change Idea: As a solid is heated, the particles of the solid move faster and slightly further apart.  As it continues to be heated, the solid reaches a point where the temperature stops changing for a little bit.  At this point, the solid is still being heated, and the particles gain enough energy to break the bonds holding them in their pattern.  They break free of their structure and are able to move around—the solid has become a liquid.  As the liquid is heated, the particles continue to move faster and slightly further apart.  There is a point where the temperature stops rising for a little bit.  At this point, the heat is being transferred into the energy of the liquid particles, and they get enough energy to break the bonds holding them close together.  A few particles break free and form a gas—the bubbles in a boiling liquid. This continues until all of the liquid particles have gained enough energy to break their bonds and move freely.