MS-PS1 Matter and Its Interactions
Key Ideas and Common Student Misconceptions
This document compiles “Key Ideas” and “Common Misconceptions” developed by AAAS Project 2061 and
the National Science Foundation. For additional information on this topic and others, check out:
http://assessment.aaas.org/topics
For similar instructional documents check out:
www.ScienceBrewer.org
Atoms, Molecules, and States of Matter
All matter is made up of atoms.
All atoms are extremely small.
All atoms and molecules are in constant motion.
There are differences in the spacing, motion, and interaction of atoms and molecules that make up solids,
liquids, and gases.
For any single state of matter, increasing the temperature typically increases the distance between atoms or
molecules. Therefore, most substances expand when heated.
When heated, solids can change into liquids and liquids can change into gases. When cooled, gases can change
into liquids and liquids can change into solids. These changes of state can be explained in terms of changes in
the proximity, motion, and interaction of atoms and molecules.
For any single state of matter, the average speed of the atoms or molecules increases as the temperature of a
substance increases and decreases as the temperature of a substance decreases.
Substances, Chemical Reactions, and Conservation of Matter
A pure substance has characteristic properties, such as density, a boiling point, and solubility, all of which are
independent of the amount of the substance and can be used to identify it.
Many substances react chemically in predictable ways with other substances to form new substances with
different characteristic properties.
When substances interact to form new substances, the atoms that make up the molecules of the original
substances rearrange into new molecules.
Whenever substances within a closed system interact with one another, the total mass of the system remains
the same.
Whenever atoms interact with each other, regardless of how they are arranged or rearranged, the total mass
stays the same.
1.
2.
3.
4.
5.
6.
7.






Key Idea
All matter is made up of atoms.
Students are expected to know that:
Boundaries:
Matter is anything that has mass and takes
1. It is not expected that students will know
up space.
about the internal structure of atoms or the
Matter includes all gases, liquids, and solids,
existence of subatomic particles and,
which make up all living and non-living
therefore, that these subatomic particles
have mass.
things.
Light and heat are not matter.
All matter—solids, liquids, and gases—is
made up of discrete particles (atoms), rather
than being continuous, and that these
atoms are the matter rather than contained
in matter. In other words, the atoms are not
floating or embedded in some other
substance, such as air or a liquid.
Matter can exist even when it cannot be
seen. For example, gases or vapors are
matter even though some of them cannot be
seen.
Anything made up of atoms is matter.
It is because atoms take up space and have
mass that all matter takes up space and has
mass.
Common Misconceptions
Cells are not made up of atoms (Herrmann-Abell & DeBoer, 2008).
Air does not take up space (Driver et al., 1994).
Atoms or molecules are embedded in matter (Renstrom et al., 1990; Griffiths et al., 1992; Lee et al.,
1993; Johnson, 1998c).
Solids are not made up of atoms; especially those without visible granularity (Johnson, 1998c;
Nakhleh et al., 1999; Nakhleh et al., 2005; Nakhleh et al., 2006).
Matter exists only when there is perceptual evidence of its existence (Stavy, 1990).
Gases are not made up of atoms (AAAS Project 2061, n.d.).
Key Idea
All atoms are extremely small.
Students are expected to know that:
Boundaries:
1. Individual atoms are much smaller than
1. The comparison with very small objects can
things that can be seen and even much
be used to test students’ understanding of
smaller than very small things, such as dust,
the relative size of atoms in relation to these
germs and other microorganisms, blood cells,
objects. Students will not, however, be
expected to know the actual size of atoms.
and plant cells.
2. This is true for all atoms.
3. All atoms are so small that billions of them
make up these small things.





Common Misconceptions
A germ is smaller than an atom (AAAS Project 2061, n.d.).
Cells are smaller than atoms (Tretter et al., 2006).
A grain of sand is smaller than an atom (AAAS Project 2061, n.d.).
Atoms/molecules are similar in size to cells, dust, or germs/bacteria (Lee et al., 1993; Nakhleh et al.,
1999).
The width of a hair is smaller than an atom (Tretter et al., 2006).
1.
2.
3.
4.
5.
6.







Key Idea
All atoms and molecules are in constant motion.
Students are expected to know that:
Boundaries:
Atoms and molecules of all matter
1. Students are not expected to know the terms
are always moving.
rotation, translation, and vibration.
This is true for atoms or molecules of solids,
2. They are also not expected to know that
liquids, and gases.
atoms and molecules of different substances
Even when objects that are made up of these
move at different speeds at the same
atoms and molecules appear not to be
temperature.
moving, the atoms and molecules that make
3. Students are not expected to know the
up those objects are nonetheless themselves
special case of absolute zero temperature
in constant motion.
where, according to kinetic theory, there
should be no motion.
The motion of atoms or molecules can
include moving back and forth with respect
to a fixed point, around a fixed point, and/or
past each other from one fixed point to
another.
The motion (speed and direction) of an atom
or molecule can change when it undergoes
collision with another atom or molecule
resulting in one speeding up and the other
slowing down.
Because atoms and molecules are continually
colliding with each other, the
atoms/molecules of the substance do not
have the same speed.
Common Misconceptions
Atoms or molecules of a solid are not moving (Lee et al., 1993; Novak & Musonda, 1991).
Molecules within a phase move at the same speed (Griffiths et al., 1992).
The atoms or molecules of a liquid will stop moving when the liquid becomes a solid (Novak &
Musonda, 1991).
The atoms or molecules of a solid move only when heated. For example, the molecules of a glass
window move only when the window is warmed by the sun (AAAS Project 2061, n.d.).
Atoms or molecules of a liquid are not moving when the liquid itself is still (Johnson, 1998c).
Atoms or molecules of a gas are not in motion (Novick et al., 1981).
The atoms and molecules of a solid object move only when the object itself moves or is moved
(AAAS Pilot testing, 2006).
Key Idea
There are differences in the spacing, motion, and interaction of atoms and molecules that make up solids,
liquids, and gases.
Students are expected to know that:
Boundaries:
1. The particles of a gas (atoms or molecules)
1. Students are not expected to know the
are much farther apart than the atoms or
nature of the bonds between atoms or
molecules of a liquid or a solid. Because
between molecules.
atoms or molecules of a gas are so far apart
2. They are not expected to know that atoms or
they rarely come in contact with each other.
molecules of a solid can sometimes move
In solids and liquids, the atoms or molecules
past each other.
are packed closely together.
3. Students are not expected to know the terms
2. Although the motion of atoms or molecules
rotation, translation, and vibration.
of a solid is severely restricted, the atoms or
4. They are not expected to know that plasma is
molecules are constantly moving back and
a distinct state of matter, and they are not
expected to know the properties of a plasma.
forth in all directions with respect to a fixed
position. In liquids, the motion of the atoms
or molecules is limited but the atoms or
molecules can still move rapidly back and
forth with respect to a fixed point, around a
fixed point, and past each other from one
fixed point to another. Atoms or molecules of
a gas move freely and spread out throughout
the container they occupy.
3. Similar to the pushes and pulls between
magnets, atoms or molecules also push and
pull on each other.
a. In solids, the atoms or molecules pull
strongly on each other and are linked
together in rigid structures.
b. In liquids, the atoms or molecules pull
less strongly on each other, are more
loosely connected, and form less rigid
structures.
c. In gases, the pull between atoms or
molecules is so weak that they do not
form structures.
4. Hardness, flow, and compressibility are
macroscopic properties for which students
are expected to know the molecular reason.
a. Solids are hard because the atoms or
molecules are linked together tightly
in rigid structures, not because the
individual atoms or molecules are
hard.
b. The atoms or molecules of liquids can
easily move past each other from one
fixed point to another but do not
move apart from one another and
that this is why liquids flow and take
the shape of their container but solids
do not.
c. Gases can be compressed because
the atoms or molecules are not as
close together as they could be, not
because the individual atoms or
molecules are soft.







Common Misconceptions
Atoms or molecules of a solid are not moving (Lee et al., 1993; Novak & Musonda, 1991).
Particles of a gas are closely packed with no empty space between them (Benson et al., 1993; Novick
& Nussbaum, 1978).
Atoms and molecules of a gas in a container are not uniformly distributed (Novick et al., 1981; Lee et
al., 1993).
The gas state of a substance weighs less than the liquid or solid state (Stavy, 1990).
Observable properties of the state are attributed to the individual molecules (e.g., molecules in a
solid are hard; molecules move in gases and liquids, but not in solids; or the molecules of the
substance change from soft to hard when a liquid freezes) (Lee et al., 1993).
The molecules of the gas state are the lightest and the molecules of the solid state are the heaviest
(Griffiths et al., 1992).
Atoms or molecules of a gas are not in motion (Novick et al., 1981).
Key Idea
For any single state of matter, the average speed of the atoms or molecules increases as the temperature of
a substance increases and decreases as the temperature of a substance decreases.
Students are expected to know that:
Boundaries:
1. The temperature of a substance is directly
1. Students are not expected to know that
related to the average speed of its
temperature remains constant during a
atoms/molecules.
change of state, which is a later idea.
2. Therefore, as matter in any particular state is
2. They are not expected to know the term
heated, stirred, shaken, etc., the average
kinetic energy or that kinetic energy is a
speed of its atoms/molecules increases,
function of mass and velocity.
which is reflected in an increase in its
3. Students are also not expected to know that
temperature.
atoms and molecules of different substances
3. As matter in any particular state cools, the
move at different speeds at the same
average speed of its atoms/molecules
temperature.
decreases, which is reflected in a decrease in
4. They are not expected to know that
its temperature.
increasing pressure will increase the
4. When the temperature of matter in any
temperature or any other application of the
particular state remains constant, the
gas law equations.
average speed of its atoms/molecules
5. Students are also not expected to know the
remains constant, and when the average
special case of absolute zero temperature
speed of the atoms/molecules of matter in
where, according to kinetic theory, there
should be no motion.
any particular state remains constant, the
temperature of the matter remains constant.





Common Misconceptions
Increasing the speed of the atoms or molecules of a substance does not change the temperature of
the substance (AAAS Project 2061, n.d.).
The average speed of the atoms or molecules of a substance decreases when the temperature
increases and increases when the temperature decreases (AAAS Project 2061, n.d.).
Atoms or molecules of a solid are not moving (Lee et al., 1993; Novak & Musonda, 1991).
Atoms or molecules of a liquid in a sealed container will stop moving (AAAS Project 2061, n.d.).
The average speed of the atoms or molecules of a substance remains the same with a change in
temperature (AAAS Project 2061, n.d.).
Key Idea
For any single state of matter, increasing the temperature typically increases the distance between atoms or
molecules. Therefore, most substances expand when heated.
Students are expected to know that:
Boundaries:
1. As the temperature of a substance increases,
1. Students are not expected to know the
the average distance between the
details of the relationship between the speed
atoms/molecules of the substance typically
of the atoms or molecules and thermal
increases, causing the substance to expand.
expansion.
2. As the temperature of a substance decreases,
2. They are also not expected to know the
the average distance between the
substances that violate this rule and shrink
atoms/molecules typically decreases, causing
when heated or that water will shrink when
the substance to contract.
heated anywhere between 0°C and 4°C.
3. This expansion or contraction can happen to
3. Students are not expected to know or apply
solids, liquids, and gases.
gas law equations.
4. Expansion or contraction due to changes in
4. Because the definition of the size of an atom
temperature can also happen to mixtures of
is varied and complex, we only expect
substances.
students to know that the size of an atom or
5. Expansion or contraction due to changes in
molecule does not decrease when the
temperature is not permanent (e.g., objects
temperature increases and that the size does
not increase when temperature decreases.
that expand when heated then contract
when cooled).
6. The number of atoms and the mass of the
atoms do not change with changes in
temperature.
7. Different substances expand and contract
differently.











Common Misconceptions
Solid substances do not expand or contract with changes in temperature (Herrmann-Abell & DeBoer,
2007, 2008).
The mass of the atoms or molecules of a substance increases when the temperature increases and
decreases when the temperature decreases (AAAS Project 2061, n.d.).
The average distance between the atoms or molecules of a substance remains the same when the
temperature of the substance changes (AAAS Project 2061, n.d.).
Heat is made of "heat molecules" (Berkheimer et al., 1988).
The number of atoms or molecules of a substance increases when the temperature increases and
decreases when the temperature decreases (Herrmann-Abell & DeBoer, 2008).
Water molecules break down when heated (Griffiths et al., 1992).
The mass of the atoms or molecules of a substance increases when the temperature decreases and
decreases when the temperature increases (Herrmann-Abell & DeBoer, 2007, 2008).
The molecules of air break down when the air is cooled (AAAS Project 2061, n.d.).
Substances shrink when heated (especially solids) (Lee et al., 1993).
The number of atoms or molecules of a substance increases when the temperature decreases and
decreases when the temperature increases (Herrmann-Abell & DeBoer, 2007).
The size of the atoms or molecules of a substance increases when the temperature decreases and
decreases when the temperature increases (AAAS Project 2061, n.d.).
Key Idea
When heated, solids can change into liquids and liquids can change into gases. When cooled, gases can
change into liquids and liquids can change into solids. These changes of state can be explained in terms of
changes in the proximity, motion, and interaction of atoms and molecules.
Students are expected to know that:
Boundaries:
1. When the temperature of a liquid decreases,
1. Although changes of state can be caused by
the average speed of the atoms or molecules
changes in either temperature or pressure,
decreases and, as a result, the pull that exists
students are only expected to know the
effects of changes in temperature.
between the atoms or molecules is strong
enough to link them together as a solid.
2. When the temperature of a solid increases,
the average speed of the atoms or molecules
increases and the pull between the atoms or
molecules is no longer strong enough to hold
them together as a solid; the atoms or
molecules are now more loosely connected
as a liquid.
3. When the temperature of a gas decreases,
the average speed of the atoms or molecules
decreases and, as a result, the pull that exists
between the atoms or molecules is strong
enough to loosely connect them together as
a liquid.
4. When the temperature of a liquid increases,
the average speed of the atoms or molecules
increases and the pull between the atoms or
molecules is no longer strong enough to hold
them together as a liquid. In this case, the
pull between atoms or molecules is so weak
that they are no longer connected to each
other, but rather they exist as a gas.
5. At the boiling point and freezing point, atoms
or molecules from anywhere in the substance
can enter the gas state and solid state,
respectively. Because of this, the bubbles
that form when the substance is boiling are
atoms or molecules of that substance in the
gaseous state.
6. Evaporation or condensation can also occur
independent of temperature; i.e., at any
temperature there are some atoms or
molecules that may move from one state to
another at the surface of a substance. This
also includes atoms or molecules on the
surface of a solid that can enter the gas state.
7. A substance is made up of the same type of
atom or molecule regardless of whether it is
in the solid, liquid, or gas state. There
is no change in the identity of the atoms or
molecules during a change of state; only the
arrangement, motion, and interaction of the
atoms or molecules change.
8. Atoms or molecules are not destroyed during
a change of state.
9. Any change of state is reversible.













Common Misconceptions
The molecules of a substance break down into individual atoms when the substance evaporates.
During evaporation, water breaks down into hydrogen and oxygen (Bar et al., 1991; Bar et al., 1994).
The molecules of a substance break down into individual atoms when the substance boils. For
example, molecules of water become atoms of hydrogen and oxygen when water boils (Osborne et
al., 1983; Renstrom et al., 1990).
When water boils, the bubbles formed during boiling contain air, not water in the gas state (Osborne
et al., 1983; Renstrom et al., 1990; Bar et al., 1991; Johnson, 1998a; Chang, 1999).
Heat is made of "heat molecules" (Berkheimer et al., 1988).
Molecules change shape during a phase change (Novak et al., 1991; Griffiths et al., 1992).
Observable properties of the state are attributed to the individual molecules (e.g., molecules in a
solid are hard; molecules move in gases and liquids, but not in solids; or the molecules of the
substance change from soft to hard when a liquid freezes) (Lee et al., 1993).
The identity of the molecules of a substance changes during a phase change (Lee et al., 1993).
Molecules change weight/mass during a phase change (Griffiths et al., 1992).
Matter is destroyed during melting (AAAS Project 2061, n.d.).
Molecules change size during a phase change (Novak et al., 1991; Griffiths et al., 1992).
Matter is destroyed during boiling (AAAS Project 2061, n.d.).
Matter is destroyed during evaporation (Bar & Galili, 1994; Bar & Travis, 1991; Lee et al., 1993;
Osborne & Cosgrove, 1983)
When water evaporates from an object, that water is absorbed into the object (Osborne et al., 1983;
Bar et al., 1991; Bar et al., 1994).
Key Idea
A pure substance has characteristic properties, such as density, a boiling point, and solubility, all of which are
independent of the amount of the substance and can be used to identify it.
Students are expected to know that:
Boundaries:
1. Note: The term “characteristic property” is
1. Students are not expected to know that the
used to emphasize that these properties are
properties of substance can be different at
defining attributes that are independent of
the nanoscale.
the amount of the sample, regardless of
2. They are not expected to know the formula
time, location, size, or shape.
for density (density = mass/volume).
2. The term “substance” means a pure material
3. Students are not expected to know that the
that is made of the same matter throughout.
atomic mass of a substance is a characteristic
This is in contrast to the common definition
property.
that equates substance with matter that
4. They are not expected to know whether
could be made of either a single substance or
specific materials are or are not pure
a mixture of more than one substance. To
substances.
make this explicit, the phrase “pure
5. Because some properties do change with
substance” is used in assessment items.
changing conditions (e.g., changing
3. A substance can be a solid, a liquid, or a gas.
atmospheric pressure affects boiling point)
4. Every substance has a set of characteristic
all assessment items will make comparisons
properties that are always the same for that
between substances where it is clear that the
substance, regardless of time, location,
conditions, such as temperature and
pressure, are constant.
shape, or size.
5. Furthermore, characteristic properties are
consistent throughout a sample of a
substance.
6. Characteristic properties with which students
should be familiar are boiling point, melting
and freezing point, solubility (i.e. how much
of the substance can dissolve in water),
flammability (i.e. the ease with which a
substance will catch on fire), odor, color, and
density (i.e., that equal volumes of different
substances have different masses).
7. Weight, mass, volume, shape, length/width,
texture, and temperature are not
characteristic properties of substances and
may change.
8. The characteristic properties of a substance
do not change when temperature and
pressure remain the same.
9. No two substances can have the same set of
characteristic properties under the same
conditions and that if two materials have
even one different characteristic property,
they are different substances.















Common Misconceptions
If two substances share one characteristic property, they are the same substance (AAAS Project
2061, n.d.).
The melting point of a substance is dependent on the amount of substance. For example, the
melting point of a ball of wax will change if a piece of wax is removed from the ball (AAAS Project
2061, n.d.).
Melting point is not a characteristic property of a pure substance.
Volume is a characteristic property of a substance (DeBoer et al., 2009).
Color is not a characteristic property of a pure substance (AAAS Project 2061, n.d.).
Boiling point is not a characteristic property of a pure substance.
Density is not a characteristic property of a pure substance.
If most of the listed characteristic properties are the same, the substances are the same (AAAS
Project 2061, n.d.).
Temperature is a characteristic property of the substance (Thomaz et al., 1995).
Mass/weight is a characteristic property of a substance (DeBoer et al., 2009).
Freezing point is not a characteristic property of a pure substance.
Flammability is not a characteristic property of a pure substance.
Shape is a characteristic property of a substance (AAAS Project 2061, n.d.).
Length is a characteristic property of a substance (AAAS Project 2061, n.d.).
Width is a characteristic property of a substance (AAAS Project 2061, n.d.).
Key Idea
Many substances react chemically in predictable ways with other substances to form new substances with
different characteristic properties.
Students are expected to know that:
Boundaries:
1. When substances react chemically one or
7. Students are not expected to know that
more new substances are formed.
chemical reactions involve the
2. If a new substance does not appear, a
rearrangement of atoms into new molecules.
chemical reaction did not occur.
This idea is addressed in a later idea (Idea D).
3. The products of a chemical reaction can be
8. Students are also not expected to know that
identified as new substances because each
nuclear reactions are not chemical reactions
product has different characteristic
nor why nuclear reactions are not chemical
properties from the original substances
reactions. Nuclear reactions are addressed in
under the same conditions.
later ideas (4E/H6* and 4G/H6*)
4. Liquids, solids, or gases can be reactants or
9. By “predictable ways,” we mean that the
products in chemical reactions.
same products will be formed when the same
5. It is possible for a single substance to
reactants are combined regardless of
undergo a chemical reaction, such as when
location and experimental set-up. Students
the substance is heated or an electrical
are not expected to predict what the
current flows through the substance.
products of a reaction will be.
6. It is not true that all chemical reactions are
10. Students are expected to know that the
irreversible.
original substances in a chemical reaction are
called reactants and the resulting substances
are called products but they will not be
assessed on these definitions.












Common Misconceptions
Chemical reactions involve two reactants (Cavallo et al., 2003; Eilks et al., 2007).
A chemical change is irreversible (Cavallo et al., 2003; Calik et al., 2005).
A chemical reaction occurs when a substance dissolves (Novak et al., 1991; BouJaoude, 1992;
Abraham et al., 1994; Ahtee et al., 1998; Stavridou et al., 1998; Valanides, 2000; Eilks et al., 2007)
A chemical reaction occurs during a change of state (Hall, 1973; Novak et al., 1991; BouJaoude, 1992;
Ahtee et al., 1998; Stavridou et al., 1998).
A chemical reaction always happens when two substances are combined together (AAAS Project
2061, n.d.).
Chemical reactions involve only the production of gas (Cavallo et al., 2003).
Chemical reactions involve liquids only (Cavallo et al., 2003).
A chemical reaction always happens when two liquids are combined together (DeBoer et al., 2008).
Chemical reactions occur between solids and liquids but not between solids and gases (AAAS Project
2061, n.d.).
All chemical reactions are inherently dangerous (Cavallo et al., 2003).
A solid substance is always formed during a chemical reaction (DeBoer et al., 2008).
A chemical reaction must take place in a laboratory (Herrmann-Abell et al., 2009).
Key Idea
When substances interact to form new substances, the atoms that make up the molecules of the original
substances rearrange into new molecules.
Students are expected to know that:
Boundaries:
1. During a chemical reaction the atoms that
1. The idea that the number of each kind of
are linked together in molecules (or arranged
atom stays the same will be assessed under
in other patterns such as arrays and crystals)
Idea H (conservation of matter).
rearrange to make new molecules.
2. Students are not expected to know the term
2. If the atoms are not rearranged, then a
“bond” or how chemical bonds are formed or
chemical reaction has not occurred.
broken during chemical reactions.
3. New substances are made of the same kinds
3. They are not expected to know that during
of atoms as the original substances (i.e., the
chemical reactions, atoms are not turned
into energy.
atoms themselves did not change during the
reaction).





Common Misconceptions
The atoms of the reactants of a chemical reaction are transformed into other atoms (Andersson,
1986).
Substances can change their characteristic properties but maintain their identity (Pfundt, 1982). The
products of a chemical reaction are the same substances as the reactants but with different
properties (Solomonidou et al., 2000).
The products of a chemical reaction, though unseen, must have somehow existed from the start in
another location, such as in the air or inside the reactants (Andersson, 1986; Solomonidou et al.,
2000).
After a chemical reaction, the product is a mixture in which the old substances persist, and is not a
new substance (Johnson, 2000b; Solomonidou & Stavridou, 2000).
The reactants and products of a chemical reaction are different and independent of each other.
There is no recognition of a change of one sample to the other (Johnson, 2000b).
Key Idea
Whenever substances within a closed system interact with one another, the total mass of the system
remains the same.
Students are expected to know that:
Boundaries:
1. When substances mix, undergo chemical
1. Students are not expected to know that mass
reactions, change state, or dissolve, or when
is not conserved in energy-mass conversions
objects are cut or broken into smaller pieces,
such as nuclear reactions or other subatomic
the total mass of all the matter will always
interactions.
remain the same.
2. Note: The words “weight,” “weigh,” and
2. Regardless of the form that the products of
“mass” are used appropriately in the
these processes may take (for example, when
assessment items. The students are not
a sugar cube dissolves in water or a chemical
expected to know the difference between
“weight” and “mass.”
reaction produces a gas), the mass will
always stay the same.
3. If it appears that the mass has changed, it is
because some material has not been
accounted for.
4. Because light is not matter, its presence or
absence does not affect the mass of the
matter.








Common Misconceptions
Mass is not conserved during processes in which gases take part (Mas et al., 1987; Berkheimer et al.,
1988; Hesse et al., 1992). If a gas is produced during a chemical reaction that takes place in a closed
system, the total mass decreases (Ozmen et al., 2003).
In a closed system, the total mass increases during a precipitation reaction (Barker et al., 1999;
Ozmen et al., 2003).
When a liquid in a closed container is heated, the mass of the liquid increases as the liquid expands
(AAAS Project 2061, n.d.).
During biological decomposition in a closed system, the total mass of the system decreases (Mitchell
et al., 1984).
In a closed system, mass decreases after a solid dissolves in a liquid (Stavy, 1990b; Ozmen et al.,
2003).
When a chemical reaction occurs, matter just disappears. For example, gasoline is used up in the car
and disappears (Andersson, 1986).
Mass increases after a solid dissolves in a liquid (Stavy, 1990b).
Matter can disappear with repeated division, dissolving, evaporation, or chemical change (Smith et
al., 2004).
Key Idea
Whenever atoms interact with each other, regardless of how they are arranged or rearranged, the total
mass stays the same.
Students are expected to know that:
Boundaries:
1. Atoms are not created or destroyed when
1. Students are not expected to know that mass
substances mix, undergo chemical reactions,
is not conserved in energy-mass conversions
change state, or dissolve, or when objects are
such as nuclear reactions or other subatomic
interactions.
cut or broken into smaller pieces.
2. The total number of each kind of atom
always remains the same regardless of what
happens to the matter (mixing, chemical
reactions, changes of state, dissolving, or
objects being cut or broken into smaller
pieces).
3. The mass of an atom does not change.
4. If the measured mass has changed, it is
because some atoms have not been
accounted for.










Common Misconceptions
New atoms are created during chemical reactions (AAAS Project 2061, n.d.).
Atoms can be destroyed during a chemical reaction (AAAS Project 2061, n.d.).
Mass is not conserved during processes in which gases take part (Mas et al., 1987; Berkheimer et al.,
1988; Hesse et al., 1992). If a gas is produced during a chemical reaction that takes place in a closed
system, the total mass decreases (Ozmen et al., 2003).
The mass of a closed system will increase if a new kind of molecule is formed in the system (AAAS
Project 2061, n.d.).
The number of different kinds of molecules, not the number of each kind of atom, is always
conserved (AAAS Project 2061, n.d.).
Atoms can become lighter during a chemical reaction (AAAS Project 2061, n.d.).
The total number of atoms, not the number of each kind of atom, is always conserved (AAAS Project
2061, n.d.).
The mass of a closed system will decrease if two substances combine to form one substance (AAAS
Project 2061, n.d.).
Atoms can become heavier during a chemical reaction (AAAS Project 2061, n.d.).
The number of molecules is always conserved (Mitchell et al., 1984).