Performance Benchmark P.12.A.1
Students know different molecular arrangements and motions account for the different
physical properties of solids, liquids and gases. E/S.
Some Background about Energy
According to the Kinetic Molecular Theory (KMT), matter is composed of tiny particles
(atoms, molecules, or ions) with definite and characteristic sizes that do not change. The
energy and organization of these particles determines the physical state and bulk physical
properties of a particular sample of matter.
For more information about the Kinetic Molecular Theory, see
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch4/kinetic4.html.
All particles are in constant, random motion. In other words, all particles possess kinetic
energy. The amount of kinetic energy is directly related to Kelvin temperature. With a
higher temperature, the particle possesses more kinetic energy. Kinetic energy is
transferred from one particle to another through collisions.
Particles also interact with each other through attractions and repulsions because they
have either permanent or temporary dipoles. Overall, molecules tend to orient themselves
to align the partial positive charge on one molecule with a partial negative charge on
another molecule (“opposites attract”). The physical separation of these charges results in
electrostatic potential energy. The strength of the attractions between the particles (the
intermolecular attractions) in a sample of matter is determined by the bonding, shape,
and polarity of the molecule.
For more information about intermolecular attractions, see
http://chemed.chem.purdue.edu/genchem/topicreview/bp/intermol/intermol.html.
The interaction between the particles’ kinetic and potential energies determines how the
particles interact, how the particles are arranged relative to each other, and the bulk
physical properties of the matter the particles compose.
States of Matter
There are different physical states of matter. Here, we focus on the three that are most
common in our everyday experiences: solids, liquids, and gases.
Figure 1. Microscopic representations of the three common states
of matter
(From http://www.ilpi.com/msds/ref/gifs/statesofmatter.gif)
Solids. This state of matter is characterized by a dominance of potential energy over
kinetic energy. In solids, there are strong intermolecular attractions that hold individual
particles together. These particles do not have sufficient kinetic energy to overcome the
strong intermolecular attractions. This does not mean that the particles are motionless. In
fact, the particles in a solid are located close to each other (due to the strong
intermolecular attractions) and vibrate about fixed sites.
Because their particles are held in fixed locations, solids have defined volumes and
shapes and do not flow. Because their particles are close together, solids have high
densities (a large amount of mass in a small volume) and small compressibilities
(particles cannot be pushed much closer together than they already are). Because the
particles are held together by strong intermolecular attractions, solids have small thermal
expansions (the bulk sample does not expand because the particles do not move away
from each other much when they are heated).
Liquids. This state of matter is characterized by potential and kinetic energies that are
comparable. The particles in a liquid are randomly packed and close to each other. The
intermolecular attractions between the particles are not as strong as they are in solids. The
particles have enough kinetic energy to temporarily break the intermolecular attractions
and slide over each other, but they do not have enough kinetic energy to completely
separate from each other.
Because their particles are close together and can slide over each other, liquids have a
definite volume but an indefinite shape (liquids take the shape of as much of their
containers as they fill). They are also capable of flow. In fact, the higher the temperature
of the liquid, the more kinetic energy the particles have and the more easily
intermolecular attractions are broken so the particles can slide over each other (lower
viscosity). The close packing of the particles gives liquids high densities (a little less than
that of solids) and small compressibilities. The intermolecular attractions between
particles in a liquid, though less strong than those in solids, keep particles close together,
even when they are heated (small thermal expansion).
Gases. This state of matter is characterized by a dominance of kinetic energy over
potential energy. The intermolecular attractions between particles in a gas are very small
(they are ignored in many simple gas law calculations), so the particles have enough
kinetic energy to move far apart from each other. The distance between particles is
usually much greater than the size of individual gas particles. The particles are in
constant, random motion, essentially independent of each other.
The particles in a gas are not limited to any specific shape or volume by intermolecular
attractions. Gases take the shapes and volumes of their containers. Because their particles
are so far apart from each other, gases exhibit low densities (relatively few particles in a
given volume, compared to a solid or a liquid), large compressibilities (because particles
are separated by a lot of empty space, they can be pushed closer together), flow (particles
can move past each other because of the very low intermolecular attractions), and
moderate thermal expansions (when gases are heated, the kinetic energy of their particles
increases and particles can move further away from each other, causing the bulk sample
to increase in volume).
Table 1. Summary of solid, liquid, and gas characteristics
Solid
Liquid
Gas
Arrangement and
Closeness of
Particles
Attraction between
Particles
Motion of Particles
Fixed; Close
Random; Close
Random; Far apart
Strong
Moderate
Little to none
Particles vibrate about
fixed sites
Particles slide over each
other
Volume
Definite volume
Definite volume
Shape
Definite shape
Density
High density
Flow
Compressibility
Thermal Expansion
Does not flow
Small compressibility
Small thermal expansion
Takes the shape of its
container (the part it
fills)
High density (usually
lower than that of the
corresponding solid)
Flows easily
Small compressibility
Small thermal expansion
Particles move
constantly and
randomly, independent
of each other
Takes the volume of its
container
Takes the shape of its
container
Low density
Flows easily
Large compressibility
Moderate thermal
expansion
Phase Changes/Transitions
Matter can change from one state to another as it is cooled or heated. The names of the
different phase changes and the heat loss and gain associated with them are summarized
in Figure 2.
Figure 2. Phase changes
(Modified from
http://www.ccrc.sr.unh.edu/~stm/AS/Common/Water_Phases.JPG)
Heating or cooling a substance changes the amount of energy its particles have; as heat is
added, the energy of the particles increases. When heat is added to a solid, the vibration
of the particles increases; but the particles remain locked in place (and the solid retains its
shape) until enough energy is added to break the intermolecular attractions that keep the
particles in place, at which point the solid melts.
In liquids, there are still intermolecular attractions between particles, but they are not as
strong as those in solids. They are more easily broken. As heat is added to a liquid, the
particles gain energy that allows the intermolecular attractions to be broken more
frequently and more easily. This allows the particles in the liquid to slide or flow over
each other more easily as the temperature increases. When the kinetic energy of the
particles exceeds the forces required to keep the molecules close together, the liquid
boils/evaporates. Any heat added at this point simply increases the kinetic energy of the
particles in the gas.
For a discussion of the difference of boiling and evaporation, see
http://www.launc.tased.edu.au/online/sciences/agsci/essoil/boiling.htm.
Heating and Cooling Curves
Phase transitions occur at specific temperatures when the energy of particles exceeds that
allowed for a given state. When heat is added to a substance, the solid to liquid transition
occurs at the melting point, when particles have enough energy to temporarily break
intermolecular attractions. The liquid to gas transition occurs at the boiling point, when
particles have enough energy to completely overcome intermolecular attractions.
When a substance is cooled and energy is removed from the system, the liquid to solid
transition occurs at the freezing point, which is the same temperature as the melting point.
The gas to liquid transition occurs at the same temperature as the boiling point, a
temperature sometimes referred to as the condensation point.
Heat added to a system can have one of two effects on a bulk sample: (1) it can increase
the temperature of the sample or (2) it can cause a phase change (provide the energy
needed to break intermolecular attractions). It cannot have both effects at the same time.
Temperature does NOT change during a phase transition. The results of heating a
substance are visualized in a heating curve (see Figure 3).
Figure 3. Generic heating curve
(From
http://library.thinkquest.org/C006669/media/Chem/img/Graphs
/HeatCool.gif)
The heating curve consists of a series of diagonal lines and plateaus. The diagonal lines
represent changes in temperature, and the plateaus represent phase changes. A sample of
matter begins as a solid when it is at a temperature lower than its melting point (segment
A). As heat is added, the temperature of the substance increases. Once the temperature
reaches the melting point, the solid/liquid phase transition occurs, during which there is
no change in temperature (segment B). When all of the sample has melted, additional
heat causes an increase in the temperature of the liquid (segment C) until the boiling
point is reached. At this point, all added energy is used to break intermolecular attractions
between particles (segment D). Once the attractions are broken, any heat energy added
will increase the temperature of the gas (segment E).
Note: Cooling curves (starting at higher temperatures and ending at lower temperatures)
can also be drawn. For an example of a cooling curve, see
http://www.wpbschoolhouse.btinternet.co.uk/page03/3_52states/Image86.gif).
For more information about states of matter and phase changes, see the Visionlearning
site about states of matter, which gives an excellent narrative description of the different
states of matter and of phase changes. It also has links to pictures of different states of
matter that can be “magnified” until representations of particles can be seen.
http://www.visionlearning.com/library/module_viewer.php?mid=120
Performance Benchmark P.12.A.1
Students know different molecular arrangements and motions account for the different
physical properties of solids, liquids and gases. E/S.
Common misconceptions associated with this benchmark:
1. Students incorrectly believe that gases have no mass
Many students believe that gases have no mass since they cannot usually be seen and
because they seem to “disappear.” Gases do have mass, although that mass is spread out
over a large area since the particles in a gas are separated from each other by large
distances (giving gases small densities).
This concept is discussed in a book about chemistry misconceptions at
http://www.chemsoc.org/pdf/LearnNet/rsc/miscon.pdf.
Several activities can be used to address this misconception, including one from the
American Chemical Society:
http://www.chemistry.org/portal/resources/ACS/ACSContent/education/wande/resourcec
hem/matter/Matter_Act_6.pdf.
2. Students incorrectly believe that the particles in a solid are motionless.
Many students believe that the particles in a solid do not move since the bulk sample does
not appear to move; however, all particles (in solids, liquids, and gases) move because
they have kinetic energy. Particles in solids vibrate about fixed sites. The vibrations are
so small we can’t see them.
For a detailed discussion of and activity about vibrational, rotational, and translational
motion, see http://academic.pgcc.edu/~ssinex/molecules_in_motion.htm. You will need
Chime (http://academic.pgcc.edu/psc/chime_guide.html) to look at the accompanying
pictures.
3. Students do not understand that there is empty space between particles in a gas.
Many students have difficulty accepting the idea that there can be “empty space.”
Therefore, since they know that particles in a gas are far from each other, they assume
that there must be something between the particles instead of nothing.
To read a paper in which researchers examined the bulk ideas about matter that makes it
hard for students to accept the idea of empty space, see
http://hi-ce.org/presentations/documents/MerrittetalNARST2007.pdf.
4. Students incorrectly believe that individual particles shrink when cooled or swell
when heated.
Students might assign bulk properties to individual particles. For example, they might
assume that particles expand when heated because the bulk sample expands when heated.
The students do not seem to understand that the expansion of a bulk sample is the result
of particles moving further apart from each other and contraction is the result of particles
moving closer together.
To see a unit about states of matter that was specifically designed to address this
misconception, please see
http://cps.bu.edu/education/vmdl/workshops/projects/2003/session2/phasechanges/word_
document.doc.
5. Students incorrectly believe that particles decompose during boiling.
Several studies have shown that students believe that the bubbles in boiling water contain
air, oxygen or hydrogen (or some combination of these) instead of steam. This indicates
that students incorrectly believe that intramolecular bonds (those within the molecule)
are broken during the liquid/gas phase change. However, intermolecular attractions are
much weaker than intramolecular bonds. During a phase transition, the intermolecular
attractions are broken, but the intramolecular bonds remain intact.
This misconception is addressed in question #6 of the Chemical Concepts Inventory and
can be found at
http://jchemed.chem.wisc.edu/JCEDLib/QBank/collection/CQandChP/CQs/ConceptsInv
entory/Concepts_Inventory.html.
To listen to an Orange County Department of Education podcast about this
misconception, go to
http://science.ocde.us/audio/BOILINGWATER.MP3.
Performance Benchmark P.12.A.1
Students know different molecular arrangements and motions account for the different
physical properties of solids, liquids and gases. E/S.
Sample Test Questions
1. A sample of carbon dioxide that undergoes a transformation from solid to liquid to gas
would undergo a change in
a. density
b. mass
c. composition
d. chemical properties
2. Which statement is NOT true?
a. To change from a solid to a liquid requires energy.
b. Water can exist in solid, liquid, or gas states.
c. Solids and liquids have similar densities.
d. Particles in a solid do not move.
3. As ice cools from 273 K to 263 K, the average kinetic energy of its molecules will
a. decrease
b. increase
c. remain the same
d. quickly increase, then slowly decrease to zero
4. Under the same conditions of temperature and pressure, a liquid differs from a gas
because the particles of the liquid
a. are in constant straight-line motion
b. take the shape of the container they occupy
c. have no regular arrangement
d. have stronger forces of attraction between them
5. Which of the following is true as a gas is changing to a liquid during condensation?
a. There is a decrease in particle order.
b. The particles gain freedom.
c. The temperature rises.
d. Energy is released.
6. The relative randomness of molecules in the states of matter is best indicated by:
a. solid > liquid > gas
b. liquid > gas > solid
c. liquid > solid > gas
d. gas > liquid > solid
7. The physical state of matter is related to the potential energy and kinetic energy
between molecules of the substance. The gaseous state is characterized by
a. high potential energy and high kinetic energy
b. high potential energy and low kinetic energy
c. low potential energy and high kinetic energy
d. low potential energy and low kinetic energy
8. Low density and high compressibility are characteristic of
a. the liquid state
b. the solid state
c. the gaseous state
d. both the liquid and solid states
9. Gases have a low density because
a. gas molecules have strong intermolecular attractions.
b. gas molecules are very far apart.
c. gas molecules take the shape of their container.
d. gas molecules have an indefinite volume.
Please Note: These last four questions come from the Particulate Nature of Matter
Assessment listed in the Intervention Strategies and Resources section.
10. A pot of water is placed on a hot stove. Small bubbles begin to appear at the bottom
of the pot. The bubbles rise to the surface of the water and seem to pop or disappear.
What is inside the bubbles?
a. oxygen or hydrogen
b. air
c. oxygen and hydrogen
d. steam
11. In a pure sample of oxygen gas, what exists between the oxygen molecules?
a. matter
b. air
c. nothing
d. atmosphere
12. Which of the following statements is correct?
a. Heat causes molecules to expand.
b. Molecular size depends on temperature
c. Gases have mass and volume.
d. Bonds within molecules are broken during melting.
13. Consider three samples of water in three phases. The first is solid water (ice) at 0 oC,
the second is liquid water at 24 oC, and the third is gaseous water at 100 oC. The water
molecules in the liquid phase _______ the water molecules in the solid phase.
a. move faster than
b. move slower than
c. move less randomly than
d. travel in the same direction as.
Performance Benchmark P.12.A.1
Students know different molecular arrangements and motions account for the different
physical properties of solids, liquids and gases. E/S.
Answers to Sample Test Questions
1. (a)
2. (d)
3. (a)
4. (d)
5. (d)
6. (d)
7. (c)
8. (c)
9. (b)
10. (d)
11. (c)
12. (c)
13. (a)
Performance Benchmark P.12.A.1
Students know different molecular arrangements and motions account for the different
physical properties of solids, liquids and gases. E/S.
Intervention Strategies and Resources
The following list of intervention strategies and resources will facilitate student
understanding of this benchmark.
1. Interactive Animation of Phase Changes
This interactive animation allows students to “melt” or “boil” a sample and then
look at the result of the phase transition at a microscopic level.
http://www.harcourtschool.com/activity/states_of_matter/
2. Phases Worksheet
This website provides a worksheet on which students can draw macroscopic and
microscopic views of solids, liquids, and gases.
http://www.colorado.edu/physics/EducationIssues/STOMP/IHAD/Worksheets/WO
RKSHEET%201--Solid%20Liquid%20Gas%20Drawing.pdf
3. Interactive Animation of Heating Curve
This interactive animation allows students to visualize what happens to particles as
heat is added to a substance. The animation is tied to a heating curve.
http://www.footprints-science.co.uk/states.htm
4. Assessment about Students’ Understanding of the Particulate Nature of Matter
This diagnostic test was designed to assess students’ understanding of the
particulate nature of matter.
http://jchemed.chem.wisc.edu/JCEDlib/QBank/collection/CCInventory/JCE2006p0
954QB/JCE2006p0954W.pdf
5. States of Matter Mini-Module
This mini-module about states of matter is well-grounded in the literature.
Molecular Workbench software allows students to visualize the particles in
different states of matter. Activities help students connect macroscopic properties to
microscopic arrangements of particles.
http://workbench.concord.org/web_content/states_of_matter/index.html (minimodule)
http://mw.concord.org/modeler/index.html (Molecular Workbench software)
6. States of Matter Unit
This module about states of matter also covers intermolecular attractions,
temperature and states of matter, and boiling and freezing points. The unit includes
assessments, a list of words for creation of a concept map, and grading rubrics.
Virtual Molecular Dynamics Laboratory (VMDL) software allows students to
visualize the particles in different states of matter.
http://cps.bu.edu/education/vmdl/workshops/projects/2004/session1/3._states_of_m
atter_and_phase_change/activity.doc (units)
http://cps.bu.edu/education/vmdl/software/vmdl.html) (Virtual Molecular Dynamics
Laboratory software)