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.

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

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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.

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.

2H2 / + / O2 / / 2H2O

/ + / / /
2 * 2.02g / + / 32.00g / = / 2 * 18.02g

Figure 4. A model of a chemical reaction forming water.

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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.

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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.

(From

In a chemical reaction, no such changes in the nucleus are observed. Recall the reaction for the formation of water:

2H2 / + / O2 / / 2H2O

/ + / / /
2 * 2.02g / + / 32.00g / = / 2 * 18.02g

Figure 7. A model of a chemical reaction forming water.

(From

The atoms combine in simple whole number ratios that do not involve the nucleus but rather the outer valence electrons.

4. Students incorrectly thinkthat chemical changes are determined by features of the change.

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

and,

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 FordamPrepatorySchool, 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.