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Middle School    |    Formative Assessment Probe

Earth’s Mass

By Page Keeley

Assessment Life Science Middle School

Sensemaking Checklist

This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

Earth's Mass

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The purpose of this assessment probe is to elicit students’ ideas about the cycling of matter. The probe can be used to determine whether students recognize that once-living matter breaks down and cycles through ecosystems without subtracting or adding mass to the Earth.

Type of Probe

More A-More B

Related Concepts

closed system, conservation of matter, cycling of matter, decay, transformation of matter


The best response is C: The mass of the Earth stays about the same. Although some mass is added to the Earth by meteorites and micrometeorites, it is so minuscule that for the purpose of this probe, it can be neglected. Likewise, some mass is lost when hydrogen atoms at the edge of our atmosphere escape into space, nuclear reactors convert matter to energy, or rockets and other materials are launched into space. But, this too is negligible as far as the total mass of the Earth. The reason why the Earth’s mass stays about the same, even though tons of dead and decomposing material are produced each minute, is explained by the conservation of matter principle. No matter what physically or chemically happens to materials in a closed system, the amount of matter, and thus the mass, stays the same. No new matter is added or taken away. Earth is primarily considered a closed system in regard to matter (not energy), even though there are some small amounts of material entering or leaving from space. This means that the amount of material on the Earth stays pretty much the same, even though its form, composition, and location can change.

Earth does not receive new inputs of elements, such as carbon, hydrogen, oxygen, nitrogen, silicon, calcium, and so on, that make up living and nonliving things. What we see is what we get. The amount of matter that made up our original Earth is still here today. Matter is continuously transformed through biological, physical, and geological processes. Carbon and other nutrients constantly cycle between living and nonliving things. For example, plants use carbon dioxide and water molecules to make sugars. These sugars are transformed into plant material and, say, eaten by a rabbit. Some material is converted back into the inorganic carbon dioxide and water of the atmosphere when the rabbit respires. Furthermore, when organisms, like the rabbit, die or parts of organisms, like leaves, fall to the ground, they are broken down by decomposers such as bacteria, worms, and insects and “recomposed.”

As decomposers use food from dead material, gases are released back into the atmosphere through respiration, and molecules are incorporated into the decomposer’s cell and body structures. Some of the material is further broken down and released as waste into the soil, water, or air. The materials within the Earth may be further transformed by geological processes into rocks and minerals. Matter in the soil, air, and water may be taken in again by other living organisms and transformed into a new material.

The most important thing to keep in mind is that living and nonliving matter never disappears (it may disappear from sight but it does not cease to exist) or is added as additional matter to the Earth’s total mass. When organisms die or waste materials are produced, the number of atoms remains the same even though the material seems to disappear or build up. All matter can be accounted for through various transformations.

Curricular and Instructional Considerations

Elementary Students

During the elementary school grades, children build an understanding of recycling that forms the foundational idea for later understandings about the cycling of living or once-living matter. The idea that materials can be reused in different forms begins with objects and extends to once-living things in the upper elementary grades. In those grades, students develop a basic understanding of decomposers and the decay process, beginning with macroscopic organisms they can observe, such as worms, beetles, mushrooms, and molds. They begin to notice that substances can change form and move from place to place but they never appear out of nowhere and never just disappear (AAAS 1993, p. 119).

Middle School Students

In the middle school grades, students become familiar with ecosystems, including the beneficial role of bacteria in ecosystems. They expand their understanding of decomposers and decay to include microorganisms and recognize the essential role of microorganisms in the decomposition process as matter recyclers. They are introduced to ideas about nutrition and matter and energy flow and identify the relationships between organisms in a food web, including decomposers. At this level, they begin to trace matter as it moves through ecosystems and should connect it to the notion of atoms. The idea of systems is made more explicit and they can now link the concept of closed systems and cycling of matter to the conservation of matter principle.

High School Students

At this level, students should have a sufficient grasp of atoms and molecules so as to link the conservation of matter with the flow of energy in living systems (AAAS 1993, p. 121). In high school, students approach decomposition from a molecular view, including the assortment of complex biological processes involved in breaking down once-living material. They recognize the role of decomposers in cycling atoms and molecules throughout the living and nonliving components of Earth’s biosphere while conserving matter. Students connect the natural process of biodegradation to human-engineered systems that solve problems of buildup of dead material and metabolic waste.

Administering the Probe

This probe can be combined with “Rotting Apples” and the conservation of matter probes in Volume 1 of Uncovering Student Ideas in Science (Keeley, Eberle, and Farrin 2005). The word weight can be substituted for mass without compromising the ideas elicited by the probe. Substitute weight if the concept of mass is not well developed with younger students, because it can interfere with students’ interpretation of the probe. Research indicates that some students confuse the word mass with the phonetically similar word massive and thus equate the probe with size rather than amount of matter.

Related Disciplinary Core Ideas (NRC 2012; NGSS Lead States 2013)

6–8 LS2.B: Cycles of Matter and Energy Transfer in Ecosystems

  • Food webs are models that demonstrate how matter and energy is transferred between producers, consumers, and decomposers as the three groups interact within an ecosystem. Transfers of matter into and out of the physical environment occur at every level. Decomposers recycle nutrients from dead plant or animal matter back to the soil in terrestrial environments or to the water in aquatic environments. The atoms that make up the organisms in an ecosystem are cycled repeatedly between the living and nonliving parts of the ecosystem.
Related Ideas in National Science Education Standards (NRC 1996)

K–4 Organisms and Their Environments

  • All organisms cause changes in the environment where they live. Some of these changes are detrimental to the organism or other organisms, whereas others are beneficial.

5–8 Populations and Ecosystems

  • Decomposers, primarily bacteria and fungi, are consumers that use waste materials and dead organisms for food.

9–12 The Interdependence of Organisms

  • The atoms and molecules on the Earth cycle among the living and nonliving components of the biosphere.*

9–12 Matter, Energy, and Organization in Living Systems

  • As matter and energy flow through different levels of organization of living systems— cells, organs, organisms, communities— and between living systems and the physical environment, chemical elements are recombined in different ways.

9–12 Geochemical Cycles

  • The Earth is a system containing essentially a fixed amount of each stable atom or element. Each element can exist in several different chemical reservoirs. Each element on Earth moves among reservoirs in the solid earth, oceans, atmosphere, and organisms as part of geochemical cycles.

9–12 Natural Resources

  • Natural systems have the capacity to reuse waste, but that capacity is limited.

*Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal.

Related Ideas in Benchmarks for Science Literacy (AAAS 1993)

K–2 Flow of Matter and Energy

  • Many materials can be recycled and used again, sometimes in different forms.

K–2 Constancy and Change

  • Things change in some ways and stay the same in some ways.

3–5 Interdependence of Life

  • Insects and various other organisms depend on dead plant and animal material for food.

3–5 Flow of Matter and Energy

  • Over the whole Earth, organisms are growing, dying, and decaying, and new organisms are being produced by the old ones.*

6–8 Structure of Matter

  • No matter how substances within a closed system interact with one another, or how they combine or break apart, the total weight of the system remains the same. The idea of atoms explains the conservation of matter: If the number of atoms stays the same no matter how they are rearranged, then their total mass stays the same.*

6–8 Flow of Matter and Energy

  • Over a long time, matter is transferred from one organism to another repeatedly and between organisms and their physical environment. As in all material systems, the total amount of matter remains constant, even though its form and location change.*

6–8 Systems

  • A system can include processes as well as things.
  • A system is usually connected to other systems, both internally and externally. Thus a system may be thought of as containing subsystems and as being a subsystem of a larger system.

9–12 Flow of Matter and Energy

  • At times, environmental conditions are such that plants and marine organisms grow faster than decomposers can recycle them back to the environment. Layers of energy-rich organic material have been gradually turned into great coal beds and oil pools by the pressure of the overlying earth. By burning these fossil fuels, people are passing most of the stored energy back into the environment as heat and releasing large amounts of carbon dioxide.
  • The chemical elements that make up the molecules of living things pass through food webs and are combined and recombined in different ways.

*Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal.

Related Research

  • Several research studies have identified children’s common notions about decay. In response to research questions related to the “disappearance” of dead animals or fruits on the surface of the soil, young children thought dead things just disappear and did not allow for ideas about conservation of matter after death. Most of these children thought of decomposition as the total or partial disappearance of matter (Driver et al. 1994).
  • Some middle school students think dead organisms simply rot away. They do not realize that matter from dead organisms is converted into other materials in the environment (AAAS 1993).
  • A study by Leach et al. (1992) revealed that 70% of 11- to 13-year-olds lacked an understanding of conservation of matter to explain what happens after organisms die, even after the topic is taught. Furthermore, they found that up to age 16, few students had a view of matter that involved conservation in a variety of contexts, including decay (Driver et al. 1994).
  • A study by Smith and Anderson (1986) found that middle school students seem to know that some kind of cyclical process takes place in ecosystems, but they tend to see only chains of events and pay little attention to the matter processes involved. Students tend to think the processes involve creating and destroying matter rather than chemically transforming it from one substance to another (AAAS 1993).
  • Some middle school students recognize recycling of material through soil minerals but fail to incorporate water, oxygen, and carbon dioxide into matter cycles. Even after specially designed instruction, students cling to the misinterpretation that materials are recycled primarily through soil in the form of minerals (AAAS 1993).
  • Generally, students are unaware of the role that microorganisms play in ecosystems, especially microorganisms’ role as agents of change, such as decomposers and recyclers (Driver et al. 1994).
  • Field-test results of this probe with middle school students revealed that students chose the correct response, but had incorrect reasons for why the mass of the Earth would stay the same. The most common incorrect justifications described a cycle of birth and death whereby every organism that dies is replaced by a new one born, thus no new matter is created or destroyed. Another common explanation lacks a cycling concept: When Earth was formed, there was a certain amount of matter, and when organisms die, the matter remains but is locked up in dead organisms. As Earth produces more organisms, more matter is used until eventually no more new organisms can be produced and all the matter will be in dead organisms and waste products.

Related NSTA Resources

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1, “flow of matter in ecosystems,” 76–77. Washington, DC: AAAS.

Driver, R., A. Squires, P. Rushworth, and V. Wood- Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

Trautman, N. and Environmental Inquiry Team. 2003. Decay and renewal. Arlington, VA: NSTA Press.

Suggestions for Instruction and Assessment

  • Instruction that traces matter through the ecosystem as a basic pattern of thinking may help correct difficulties students have in understating conservation of matter in ecosystems. Students should be encouraged to consider where substances come from and where they go (AAAS 1993).
  • Explicitly link the notion of the breakdown of molecules and the reassembly of atoms to food webs and cycles of matter in ecosystems.
  • The notion of reusable building blocks common to plants and animals is difficult to understand if students do not first have an understanding of atoms and molecules. Once they grasp a particulate model of matter, have students use their understanding of atoms and molecules to follow matter through ecosystems (AAAS 1993).
  • It is important for students to understand the concept of a closed system. Help students understand why Earth is considered a closed system and to use systems reasoning to explain why Earth does not gain or lose matter.
  • Use examples of oil and coal beds to show ways in which material has been changed by the natural environment, other than recycling by decomposers. Help students see how burning large reservoirs of fossil fuels does not decrease the mass of Earth, but releases the matter primarily in the form of carbon dioxide back into Earth’s atmosphere.
  • Use decomposition chambers, such as those developed by Bottle Biology, www., to design investigations that show that matter in a closed system is conserved during the decay process.

American Association for the Advancement of Science (AAAS). 1993. Benchmarks for science literacy. New York: Oxford University Press.

Driver, R., A. Squires, P. Rushworth, and V. Wood- Robinson. 1994. Making sense of secondary science: Research into children’s ideas. London and New York: RoutledgeFalmer.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press.

Keeley, P., F. Eberle, and L. Farrin. 2005. Uncovering student ideas in science: 25 formative assessment probes. Vol. 1. Arlington, VA: NSTA Press.

Leach, J., R. Driver, P. Scott, and C. Wood- Robinson. 1992. Progression in conceptual understanding of ecological concepts by pupils age 5–16. Leeds, England: Center for Studies in Science and Mathematics Education, University of Leeds.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.

Smith, E., and C. Anderson. 1986. Alternative student conceptions of matter cycling in ecosystems. Paper presented at the annual meeting of the National Association of Research in Science Teaching (NARST), San Francisco.

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