Content Benchmark P.8.A.5

Content Benchmark P.8.A.5
Students know mass is conserved in physical and chemical changes. E/S
This benchmark contains several key ideas that can be difficult for middle school students.
Students must understand what physical and chemical changes are. They need to be able to
differentiate between them by providing credible examples. Students must understand that as
physical and chemical changes occur, matter is not created or lost and there is no change in mass
between materials before or after the changes.
Figure 1. An example of the conservation of mass is shown in this stream of water that is fatter near the mouth
of the faucet and skinnier lower down. This is because the water speeds up as it falls. If the cross-sectional area
of the stream was equal all along its length, then the rate of flow through a lower cross-section would be greater
than the rate of flow through a cross-section higher up. Because the flow is steady, the amount of water between
the two cross-sections stays constant. The cross-sectional area of the stream must therefore shrink in inverse
proportion to the increasing speed of the falling water.
Physical changes are those changes that materials undergo that do not alter their molecular
structures and do not change their identities. They may also include energy changes. Examples
include phase changes, such as ice to water and water to steam. When 20.0 grams of ice melts,
there is 20.0 grams of liquid water that results from the melting. This is an easy activity to give
to middle school students, which will help them understand the idea. Similarly, physical changes
can involve pounding a piece of metal into a sheet, tearing paper, or cutting a tree into logs. In
these examples, there has been no change in identity or mass. There has been a change in shape
and form. Mass is conserved and students can easily account for the pre and post changes.
To learn about conservation of mass in physical changes in solids and fluids, go to
Chemical changes are particularly problematic for middle school students. Chemical changes
involve changes in outer energy level electrons (ionic and covalent bonding). Explanations of
chemical reactions are often abstract and students need to witness a variety of examples to fully
understand that matter and mass are conserved. Simple reactions that include aqueous reactants
are helpful. Students can mass reactants before the reaction and compare them to the mass of the
products after a chemical change. It is not necessary at this level that students balance chemical
equations although the practice is helpful for teachers in furthering their understandings that
atoms are conserved during chemical equations. Choose simple reactions for students to balance
such as 2H2 + O2  2H20. The use of manipulatives to represent the hydrogen and oxygen
atoms is helpful to convey the concept that quantities of atoms remain the same after a chemical
change has occurred.
For an easy to understand tutorial about balancing chemical equations, go to
Figure 2. Reaction between hydrogen and oxygen to produce water
As shown in Figure 2, students can easily develop models of hydrogen and oxygen atoms and
analogously witness that in a chemical change the atoms rearrange themselves to produce water.
Students do not need to worry about the type of bonding as indicated in the graphic. Atoms
touching each other are considered to be bonded.
It is important to point out to students that philosophers and scientists have long considered the
concept of matter and its various properties. Democritus (460-370 BC) coined the term “atomos”
when he attempted to describe the smallest form of matter. John Dalton furthered this idea with
the development of the first atomic theory in the 18th century. Both recognized that the atom is
the smallest unit of matter that is indivisible. This is the concept which leads to the development
of the Law of Conservation of Mass in a chemical reaction. The mass of the reactants is always
equal to the mass of the products in a normal chemical reaction.
An excellent discussion of the development of atomic theory and it relationship to the
conservation of mass is found at
To reinforce the idea that gases have mass, students should be given the opportunity to
experiment with reactions in which gases are produced. If students place an alka-seltzer tablet in
water, they will observe a gas being produced. Weighing the water and the alka-seltzer tablet
before and after the reaction, will allow students to see that the masses change because the gas is
lost to the atmosphere. Repeating the reaction in a Ziploc bag will demonstrate that matter was
conserved as the masses are conserved and should naturally lead to a conversation about gases
having mass.
Figure 3. Schematic of a simple experiment demonstrating the conservation of mass
For a more detailed explanation of chemical and physical changes including a short video, go to The purpose of the project is for
students to learn about the properties of matter, including phase changes in matter, physical, and
chemical properties of matter, and physical and chemical change. The students demonstrate their
understanding by creating a movie that can be shared with others.
Content Benchmark P.8.A.5
Students know mass is conserved in physical and chemical changes. E/S
Common misconceptions associated with this benchmark
1. Students incorrectly think that gases are not matter because most gases are invisible.
Similarly, students believe that gases do not have mass.
Since their experiences with gases are limited, students are often confused by the characteristics
and the properties of gases. One of the major misconceptions is that gases have no mass.
Teachers can assist students in confronting this misconception by allowing them to perform
some simple experiments in which they can mass a particular volume of gas. One good example
is the sublimation of dry ice. Providing students with a small amount of dry ice that they can
mass and then place in a Ziploc bag to observe its sublimation will allow them to see a solid
actually sublime. They can observe the phase change, witness that no chemical change has
occurred and then re-mass the substance to see that its mass has remained constant will help
avoid this common misconception.
Annenberg’s “A Private Universe” will assist the
teacher in understanding the difficulties that middle school students face when trying to
understand properties of gases.
2. Students are confused about the conservation of particles during a chemical change.
Also, Students frequently disregard particle conservation and orderliness when describing
physical changes.
Students have limited experience with chemical changes and often believe that substances
mysteriously appear and disappear in chemical reactions. Some of these misconceptions are
media induced. Having students develop models of chemical reactions before a chemical change
and after a chemical change will assist them in developing more sophisticated ideas regarding the
conservation of matter.
One example of a model that students can build is exemplified in the table below.
2 * 2.02g + 32.00g
2 * 18.02g
Figure 4. A model of a chemical reaction forming water.
To get a listing of this misconception and other physical science misconceptions, go to
3. Students incorrectly think that chemical and nuclear reactions are the same.
Students may inquire about nuclear reactions and the teacher needs to be able to differentiate
between a nuclear change and a chemical change. Nuclear changes involve changes in the
nucleus of an atom and can result in changes in mass. Chemical changes involve only outer
energy level electrons. It might be useful to point out to students that nuclear fusion occurs in the
Sun producing helium from the fusion of hydrogen nuclei. The change does not involve
electrons, but rather the changes occur in the nuclei with the release of large amounts of energy.
Figure 5. A cut-away of the solar interior showing the core, radiative zone and convection zone.
Energy is an important by-product of nuclear fusion as witnessed by the power of the Sun. The
nuclear process is demonstrated by the following reaction diagram.
Figure 6. The Proton-Proton Cycle, protons colliding
to produce a deuterium nucleus and energy.
In a chemical reaction, no such changes in the nucleus are observed. Recall the reaction for the
formation of water:
2 * 2.02g + 32.00g
2 * 18.02g
Figure 7. A model of a chemical reaction forming water.
The atoms combine in simple whole number ratios that do not involve the nucleus but rather the
outer valence electrons.
4. Students incorrectly think that chemical changes are determined by features of the
For example, if a piece of paper is ignited within a closed container, students observing the
smoke will believe the flask should weigh more after the paper combusts. Teachers can point this
misconception out to students by weighing materials before and after the change.
To learn more about this misconception and other science misconceptions, go to
Content Benchmark P.8.A.5
Students know mass is conserved in physical and chemical changes. E/S
Sample Test Questions
Questions and Answers to follow on separate document
Content Benchmark P.8.A.5
Students know mass is conserved in physical and chemical changes. E/S
Answers to Sample Test Questions
Questions and Answers to follow on separate document
Content Benchmark P.8.A.5
Students know mass is conserved in physical and chemical changes. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Science Help Online Chemistry, Physical and Chemical Properties and Changes
Developed by Fordam Prepatory School, this site contains a further explanation of chemical and
physical changes, including worksheets, assessments, and answers. Some of the activities may be
more appropriate for advanced students; however, parts of the worksheets are completely
appropriate for the “meets” designation by the Nevada State Department of Education.
To access these activities, go to
2. Physical or Chemical Change?
This activity was created by a Quia Web subscriber and contains an online quiz regarding
chemical and physical changes. It can be used as a pre or post quiz on physical and chemical
change. The site does not provide feedback for students.
To access these activities, go to
3. Beacon Lesson Plan Library, Conservation of Mass
This site provides a well-designed lesson focused on the Law of Conservation of Mass. Students
observe a chemical reaction, determine that a gas has mass, and confirm the law of conservation
of mass and energy. After completing the lab activity, a multiple choice assessment is provided
for the teacher.
To access these activities, go to
4. Conservation of Matter and Balancing Chemical Equations
Developed by the University of Virginia, Physics Department, this website contains a lesson unit
on the conservation of mass and includes several equations for students to balance. There is also
an assessment with answers provided. One note for the teacher is that although the Nevada State
Science Standards do not explicitly state that students should be able to balance equations, they
can accomplish equation balancing using manipulatives or by drawing pictures which model the
To access these activities, go to
5. Changes in Matter: Mission Impossible
This website contains a lesson developed to meet the science content standard for Conservation
of Mass for 8th grade students in Texas. The lesson is activity based, contains notes to the
teacher, a scoring rubric for the activities and an assessment.
To access these activities, go to